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18.5 hp B & S twin, L head with electric start and alternator,

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18.5 hp B & S twin, L head with electric start and alternator, 1997 model 42D777.

This engine has surged for many years. It's annoying, but I've lived with it. It cranks over fast, and while it could start more quickly, it usually starts on the first attempt-unless it's been sitting for too long since the last start, and once started it runs ok despite the surge. While under load and while mowing the surging isn't noticible.

I've tried adjusting the idle speed and fuel mixture, do regular oil gas and air filter changes, plug cleaning/gapping, tried EZ-Start plugs, checked and adjusted the float level, changed the fuel inlet needle, replaced the fuel pump diaphragm and the vacuum line to the fuel pump, and a couple year s back decarboned the heads after many hours of running time (not much carbon-and not a hard acculmuation), but nothing I do seems to get rid of the surging when it's not under load.

The plugs have always seemed to be indicating a reasonable fuel mixture from the insulator's color and the slight amount of carbon that accumulates on the shell. I attribute that carbon buildup to my use of multigrade 15W40 oil-all year round in east Tennessee, as a compromise to switching from straight 30wt to lighter oil in the Winter.

Interestingly, it runs a little smoother at partial choke (no matter how clean the air filter and pre filter happen to be) but it still surges, which is contrary to what I would expect if it was a jetting issue from altitude-we're at about 2600' above sea level.

Lately, I'v ehad a few occasions where the engin will not RE-start after mowing for 2 or 3 hours and it runs out of gas. At those times there is no spark I'm starting to think that it may actually be an electrical issue-one that's possibly related to a wear pattern that repositions the crankshaft slightly from idle to load conditions-and that the airgap of the armature changes when that happens and also when the engine is HOT from heat soak after being shut down.
Any thoughts?
Submitted: 2 years ago.
Category: Small Engine
Expert:  Curtis B. replied 2 years ago.

curtis b :


curtis b :

The surging can be just about anything, but the hard to start when hot is usually a valve issue. With the flat head, the only way to change the valve gap is to remove the valves and grind off the valve tips to get more clearance. I would suggest checking the valve clearance, behind the manifold, in front where the breather cover is on one side and a blank cover on he other. If you have no clearance, you need to adjust the valves. If you have any , it should work. The true setting is .006in. both intake and exhaust.

curtis b :

I have used a small file and filed the valves through the breather hole, but it is not easy.

curtis b :

Let me know. Thank you.

Expert:  Curtis B. replied 2 years ago.
Did you find your problem?
Customer: replied 2 years ago.

Well, what you say may be what's happening in re. the valves, and especially with a high-hour engine it makes sense that valve face/seat wear could be happening (even with exclusively semi-synthetic diesel-spec oil use, which I hadn't mentioned) and regapping may well be necessary but I'd SAID that I've been living with that....


As I also said, the only time that it's not started reliably, even if it does surge, and even when it took a long time cranking to get enough gas to refill the carb. after running out of gas, began very recently. Not starting is a LOT harder to deal with than running with a surge.


My current concern was getting info about the no-start situation.


If it doesn't start, and if there's NO spark getting to the plugs (which I said) it can't possibly start. I also said that I suspected that it was an electrical issue, possibly involving the armature.


Monday it died while running-with a 2/3 full gas tank-and I haven't gotten a spark since then-to either spark plug.


Yesterday when I was in town I ran into a friend that I see maybe twice a year (he's an over-the road truck driver with an erratic schedule) and he said that this no-spark situation has nothing to do with the valves (which was obvious), but he then went on to say that said that not only should I check the armature/coil gap itself, but to check it when the flywheel was at several different positions-which seems logical if it happened to be a wear related situation that affects the flywheel's position) but if the gaping is reasonable every flywheel position that I check it, to then check the wire that goes from the armature/coil to the chassis.


He explained that if there's any problem relating to any safety equipment that is causing the electrical problem, such as a seat switch that's malfunctioning, or if the safety-related wiring has insulation worn off so that there's a short to the chassis anywhere in the circuit, that disconnecting that wire will let me bypass all that non-ignition-essential circuitry while still allowing a spark from the armature to get to the plugs and since it was running before the spark disappeared, that the engine should start-IF the armature is still good and IF it's gapped correctly (.008 -.012).


And if there's still no spark then it will be the armature.


Since his advice is more to the point, I'm going to try that first, but I still have to do our taxes and they can't wait, so now I may not be checking the mower for a few days-unless I need a "recreational" break from that.


I'll let you know how the armature situation turns out.

Expert:  Curtis B. replied 2 years ago.
He is correct on unplugging the kill wire at the armature and testing for spark. But I have seen Where you didn't see spark, have replaced the armature and still could not see it. Then found out that spark was there and had bought armature(coil) for no reason. When looking for the spark, unit being out of the sun is a must. Reset the valves and unit has not quit sense. Take your time, the coil is an easier replacement than regapping the valves. Have a good day. Thanks for the reply!
Customer: replied 2 years ago.

Taxes done, back to dirtier work., and this is on my short list now, hopefully this weekend I'll have the answer-or at least be able to narrowdown the possibilities.


This tractor and engine has probably between 1500-2000 operating hours on it over 14 years, and while I've previously replaced the battery and starter solenoid twice, the ignition switch once, and worked on the starter drive and starter a few times, the rest of the electrics are still original-so it wouldn't surprise me if it actually turns out to be something other than the magneto's armature.


I use an inductive strobe timing light on sparkplug leads to verify spark and whenever there's room-which this one has. It's faster, easier, and doesn't require disturbing the plugs, plus a miss is very easy to spot. It can safely be used on a hot engine too-not that that's a factor right now.....



Expert:  Curtis B. replied 2 years ago.
O,k, I'm interested as to what you find. I went down this same road with my sons mower, new coil, and finally did the valves and hasn't asked up since. Glad taxes are over myself!
Customer: replied 2 years ago.

Cranked it over-no spark, no start.


Took the side and top covers off earlier today, and blew off the dust and other crud with pressurized air, and brushed off the areas above and around the armature.


Disconnected the kill wire, it cranked and started right up-ran it for a few seconds in the garage. And no missing.

Reconnected the kill wire and it wouldn't start-no spark. OK.......


Just to double check before doing anything else, I disconnected the wire again and it started-ran it a while longer this time. No missing.

Then, just to double-double check I RE-connected the wire and it ...... started, and ran with no missing.




Checked the armature gap before pulling it to take a closer look, and it had opened up a little over the Briggs spec of .008-.012 -up to .016


Pulled the armature off to look it over closely.

Found RUST on the bottom of the armature, -especially directly above the connector for the kill wire.


I scraped/filed the terminal that the kill wire slides onto (it had some rust on it), burnished teh connector by sliding it on and off several times, then put dielectric grease into the connector and reattached it.


That's as far as I got before coming in to eat at 8pm.


Since I have the gaskets on hand, I'm going to decarbon the heads-hopefully tonight-if not, maybe tomorrow. And If I have time, I'll check the valve clearances too.


Next up I have to pull the deck, change the oil and ground drive belt, clean the deck and swap blade sets. Don't know when I'll get the time to put the new back tires on.


I lost most of Sunday when my bigger rototiller started acting up and ultimately wouldn't keep running. After an hour or 2 of trying the obvious things and not being able to get it to stay running consistently for more than a couple minutes at most-no matter how the carb was adjusted, I pulled it into the garage and parked it. Then I had to use the lighter tiller-which takes 2-3X as long. In between tilling sessions, I took the tank, carb and various bits off, checked and filtered the gas-which was actually pretty clean.


This tank had been severely corroded several years ago and I'd done the old shake shake shake routine with chain and assorted bits of hardware and ball bearings inside with kerosene and flushing it out in between sessions several times to remove the loose rust-then air-dried it and used rust remover. About 2 years later I did it again-this time following it up with an industrial-strength phosphate conversion solution over about 2 days with the tank positioned every way possible.


That was at least 3-4 years ago, so I was half expecting to see rust popping up again, but didn't see any in the gas when I filtered it. There was a little bit of other tiny particle sized stuff on the filtering media that I poured the gas through, and some on the carb's intake screen too, so it may be some of the phosphate coating that's been sloshed and shaken enough to work loose mixed with more common contaminants-the combination might have been compromising the filter screen enough to not be able to flow gas fast enough for the engine to run.


One of the things that I'd tried was swapping in a new diaphragm and the old one looked suspiciously clean. (I checked my log book later and I'd already replaced that one the last time I had the tank off a couple of years ago.)

The carb was very clean inside. I checked the valves and they're still within .001 and .002 of spec. I have a gasket set ordered for that one and I'll replace some of the same gaskets that I did a couple of years ago and a couple others too-like the one on the valve cover, and see how it goes. I noticed that the rubber elbow for the venting tube was cracked, so I have a couple of those on order too (the other tiller has almost the same 5hp engine-and that one was almost as bad).


Did a few more hours of tilling today-and after 10+ hours over the last few days behind a big and then a medium front-tine tiller, in clay-heavy fairly dry, somewhat gravelled soil, my hands are really starting to feel it. I hope the forecast rain misses us tonight/tomorrow, so I can get at least some of the potatoes in the ground.


I'll let you know how the Poulan turns out.



Expert:  Curtis B. replied 2 years ago.
sounds like your plate is full. Take your time, I'm retired and just do this for something to keep me out of trouble. Have a good day.
Customer: replied 2 years ago.


After the tractor restarted successfully several times, I figured that I should stop while I was ahead and I might as well switch over to reassembling the tiller, since the parts for it had come in.


That one started, but it wasn't easy ( I used spray carb cleaner instead of ether starting spray) -and even after tinkering with the mixture and upping the idle speed setting a lot while it was running it didn't like staying running except at near top end speed and was still missing both at both at high and at lower speeds-and restarting it was just as hard as initially starting it too. So after it cooled down I pulled off the head and cleaned/polished that and decarboned the piston top (not too bad) and areas around the valves and corresponding areas on the head (around valves and above them in te head had thick accumulations) and reset the armature gap.

later I read about and then checked that the governor setting was correct, tried moving the governor linkage rod to a different hole and tried it again the next day. Still difficult to start-but at least it was running better at a wider range of speed and missing less-but still running best in the middle to upper part of the speed range. Ran it for about 10 minutes while tweaking the idle mixture again.

Since it still has that miss, I think I should pull the flywheel to see if the key is damaged or if the points need resetting-I've never done that before but I read up on doing it, and these 2 things are about the only things left that I can think of that would be very likely to contribute to a miss, to hard starting and to making it difficut to balance the mixture for a full range of speeds. Come to think of it, even having a chain drive instead of gears there's not a lot of shock absorption in the drive train, and given the length of the tines, and our rocky soil, that could easily be enough to progressively damage that key over time and change the cam to crank timing a bit-even if the points' setting was still in the ok range. And they're BOTH out of whack-it could change that a LOT.


Getting back to the rider mower, since it was about to rain, I left the engine covers and hood off and figured that even though the dew hadn't burned off yet, that I'd mow at least the area over the septic field and closest to the house until I started getting wet. It didn't run that long. It ran progressively worse over about maybe 5-10 minutes. The mower deck was also slowing down as if it's drive belt was slipping, so I rode it back to the garage, took a few minutes to tighten the drive belt and then it wouldn't start. Checked for spark-nothing, either with or without the kill wire attached.

Now it really looks as if it's the armature.

I'm going to see if the repair shop in town has a take-off that they'll lend me to test it, and bring along a new one if that one works.


One more thing.

While I had the mower down, I pulled one head off and cleaned the piston and the head and polished the combustion chamber. There was carbon, and some of it was hardened, but neither had a lot of thick and hard deposits (the relatively small amount of which I attribute to the semisynthetic oil) but while I was decarboning around the intake valve head I thought that I detected some movement. So I checked closer and noticed that I could EASILY manually rotate the intake valve while it was in the supposedly fully closed position-like virtually no spring pressure was there, and if that's happenning on one intake valve, the other one probably has little or no lifter clearance left either, and probably less than spec for the exhaust valves too..... never rains but that it pours I guess. I've got a complete valve gasket set on order now. I probably should check the flywheel key on this one too......


So at this point it looks as if your initial thought about the valve clearances being a potential contributory cause for hard starting may turn out to be right.

Expert:  Curtis B. replied 2 years ago.
Good to here from you! Lets hope the valves will fix it up.!!
Customer: replied 2 years ago.

This week has been more of the same electrical no-start problem with the Poulan.


I was able to swap in a working used armature, gapped it to the middle or top ened of teh specification range (.008-.012 in the book) and I thought that doing that had solved the problem because the engine started immediately and a couple hours later (after rebuilding a blade drive and lunch) the engine ran without the engine side covers (or the hood-which has been off for a couple of years in case I hadn't mentioned it before) for between 2 and 3 hours in one continuous mowing session.


Before doing that mowing I'd replaced the ground drive and the deck drive belts, cleaned the deck, swapped blades for a brand new set-while waiting to get the armature, so I'm sure that the engine wasn't working as hard as it had been recently. I was mowing up steep grades, across grades, through very tall, thick, sometimes pretty damp grass. The engine never even seemed as if it was going to die. I didn't shut it off until I got back to the garage, where I drained the oil overnight.


Next day (yesterday) after changing oil and filter,I was up mowing in the orchard upper field for a little over an hour (much of it in taller weed & grass covered but on less steep ground than the day before) and got something in my eye and shut off the mower for a few minutes while I dealt with that. You guessed it, it wouldn't restart. I tried on and off for maybe 15-20 minutes-not even a sputter. Luckily this time I'd stopped in a place that let me coast all the way back down to the garage.


Checked for spark-none. Checked the armature gap "just in case" I'd forgotten to tighten it down enough, and noticed that the air gap had grown a couple of thousandths to something that was now over .012 -which is what had happened with the other armature. Probably at least 45 minutes had passed by this time since I'd shut it off-maybe even more. On a hunch, I tried starting it again without changing anything-NO SPARK, then adjusted the air gap down to .008, and it started-easily in fact. Hmmmm. I opened the gap to .012-NO SPARK AT ALL. Closed it back to .008-started easily.


I thought that this was strange-that going from one end of the spec to the other could go from no spark at all to an engine starting fine, so I did some more research online and finally after a couple of hours came across a B&S article on their own website that one of their technical people wrote.


This is what I got out of that article, and it sounds like a possible explanation of what's going on here. It may not actually be the armatre itself, but replacing that witha a NEW one may solve the problem-or not. I have to eliminate a couple of the suspects and do some resistance testing to narrow things down.


He says that the armature could be moved quite a bit further out from spec (on engines with a Magnetron all-in-one type armature) and that even so if the armature was operating properly, and the wires and connections in good condition, that the spark should still be there, but that the timing of the spark's firing might be VERY SLIGHTLY off (retarded), due to the small additional time that it takes the magnetic field to build within the coil.


In fact he said that (because of the way that the armature is slotted and secured I presume) it was almost impossible to move the armature out far enough for there to NOT be a spark-IF the engine is turning over fast enough, the magnet(s) were still strong and the armature and spark plug wires were in good condition.


He said that if the engine is cranking fast enough or starting, to first test with the B&G in-line spark tester (which I don't have)-which has a much bigger gap than the plugs do and takes more energy to cross. If it fires across that bigger gap than the coil/armature is in good shape.


He went on to tell how to test the resistance across the combined coil and the secondary wiring, (from the lamination stack ground to the spark plug terminal) and gave a range of 2500-5000ohms as being acceptable. If it's much LOWER than 2500 ohms there's an internal short in the coil-replace it.


He said that if there is NO continuity at all between those 2 points that there must be an open circuit and something between where the test leads are attached is bad-and to replace the coil.


To explain further he says that if there is NO continuity, but that the engine runs, there is a problem (poor connection or actual break) in the connection between the coil and the spark plug lead, or in the spark plug lead wire itself, or in the connector from the sparkplug wire to the spark plug-but that the coil will often have enough voltage available to cross that break.

The internal arcing that takes place from this situation will eventually create enough resistance that ignition system performance will suffer.


In short, the magnet(s) may be weak, there may be an internal short in the armature, or a weak/broken connection in either end of the spark plug wire(s) or in the spark plug wire itself. I'd never considered the weakened magnet possibility so that's one additional candidate for being the weak link. I'll know more after doing the resistance testing, but if it starts at all I'll be doing more mowing first.


After thsi experience, I'm looking to pickup an actual spark tester-I used to have one that had an adjustable gap, but can't find it. And since idleing is a problem with the tiller and I can't measure it with the meters that I have on hand, I was looking and came across a vibratory type tachometer, called a treysit sirometer-it works with no electrical connection or other power source-other than the vibration of a machine. It's sold by Briggs (among others) as part #19200-they're calling it Vibra Tach.






Expert:  Curtis B. replied 2 years ago.
Wow, the tach can be bought off Ebay.
Customer: replied 2 years ago.

The link you sent for ebay didn't turn up when I searched the other day, but it's there now-and cheaper than any other place I'd found-thanks.


Next chapter in the saga.


I did the resistance testing a couple of days ago on the original magnetron and then on 2 used, supposedly "good" B&S ones that came from an ebay seller and were able to start the mower with no problem. but they ALL tested as bad. (I didn't even bother swapping in the second used one after seeing what had happened with the first one-starting and not restarting when hot.)


After that, just to be sure that my meter was working right, I tried testing the Magnetron on that tiller that had been giving me grief-and that one tested right where B&S said that it should. (That tiller is running better again, BTW.)


The seller had sent one then another one to replace the first one at no cost when the first one he'd sent wouldn't restart when hot-just like mine had done before it conked out entirely. I'd told him about what Briggs's site had said-but he sent out the second one without testing it either anyway.

I got full credit for the used ones (he didn't even want them back) then picked up a new genuine B&S one in town yesterday, ($54.80 with tax) but haven't gotten it in yet. Wish me luck.

Expert:  Curtis B. replied 2 years ago.
good luck!
Customer: replied 2 years ago.

No luck-the new one worked for one mowing. The next day it wouldn't restart the engine-no spark.


I did some more research and came across a few references to Briggs' armatures/coils fine burning out from getting 12V-via the kill wire. Now I'm thinking that this may have been what's been actually going on all this time. It would explain a lot. I may dissect one of the bad coils just to satsfy my curiosity. (If this turns out to be the case-I'll be sending out an ebay refund to that seller fro his coil.)


Anyway, in these cases it's supposed to be getting there via a 12v power to ground path that isn't supposed to exist, so with bundled groups of wires and some unprotected open wires running to the various parts there are a lot of possible sources-bare insulation, shorted connectors, bad switches, etc., and lots of places that it could be happening.


So, because this engine has had a midlevel engine case oil leak since about age 2, and accumulates oil-held dust&gunk I started by degreasing the entire engine and the chassis ( followed by a water flush at low pressure and air drying + fans for a couple of days to be surevall the water was gone. I didn't do under the flywheel but had already blown that area out with compressed air beforehand. (I usually do the degreasing routine at least once a year, but it wasn't near the end of the season last year.)


Then I checked the DISCONNECTED kill wire for ANY voltage and found that it was indeed there momentarily (at differemnt levels because it's only a spike) while moving the ignition switch through the different positions.


I pulled the ignition switch and meter teseted it for function-it had proper continuity in all positions (which didn't surprise me because everything else works), the resistance at different positions seemed to be appropriate too, but I couldn't find any guidance about testing it in isolation for an improper INTERNAL voltage path when energized, so I started through the wiring harness, disconnecting and examining things, to see if I could find the unwanted voltage source that way-maybe by a frayed wire.

Didn't find any frayed wires, or any hardened/discolored or obviously burned insulation in any place that I looked, but I'll admit there were a few protected bundled runs that I didn't take apart.

However, even with all the safety switched unplugged, both individually and also all at one time, voltage was still getting to the kill wire, so the switches themselves appear to be ok, (even if a connector or wire could be bad without it being obvious).

I cleaned out all the connectors out with spray solent and didn't find any oxidation or evidence of arcing.

Unfortunately I have to have the ignition switch installed to be checking for kill wire voltage in the several circuits when it's at it's different positions, and the starter solenoid also has to be installed to have a complete circuit in START-so at this point I'm hoping that it turns out to be an internal problem with the ignition switch-or starter solenoid. I checked my maintenance log and even though I've previously replaced each of those individually (solenoid twice/ign switch once), the last time I did either was replacing the ignition switch in 2006, and based on the prior parts' longevity it isn't too soon to be thinking about swapping them out for new ones on general principles-so I have both parts coming and I'll try the ignition switch first.


Even if neither part eliminates the kill wire voltage, at least I'll be able to eliminate them as possibilities. unfortunately that doesn't leave much other than the charging system parts hiding under the flywheel. Come to think of it, now I don't remember for certain if I tried disconnecting/isolating that circuit. Have to double check the wiring diagram to see if I can tell if I can do that without needing to have anything else disconnected.


For that matter, even if f it turns out to be in the charging system, I could probably live without ANY charging system. I don't use the lights anymore (the hood has been off most of the last 5 years or so), the ignition system can't be drawing very much, so it's basically the starter as the biggest cumulative draw and since the first replacement it's always had the bigger than OEM capacity 350 cranking amp battery-which I could start charging routinely......


Expert:  Curtis B. replied 2 years ago.
Yes, unless you have an electric clutch, you basically do not need the alternator.
Customer: replied 2 years ago.
Now that I reflect on it-this type of ignition doesn't require any battery power-and if I didn't have the electric starter this tractor wouldn't need any battery at all-and there'd be no 12v to fry the armatures....... ahh for simpler times.
Expert:  Curtis B. replied 2 years ago.
You could put on a pull start! Yeah, right!
Customer: replied 2 years ago.

Right about now that doesn't sound like a completely ridiculous idea-except that it's hard enough for me to pull over our 11 or 12 hp Briggs on our emergency start generator and that's only 2/3rds of the way there. (I don't know if it has a compression release system.) My back probaby wouldn't be too happy about that-although it does seem to start without any drama.



Expert:  Curtis B. replied 2 years ago.
When I work on The Polaris 4 wheelers, the first thing I do is cut the pull cord off and be sure the electric start works!
Customer: replied 2 years ago.

Kinda burning your bridges that way aren't you? What happens if the machine is in the boonies somewhere and its battery goes low-or gets too weak from cold to start the engine as it declines?



Well, you may have been right all along about the no start with no clearance on the valves. AND it looks as if you're right about the wisdom of using an "official" (or at least adjustable) type of spark plug tester too.


Now it looks as if 2 (or more) different things are probably going on here at the same time. The valve clearance issue AND the ignition-related issue. At least.


Today I tried unplugging the alternator wiring and that made no difference-there was still a momentary flash of what looked to be a low voltage to the killwire when turning the old ignition switch from run to off. (Not from a running engine to off. )


I got sidetracked before testing the NEW ignition switch, because right about then while digging out some ar compressor accessories to inflate a flat tire on our driveway, I finally found my "official" (adjustable gap) spark plug clip on tester, and just for hell of it (after the tire) clipped it onto #1 spark plug lead with the same (new) Magnetron installed that I'd thought had completely cashed in. No spark-until I closed the gap down to about .030. Then there was a spark. ( Maybe the coil is only weak but at least it's still got some life left.)

I double checked the flywheel air gap and once again ran into the same dilemma that I've had for some time-which I probably should have mentioned earlier-a LOT earlier.


I ALWAYS set the gap with both legs of the Magnetron simultaneously straddling the flywheel magnet, (which has them overlapping the adjacent body of the flywheel on both sides on this engine) but when I do that I have a choice between setting the gap while letting the feeler gauges cross the entire face of the magnet and overlap onto the flywheel OR set the gap while ONLY letting the feeler gauges lie between the magnet and the Magnetron. Never read anything that indicated it would be a problem either way. So, I've been under the assumption that it shouldn't make a difference, because the air gap is supposed to be between the magnet and the legs.


The dilemma comes up because on this particular flywheel, the face of the magnet is very slightly crowned (higher in the middle and slightly lower at the outboard edges where the Magnetron lines up with it when it's being set.).

In addition to the crowned configuration, at the magnet's outboard edges it's actually several thousandths of an inch below the adjacent front metal surface of the flywheel.


So, if I set the air gap while the feeler gauges are overlapping the body of the flywheel it's several thousanths wider than if I set air gap while holding the feeler gauges so that they lie between the magnetron and only the magnet. In fact, currently doing it the second way allows the Magentron to scrape the flywheel's light coating of surface rust ever-so-slightly at a couple of places around it's perimeter when that gap is set to specification.


I'd been setting it to the "no-scrape" result-until today.


After seeing that spark in the spark tester when it was set at well under what Briggs official one's gap is supposed to be (.070" I think) , I closed the armature air gap down slightly. It was now .010-but in the flywheel scraping position. The gap in the spark plug tester could then be opened up quite a bit-it was now jumping a gap of about 1/8" possibly more-while cranking....hmmm.


Leaving plug wire #1 loose, I swapped the spark plug tester over to cylinder #2 and reconnected the alternator wiring,and cranked the engine again-good 1/8"+ spark on that one too.


Reconnected spark plug wire #1, cranked and the engine didn't start. Hmmmmm

Went over to #2 cylinder, reconnected that wire too and the engine started. DOUBLE hummmm.


DISCONNECTED #2 spark plug wire-no start. Aha.


Either it's the phase of the moon, or the barometric pressure, and this will be completely different tomorrow-or it's starting to make sense.


#1 cylinder is the one who's head I'd pulled 2-3? weeks ago and had noticed the valve being able to spin when it was supposed to be seated.


So it looks as it cyl #1 NOW is not firing while starting, so maybe I co-incidentally cleaned the seat areas too well on #1- and the intake valve is even looser in the seat no than it had been. And that the air gap has to be set so that the armature is scraping the flywheel in order to start.


I've been doing almost all my SPARK checking from the cylinder #1 accessible side where the armarture is on this engine (with the inductive timing light.) So maybe cylinder #2 had actually been getting combustion more of the time, and the engine was runnning on one cylinder part of the time lately-which might have helped overheat things and helped do in the armature? Neither plug ever seemed either burnt or badly fouled.


Now I'm wondering, if the flywheel magnet can be shimmed, maybe I could raise it and eliminate the scraping while keeping the proper gap. I see what looks like a flush Phillips screw recess in the center, but don't know if that is anything which can or should EVER be unscrewed-but I'm not going to try doing that before getting more info.


Have you ever heard of a flywheel magnet being like this on a 18.5hp or similar Briggs, and is there anything that could be done to rectify that besides shimming the magnet, replacing it, replacing the entire flywheel or or turning the flywheel on a lathe to even out it's surface? I still think that a GOOD armature would be able to jump a wider gap than .010", but it would be nice to be able to set it correctly and not have the scraping.


Luckily it's been raining here, so I wouldn't have been able to mow much lately.



Expert:  Curtis B. replied 2 years ago.
Just back the armature away just enough to clear the flywheel. I doubt your talking less than .002in. I think it will work. I always say .010in, but I have been told that some will not work at ten , but will work at .008. Maybe, I haven't spent much time on worrying about. Thanks.
Customer: replied 2 years ago.

Tried backing it off about .002, and that only weakened the spark considerably. (At that point I was still within the OEM spec for airgap too.)


That started me looking at things a little bit diffferently though, so after taking a couple days to weld a couple of cracks, derust, grind and paint the underside of the mower deck, and fix another spindle, I tried a couple of other things.


I hand sanded the flywheel to clean it, (very gently also sanded the surface of the magnet to clean it) and concentrated my efforts on the highest areas of the flywheel-(the ones that come closest to the armature) which turned out to be on either side of where the magnet pocket is machined out. Which makes sense-if the flywheel isn't perfectly round, or if it wobbles, it won't necessarily start hitting the armature.

I only took off something quite minor like .002" from those areas.Whatever it was it effectively raises the magnet by that same amount for airgap setting purposes when the airgap is being set. I double checked the profile of the crowned surface of the magnet and teh center was maybe .004-.006" above its outboard edges.


The curvature of the legs of the armature were also not close to being an exact match to that of the flywheel either-even before doing the flywheel sanding. I'd noticed this before but hadn't tried doing anything about it. There was something like a .010"-.012" difference from the outer edge to the inner edge of this one.


For a while I was considering that maybe Briggs did this so that the armature legs would match the profile of the magnet better, since it's curvature is ALSO for a smaller diameter than this flywheel's diameter but the 2 curvatures didn't match.Then I figured that they more likely just used this armature on some smaller diameter flywheels too. And PROBABLY also used the same flywheel magnet on at least some smaller diameter flywheels too-which would be one explanation of why it's crowned in the first place. I'm going to check the leg curvature of my original one sometime to see if it's any different-maybe they went to fewer parts to cover more engines since 1997.


Anyway, I loosened the Magnetron and held a piece of emery cloth between it and the flywheel and rubbed the legs of the armature enough with the bolts keeping it from wobbling to begin to change the profile-which only touched the outer edges at first but gradually lowered the outboard edges by at least a few thousandths and now goes inwards from there to something like 1/3 of the way across the curved part of the legs.

(That took a lot of doing-maybe I'll get ambitious and do it more later.)


This now places the inboard edges of the armature that much closer to the magnet edges when the gap is being set. This also makes the crown of the magnet be closer to a litle more of the leg surface of the armature as the flywheel rotates.


This was the same Magnetron that needed to be scraping the flywheel slightly before to produce a big spark, and it now didn't need to be that close to produce a big spark.


It started, (seemingly on one cylinder and then the second one evidently started as it got up to speed-the 2 cylinder heads both heated up pretty quickly, so I mowed. It didn't restart when hot after I'd finished mowing. I left it overnight.

Next day (yesterday) without changing anything it started, so I mowed.


I'm now hoping that after correcting the valve clearances that it may restart when hot. I believe that the valve stems may be lengthening just enough from heat to make a big difference when the engine is hot enough. I'm caught up enough on the mowing now that I hope to do that this coming week.


Expert:  Curtis B. replied 2 years ago.
sounds like you are making progress. The valves will make big difference. Thanks.
Customer: replied 2 years ago.

Checked the valve clearances on both sides the other day and the exhausts weren't too bad-about .004 each, but the intake on #1 was less than zero-(that's the valve that was staying slightly open off it's seat) and on #2 it was less than the smallest feeler gauge leaf that I have which is .0015".


Decarboned/cleaned/polished the ports, the heads, and valves.

The valves themselves were/are in surprisingly good shape, hardly any dimensional wear to the stems or valve guides and no noticible pitting on the sealing surfaces of the valves or the seats, so I'm going to lap them in by hand and then do the stem shortening. With any luck I'll be putting it back together today.

Expert:  Curtis B. replied 2 years ago.
Good deal!
Customer: replied 2 years ago.

Finished clearancing the valves th eotherday and yesterday got the engine running and fine tuned the idle with that no-contact vibroTach.


It started easily but ran very rough and was missing a lot the first few minutes of running, so I didn't know what to think at first, but after I added a bit of carb cleaner to the gas it got a lot better-much less missing. Then adjusting the idle mix smoothed it out more.


I was able to set the high speed rpm fairly accurately with the Vibrotach and with practice could detect about 100rpm changes in it's running speed with that when fine-adjusting the idle mixture-I was impressed by how simple this instrument is-and that it works.....


.It's now running with hardly any detectable surging or missing throughout the entire rpm range. I have to check the specs, but according to the Viro Tach at near dust last evening that range was indicated as being about 1800-3400rpm.


More importantly, I was able to lower the idle speed quite a bit-it's running very smooth at idle now with no noticible surging and only an occasionaly miss (which may be from the gas) which is a BIG difference from before at lower speeds. It's no longer surging to the point that it's obvious to anyone, altough it shows up on the Vibro-tach as about a 100rpm range at top speed. It's now runing better than it has in years. Many thanks for the valve adjustment suggestion.


I ended up with .001 over the max installed spring clearance spec for each-intentionally. I figure that that first .001 will cover the wear-in and buy me some more running time before it needs another adjustment. Heck, if it took 15 years to lose all the clearance on one of the intake valves and very nearly all on the second one, I may never need to do it again.


Unfortunately, it still didn't want to restart at 2 or 5 minutes after about an hour's worth of running at mostly high speed...................but restarted after about 70minutes of cool down time in the garage last night. So the most irritating original problem is still there.


Because adjusting the carb mixture makes very noticible changes with minor adjustments and it isn't surging or missing much at all, I'm working on the premise that the carb isn't either significantly plugged up that the float level isn't LOW by much-although I suppose that if it's too HIGH and the carb is getting hot enough the gas COULD potentially be expanding or boiling and flooding the engine-which would obviously make the engine hard to restart. But that would raise the question of why it's getting too hot.....So checking the float level is on my short list now too.


Right now, the next thing that I'm going to try is closing down the spark plug gaps slightly (from .030 by about .002 at a time) to see if lowering the voltage requirement for firing them helps any with restarts when it's hot. If so, the coil would still be the weakest link. (I've had the EZ-Start fine wire plugs in there since shortly after the no-restart problem began.)


Expert:  Curtis B. replied 2 years ago.
Customer: replied 2 years ago.

Just wanted to check in with you.

can't find a record of having told you this or not, but after one of he last replies I happened to be in justthe right position to notice that the muffler is rusted through into what's essentially a narrow " slot" along a good part of the back side-the side closest to the carb.


Haven't had a chance to get or install a new muffler or check the carb's float level, but I'm going to credit you mainly based on your valve adjustment idea, as that's something that has made a difference in how well the engine runs, irrespective of how well it starts.....



Expert:  Curtis B. replied 2 years ago.

so you think maybe the muffler is causing vapor lock in the carburetor. That is a possible! Thanks for the update!
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