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senatorperkins 08-23-2012 12:48 PM

R100 Sputtering At Speed
Hi.. I have a 1978 R100/7 with 32K miles on it. I'm having a problem with it sputtering/cutting out/stumbling at high speeds, and more recently at lower speeds. I'm going to try to get all the necessary information out of the way in the OP, so sorry for the length. Here's a picture for fun..

So I just took it for a 1600 mile loop through Michigan/UP/Wisconsin with camping gear, luggage, and my girl on the back. From day one it would stumble very briefly while on the highway (55-70mph, 4th or 5th gear, 3800-4200RPM) maybe only once every 30 minutes or so. The sensation is very brief loss of power, a sharp deceleration almost like you're hitting the reserve on your gas tank, but it recovers immediately.

(Backstory: This happened on the first trip I took it on after I bought the bike at the beginning of this summer, on a hot day from Ann Arbor to Cincinnati. First occurred after about 45 minutes of riding, the frequency of the sputtering increased to a couple times a minute so I pulled over and did some damage control, thinking the engine was overheating but hoping to make it all the way down. I raised the float level thinking that the bowls were running dry, then re-gapped the points and retarded the ignition a little bit, and swapped the plugs back to some Bosch plugs that came with the bike, having previously put in new NGK BP6ES plugs. Kept riding, and the problem resolved itself. Checked the timing with a light down in Cincinnati before heading back, and it was dead on (which is strange because I had checked it before leaving and then retarded it at the side of the road). The ride back was flawless. I then went through some more maintenance, discovered I was getting a frustrating dual image, bought a used mechanical advance in decent shape because the one I had looked a little worn, though the new advance had the same dual image problem, took it to a local airhead expert who installed new points which (strangely) solved the dual image problem, he reset the float level by setting the line on the floats parallel to the carb bottom when the valve closes, re-timed and synched my carbs. Oh, also at the beginning of the summer I measured the compression at 130psi in each cylinder.

Everything was great for about 1000 miles, though I noticed oil consumption was high at 1500miles/liter.)

So back to the trip I just got back from.. the first few days up into the UP the sputter was regularly occurring every 30min to an hour of highway riding. The first 45min to an hour of the day usually had no issues though. Then after holing up at a cabin for 3 nights without riding, the sputter increased in frequency to once every 5 minutes or so. I thought maybe my airfilter had become clogged from riding on some dirt roads, and I realized I had never checked it. This continued until I got to Chicago, though strangely the last hour into Chicago in the rain on I-94 at 70mph was sputter free. Had a chance to do some maintenance in Chicago, and upon pulling the plugs saw that they were extremely dark with sizable deposits. Here's a picture:

My thought was that this explains my high oil consumption (in addition to pushrod seal leaks), and may be compounded by a rich running condition from the clogged airfilter, causing intermittent fouling. So I went down to Motowerks Chicago, which is a great place by the way, and got a new airfilter and showed them that plug. The fellow recommended some higher temp plugs, NGK BP6ES. So I installed fresh plugs and the airfilter, and when I took the old airfilter out I noticed... about half a cup of dog food under my airfilter inside the airbox. Which blew my mind. I've seen mice nests in airboxes before, but never with dog food and nothing else. So I thought some asshole had at some point deliberately put dog food down the air snorkel, but who knows, and I don't think it caused any problems. Just an odd digression for you.

Anyway Over the next 150 miles, the problem came back, and much worse, stumbling sometimes several times in a row. Often the stumble didn't make any noise that I could hear other, but occasionally now if it did it several times in a row I would hear a sharp sound, though not apparently a backfire. It would happen at any throttle position, always feeling the same when it stumbled, and sometimes going faster or slower would momentarily relieve the problem. Re-timed with a friend's timing light in Grand Rapids after swapping out the condenser for an old one, thinking that my condenser was going, and this time it was a good 10 degrees retarded at full advance, and the split timing was back. Checked the plugs, which looked like below, except half of the insulator was rather white. Blew out the fuel lines, checked the flow from the petcocks, cleaned out my carb jets, and nothing seemed out of the ordinary. The last 150 miles were smooth for the first hour, then it got way worse, and we just barely made it home, limping. Now it was sputtering as low as 30-45mph, when it hadn't before. Since then I have ridden it down the street and it has the same issue even at low speeds in 1st gear. Idle seems totally fine.

The new plugs, with 300 miles on them look like this:
so it seems they're going the same way as the last plugs and oil fouling, though I should mention this picture was after idling for half a minute with the choke on and going up and down the street.

Okay so that's the story.. here are my thoughts, and what I've done since then.
I thought it was pretty clear that this is an ignition problem, even though it's tough to get a good read on the carb mixture with all the oil burning. So having re-set the points and the timing, swapped a condenser, I thought that the coils were failing. My bike has the two stock 6V Bosch coils, like the one on the left on the first picture on this page:

So I disconnected all the coil wires and used a digital multimeter to check the resistance of the primary and secondary windings, finding that the primary coils for both were 1.2-1.3 ohms, within the specs in my Clymer manual, but the secondary windings were 6.68k and 6.73k, both just below the spec of 7.5-9.15k in the same book. Could I have found the problem? I did the test sequence explained here: and everything checked out. The spark looked good, I think. I checked the resistance of the plug wires, which both read close to 5kohms, and cleaned the spade connectors to the coils and the battery's terminals and engine ground (none of it resolved the issue).

That's my hypothesis for now, does anyone else have any ideas of what to check before I order new coils? Should I just order an electronic ignition at this point? Maybe the oil I'm burning is causing the misfires and my coils are okay?

P.S. I just noticed this while photographing the plugs, the cap end of the old plugs with about 3k on them has this distinct black mark:
I checked the inside of the plug caps, and the brass part this presses into does look a bit corroded. Strange thing is, the new plugs don't show evidence of this, and the problem persists. Maybe there's so much grime in the caps that some of the spark is being wasted through this connection? I'm going to get some contact cleaner and check..

disston 08-23-2012 01:16 PM

I think your plugs show an over rich condition. The idle mix screw starting position should be 1 turn out. You tune for best running starting at 1 turn. This according to the Bing manual.

Overly rich can be caused by floats that don't float. No mater what they are set at if they are sinkers instead of floaters the gas will not be regulated. You can try floating them in some gasoline to see if they look like they have enough buoyancy but this is a judgement call. You can weight them but you need a good set to compare against. I change mine every few years. BTW several people on this list tel me that's too anal.

The jets and needles in the carbs also are wear items. Some don't see this point but as the needle goes up and down in the jet it rubs. It wears. You might consider at this point a complete carb rebuild. If your carbs are flat tops the diaphragms are rather expensive, they aren't cheap for the other carbs too, I try to keep the diaphragms as long as I can.

senatorperkins 08-23-2012 01:36 PM






mykill 08-23-2012 01:45 PM

Your bike needs some loving.
Clean and flush your fuel tank, maybe change the fuel lines. Go through the carbs as suggested by Disston. Fuel supply seems to me the MOST likely suspect to me.
I would also check the resistance of your caps and wires as well as inspect the coils and connections. Wirewrkr has good deals on caps and wires. Points should be inspected and replaced if badly worn, then adjusted and timed to perfection. No place to be impatient!
All of this is assuming your heads are torqued and valves are adjusted to spec. Then dial in your mixture and carb balance.

senatorperkins 08-23-2012 01:55 PM

mykill, to avoid some confusion I should clarify that I have just done everything you just suggested, save for flushing the tank and changing the lines, to no effect as explained in the OP. I did clean and inspect the carbs when I first got the bike, and now I will be ordering new jets and needles, possibly floats.

Also, I should be clear that the stumbling seemed to occur for just one engine cycle most of the time. It was more of a very momentary loss of power than a prolonged sputter, leading me to think that it's not the fuel delivery, but I'll be careful not to rule it out.

EDIT: Just weighed the floats, one was 12.9g, the other 13.0g. I made sure they were dry by leaving them in the sun for about 30 minutes. The previous owner replaced them at some unknown point, and the weights match the listing of similar bing carbs exactly. I'm thinking I might not replace them.

disston 08-23-2012 02:17 PM

Another troubling issue is the double timing image that went away. Usually a problem in much higher mileage machines I'm still perplexed by the fact it went away. When you put a timing light on the flywheel and check the timing mark you should watch the mark climb in the window as you raise the speed of engine with the throttle. End result should be full advance and a solid mark. Just a little fuzziness might be one degree. Anything less than 2 degrees is still OK I think. Snowbum talked about this several years ago but I don't know if it's on his Web Site. I could go look but I might not make it back. :freaky

A '78 bike has ordinary points, not in a can, correct?

senatorperkins 08-23-2012 02:39 PM

disston, this troubles me too, and I was especially confused when it went away when the points were replaced, and not when the advance unit was replaced. Then I was frustrated when it was back last time I checked. Yes, just ordinary points. I read all I could about double images.

Here's what I see: at idle, there is a double image, though it's hard to tell exactly where either image is. The separation appears to be maybe 3mm. Just off idle, the image sort of gets clearer, the double S and lines get clear, become just one image. Then the dot comes into view just before 3krpm and becomes centered in the window, but it's not as solid as the S was as it was 'leaving'. When I timed it in Grand Rapids just before the final leg, I saw a 'ghost' dot pass through the window before another dot became centered at full advance.. separation maybe greater than 10-15mm. Which worried me greatly and is why I retarded it until (I think) only the ghost dot becomes centered. That's the best I can describe it, it's hard to tell exactly what's going on. I wonder if I could find a non-inductive timing light that hooks up to the points to see if I get the same result.

And that's also why I was leaning towards an ignition issue, but I have no idea why it's intermittent when I have observed the same timing effect in the past and the bike ran perfectly for long trip(s). I should mention that after 1500 miles, the new points have already started pitting a little, forming a mountain and valley, though not as pronounced as the previous set. Again, the condenser was replaced with the points. Is this normal progression?

disston 08-23-2012 03:12 PM

I now use a booster box. No reason not too, once you get the thing working right. I wouldn't put the box on till I had the points working right though.

It sounds more like you have the slight variation of bouncing points that aren't really stable. It sounds like it, I can't be sure. If you are getting one solid mark above idle it's OK. A little jumping around idle can be normal. Or so one of the Gurus told me when I had this problem.

Another test I think is how the bike runs. A smooth running BMW has an even firing engine. When My Beemer had the double timing image it had a lopey idle. It almost sounded like a Harley. Almost, but not quit.

It's true that Duane mentions changing advance units as one way to sometimes eliminate a double image. My experience with that was it made no difference and I had about 5 advance units to go though. You have read Duane's articles on this problem?

Another inmate here recently was able to measure the run out of the cam tip with a dial gauge. Set this to zero and thereby eliminate a double image.

Not sure any of this is about your main concern. The problem is still intermittent cutting out at speed?

senatorperkins 08-23-2012 03:43 PM

Disston, I share your thoughts exactly, though my old advance was pitted and I think it was causing the old points to close rapidly due to the wear on the rubbing block.

I did read Duane's article, and I was hoping that uneven wear on the two lobes on the advance was causing the split image, but it appears it is not. I would have thought the camshaft tip had runout (I don't have a dial gauge to measure this yet), but then the image was corrected with new points. And now it's back to acting strange. The only explanation I can think of for the points is that the pitting that develops somehow causes the points to fire at different times every half revolution or so. Or maybe the pitting causes issues with possibly weak coils, and/or corrosion at the cap/plug contact.

The funny thing is it starts and idles fine, always returning to a steady 1000rpm, and never any misfires at idle. So yes, the problem is still cutting out at speed, I think I'll ride it some more to see where it stands now.

Any word on the slight out of tolerance of my secondary coil? 6.68 and 6.73k ohms instead of 7.5-9.15k. I thought the failure mode is that the secondary coil would break and the resistance rise or become an open circuit, but maybe not? Oh I completely forgot to mention: the tachometer needle tends to weave back and forth at my cruising engine speed of 3.8-4.2krpm (where most the stumbling occurs), not erratically, perhaps back and forth twice a second or so. Amplitude is about that same range (3.8-4.2krpm) I don't think it does it at other speeds, but then again I don't usually look at my tach unless I'm cruising.

I'm hoping the local airhead tech has some spare coils I can try without having to buy a new set only to find that it was something else in the ignition or fuel delivery.

disston 08-23-2012 04:12 PM

The coils on our vintage machines are very robust and rarely cause problems but your symptoms could specifically be related to the coils so I think it worth trying another pair. I did this once also, no change. I forget what I was chasing that time.

senatorperkins 08-25-2012 10:22 AM

Okay, some updates here:

Went on a 45 minute ride at 50-60mph and it only stuttered once, other than some roughness in warming up. I've been talking to the local mechanic and we're thinking that because replacing the points before fixed the double timing issue (as odd as that seems), it's worth another try. I'm pretty sure one side sometimes fires 10 degrees or so retarded, giving me a ghost image, and it's somehow due to the points and not the camshaft nose or mechanical advance. I ordered new points and condenser, along with pushrod seals, etc.

Here's a new picture of a plug, after the ride instead of after a choked idle:
What do you make of that? The insulator nose is divided near in half black/white. The left side plug is a more normal Brown/Tan. White half faces the exhaust side.

I checked my valves and they were all okay, the LS intake was about a thou loose. Inspected my diaphragms and they looked flawless - no defects, dry spots, holes or tears. I noticed that the plastic hose sections had a buckle where they meet the carb, with the right side's more pronounced, likely enough to cause an air leak. I have some new ones on the way, I wondering if the RS had an air leak, which would maybe explain the difference in plug readings?

EDIT: Also, maybe this is normal, but I noticed some oil in the RS intake tube, which makes sense because the oil recycling tube exits into the right side. Left side is totally dry. I saw a drop at the end of it when I replaced the airfilter last week. Is there a way for the breather to go bad and start shunting more oil to the intake?

disston 08-25-2012 02:30 PM

There is a way for more oil to reach the carb because it is making more blowby or the valves are more worn. I don't know what normal is tho. I have a little oil in that tube also. Do you know if you have the original Blowby valve, sometimes called the Turkey valve, or the updated one? I think the test for blowby will be a leak down test. Sort of like a compression test but better. You could start with a compression test if you have one of those.

Air leaks are on the down side of the carb. Above the carb, on the side that goes to the air filter, dirt or grit can get inside but this is not a vacuum leak. On the side that connects to the cylinder head is a rubber attachment tube. These get cracked and stiff with age. They can be the source of a vacuum leak that effects how the machine runs. With bike running at normal temp spray WD-40 around the carb intake tubes to see if idle changes.

That plug looks like it is getting too rich fuel. It is washed on one side by raw gas and black from burnt gas on the other. Can't think of any other reason for it to look like that. Maybe it means something else to somebody else.

When was the last time you rebuilt these carbs? New O-rings and complete cleaning. If the diaphragms really do look good you can skip them. I don't think they cause high speed misses.

I think your main complaint is the high speed miss that seems to be getting worse? You say you have reset the points? This is generally not a good idea. Points do wear and in normal operation will have a hill formed on one point and a valley on the other point. This can not be set with a feeler gauge. It can be done with a dwell meter. At this point I install new points if I think I have an ignition problem.

If you want the ignition points to last use a Dyna Booster box.

senatorperkins 08-25-2012 05:09 PM

Thanks again.. as far as I know my model has the reed valve breather, but I will check it out next time I get at it. I wonder if the tiny return hole under the breather is plugged.

I suppose that isn't a proper air-leak, but if the carbs were jetted with the resistance of the air filter upstream in mind, then wouldn't a leak between the air filter and carb cause the mixture to be leaner? Maybe not by much..

There's still a bit of room around the hill to get a reading from the feeler gauge, but I think if my double image goes away with these new points I'll get a points booster. If not, I'll get an electronic ignition. I think both the condensers I've been swapping are bad, this pitting occurred in less than 2000 miles. I've heard I should be getting many more miles before they go bad.

A previous owner rebuilt the carbs, but that couldn't have been later than 2003. I will be getting the rebuild kits, never a bad idea. Wish they weren't so pricey though.. I think I see some leakage from around the enricheners, at least that's where dust is collecting.

Perhaps the difference in plug readings and that funny black/white pattern on the R plug is from uneven timing between the cylinders? Like I said, it's totally possible they're running 10-15 degrees apart, at least some of the time, based on my timing light readings.

Anyone else have insights about this plug?

disston 08-26-2012 06:01 PM

You said earlier that the timing problem was fixed and then you said it was 10-15 off. So which is it?

If you have a double image you may be able to eliminate this by tuning the bike. New points. You can not gap old points. You can not file the gap like we used to years ago. In today's world it is new points every time you turn around.

If you do have a double timing image this must be fixed before anything else can be attended to. This is the article on Duane Ausherman's pages that tells the story about the dual image on /2es. They are a lot different but they are also a lot the same, these old bikes and ours.

That article is really hard to find because Duane's index isn't very useful, IMHO.

Vacuum leaks are below the carbs and cause poor performance and bad idle. They seldom cause a miss at speed.

I know I sound like I'm yelling but I just talk loud.

senatorperkins 08-26-2012 06:23 PM

All good, sorry for the confusion.

New points 2000 miles ago fixed the double image, which doesn't seem possible. But they did, and now they're rather pitted, and weird timing things are showing up, so I'm hoping the new ones and condenser I just ordered will fix it again, and hopefully the stumbling as well. If they do, I'll add a booster box so they don't pit, and if they don't, I'll try an electronic ignition.

Here's how I think it stands, in order of likeliness in solving the stumbling issue:
1. Bad points and/or condenser causing misfires/dual image.
2. Carbs need rebuilding
3. Bad spark plug wires
4. Bad coil(s)
5. Clogged petcocks
6. Clogged gas tank vent (only putting this last because I'm fairly certain it has stumbled while I had the gas cap loose)

I'm going to go through each one at a time to pinpoint the issue. Anyone have other things to add to the list? Floats seem good, set to be parallel to the base of the carb when the valve closes. No surging before cut out indicating a lean condition. I could set them by checking the fuel level in the bowl, but doesn't that depend on how much is in your fuel lines, as a fair bit dribbles out when removing the bowl?

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