Ask Ray - letters from July 2012

Ask Ray - letters from July 2012

Tips from Hemmings Muscle Machines

July, 2012 - Ray Bohacz

I love reading your articles and look forward to reading your racing and gearhead stories.

In HMM#95, August 2011, you wrote about idle speed. I have just started tuning my new fiberglass '29 Ford Roadster with a 350 Chevy. I am using a vacuum gauge on the Edelbrock 1406 carburetor, on the driver's-side full-vacuum port.

Yesterday I was able to get the engine to run at 700 RPM. It is smooth-running and starts cold and warm with very good pedal response. However, during the test drive, the carburetor backfired upon applying the throttle after slowing down. Is this a too-lean condition? Thanks.
Steve Gerken
Queen City, Arizona

A backfire through the carburetor is almost always a lean condition. This can be caused by either a weak accelerator pump shot, or because the power enrichment circuit is not delivering enough fuel. It is not impossible (though also not probable) that the mixture screws would have sufficient authority to impact the tip-in fuel delivery while still producing a smooth and good quality idle. The first sign of an excessively lean idle mixture circuit would be a degraded idle quality. You claim the engine runs well, so I do not believe the idle mixture adjustment you performed is directly causing the problem. But there is a caveat that can apply.

Let us work under the assumption that the carburetor idle mixture prior to your adjustment was excessively rich. The engine would run fine, but would have high carbon monoxide emissions and be prone to polluting the crankcase oil. This rich mixture may very well have masked the accelerator pump or power enrichment circuit lean condition. Now that the idle mixture is adjusted properly, the lean tip-in causes a backfire.

Since the problem only occurs after a coast-down, when the engine vacuum would be very high, I tend to believe the problem lies with the metering rods/power piston sticking in the down position. These parts may be gummed up with varnish or misadjusted for your application. I would take the air horn off the carburetor and check the metering rods, along with the float level. If it were the accelerator pump delivery, the hesitation/backfire would be more prone to occur as the vehicle is driven away from a stop than after being coasted.

As an aside, a vacuum gauge is not the best tool to use to adjust the idle mixture, but it will work fine. The problem is that the resolution is dampened versus a tune-up style tachometer with graduation of 20 RPM or so. When fine-tuning to that level, you may be able to tweak the mixture slightly and pick up another 10 to 15 RPM with no discernible impact on the vacuum gauge. For this reason, I like to use a high-resolution tune-up tach-and-dwell meter to adjust a carburetor or to look for a vacuum leak using a propane tool.

I have a 1973 Mustang convertible with a 302 and an automatic, which has factory warning lights for the alternator, oil and temperature. I would like to install an aftermarket coolant temperature gauge while leaving the temperature warning light functional. Can this be done? Do you recommend an electrical or mechanical temperature gauge? Where is the installation location for the sending unit for the gauge?
Tony Mitchell
Elkhorn, Nebraska

There will be no problem keeping the coolant warning light while installing a temperature gauge. The warning light uses a calibrated switch to ground that closes at a designated temperature to illuminate the dash overheat indicator. In contrast, a temperature gauge employs a sending unit that varies in resistance when exposed to thermal change. This signal is sent to the gauge, and when calibrated as a pair, identifies an actual temperature. The only issue you may have is trying to find a spot to place the gauge's sending unit. There are a number of approaches to this.

In some applications, you may be able to mount a fitting into the engine that allows both the coolant switch and sending unit to be mounted together. It is common to find a threaded hole in the cylinder head or engine block water jacket that will accept the sending unit. You may want to investigate where Ford placed the sending unit on that engine when factory-equipped with a coolant temperature gauge.

Keep in mind, the coolant is not the same temperature throughout the engine. It will be hotter near the combustion chamber and cooler in the block. As it is pumped back from the radiator, it will be cooler yet. That is one of the reasons why the auto industry commonly used a gauge with no numbers and only lines, which identified cold and overheat. There may be a 20- to 30-degree difference in the coolant in one location than in another.

A good place to put the sending unit would be near the thermostat housing, since that will note the temperature of the coolant as it exits the engine and is pumped into the radiator. That is usually the most accurate spot.

You may then choose to put the warning light switch in a less desirable location, since it just needs to be mindful of an extreme condition. Do some research on the warning light switch you have and at what temperature it closes. That way, you will know the maximum coolant temperature Ford would accept for the location where the switch was originally installed. That would be a good reference point for the gauge reading. For example, if the Ford switch identified an overheat as 240 degrees Fahrenheit, then you will not need to be concerned if, on a hot day in traffic, the engine goes to 230 degrees with the sending unit in the location that was previously occupied by the warning light.

I have a Buick Centurion that is doing well, though recently idling and running like it is a hair out of balance. I will keep seeking the specific cause until I run out of clues and then seek professional help. At 160,000 miles, the engine has already run well past its intended life without a rebuild, but I do not plan to do that until it actually needs it. The hunt is half the fun and there is a lot to learn along the way. The engine seems smooth when cold, but steadily shakes slightly after warmup. I thought maybe it was a vacuum leak, but could not find anything, and the problem is only at idle. Next up for evaluation is the coil. What do you think?
Victor Kohanski
Sugar Land, Texas

An idle instability issue on an older engine is not uncommon, especially one with higher mileage. If your Buick is equipped with an EGR valve, I would remove it and check to see if the pintle is sealing properly. The best way to do this is to clean the carbon from the valve. Then, with it upside down, fill the cavity with a thin liquid such as carburetor cleaner or penetrating oil. It should not leak past. If it does leak, then the valve is bypassing exhaust gas all the time, and that will skew the idle quality and stability.

I don't know what method you employed to look for a vacuum leak, but the best way is to use a tune-up tachometer and a propane enrichment tool.

Another very possible reason for the idle issue would be carbon deposits on the intake valves. It would be rare for these to build uniformly on every valve. This results in a different delivered air/fuel ratio to each cylinder. The carbon causes two problems: It is an impediment to airflow and it wicks in fuel from the incoming charge air. The deposits impact the engine at idle more than they do when you're going down the road. The intake port velocity is very low during that operating state. As the amount of deposits grow, the condition will eventually create a lack of power. An effective way to remove the deposits is to purchase a drip-in style tool such as the one produced by the Run-Rite company. You can then use their excellent chemical treatment or the replacement that GM offers for its old Top Engine Cleaner. It will take a few treatments to remove 30-year-old carbon, but the service is very effective on both old and new cars.

If you can find a shop that has an ignition oscilloscope and has someone who knows how to read it, that would be the most accurate way to tell what is going on with your Buick. The ignition coil can be checked with a mere glance of the scope pattern. On an old breaker-point system, you would normally see three to five coil and condenser oscillations after each plug firing.

Also, a mixture distribution issue could be immediately identified by a spark line that dances upward. As the mixture skews lean, the spark line will shorten and go higher; as the delivery goes rich, it will go longer and become straighter. With a scope, you will be able to immediately see if there are any fuel distribution issues and identify the exact cylinder in the firing order--there is no guesswork. The scope was a wonderful tool that simply does not exist today. A true lost art.

This is not a very technical question, but I hope that you can help. I have a 1971 Camaro with a 250-cu.in. inline six-cylinder with the stock one-barrel carburetor. I just had the engine and carburetor rebuilt after 200,000 miles. The car runs like a top, but it only gets 13 MPG around town or on the freeway. That does not matter, but I admit I'm curious. My son has a 1973 Dodge Charger with a 318 and two-barrel carburetor. It has dual exhaust with Flowmaster mufflers and fat tires on the back. Everything else is stock. He only gets 12 MPG.

Can you tell me why these cars get such poor gas mileage, and is there anything that would help increase the mileage? Am I just stuck with it? I love your magazine and the advice you give. Thanks.
John Bremer
San Jose, California

Fuel economy results from the combination of many different elements that can have a positive or negative impact on the car's overall performance. From my experience, there are usually multiple factors that, when added together, result in excessive fuel consumption. The most important factor, though, is driving style, and no one wants to hear that: Most people want to blame the car and not themselves.

As I have written in this column before, in extreme cases, there can be as much as a 50 percent difference in the distance traveled on a gallon of gasoline, caused by varying driving style and the amount of idle time. Since that is an intangible, I will discuss the mechanical aspects of the two cars.

Tunable areas, such as the carburetor setting, along with the amount of spark lead, can have a huge impact on fuel consumption. For this reason, you will need to get some instrumentation on those engines. A dyno session using a linear air/fuel meter while the vehicle is being driven normally and not for power will tell you a lot. If that is not available, then a good, old-fashioned exhaust gas analyzer will tell the story.

On the dyno, you will want about 15:1 air/fuel ratio under light load and cruise. If you are looking at an emission analyzer, then the CO (carbon monoxide) will be the key: It should be very close to zero during a drive cycle. When I taught emissions many years back, a good rule was for every one percent reduction in CO you provided the customer, they would gain one mile per gallon. It was very common back then to find a car with eight to nine percent CO, and if you got it down to near zero, the vehicle could pick up seven or eight miles per gallon in a rare case; five to six extra miles per gallon was very common.

The same holds true with the ignition lead. I would feel comfortable in betting that both distributors are tuned with the advance curve they left the factory with. Not only has the engine changed from use and wear, but more importantly, the chemical composition of the fuel is so different today that the old spark advance curve can now be nowhere near right. You add this all up and the fuel mileage becomes disappointing.

Other factors such as the wide tires on the Dodge are probably impacting the gas consumption, along with a low compression ratio in each engine. But given all of that and my experience with both engines from years back, I feel that if tuned properly and driven with an eye toward economy, the Camaro should be getting about 17 to 18 MPG, while the Charger should turn in about one less mile per gallon--a substantial difference from where you are now. But do not forget, anything that requires more power to move the vehicle all has to come from the gas tank. Tire pressure, wheel alignment, brake drag all are overcome by the engine having to use more fuel to travel the same distance.

I am restoring my 1971 Road Runner hardtop, and now would be the time to fix the "hinge in the middle" handling of the car. I am upgrading the front and rear suspension systems with Magnum Force components, including their frame stiffeners. After scraping off the undercoating, I discovered that most of the factory spot and seam welds are of very poor quality. Would there be any disadvantage to properly welding all body seams, torque boxes, frame rail, etc.? The folks at Magnum Force strongly recommend I weld the car together as tightly as possible, but there may be other factors that I am not considering and so I would like to hear your opinion. Thanks in advance.
Mike McDermid
St. Louis, Missouri

Strengthening frame/unit-body welds is never a bad idea and can only make for a stiffer car. However, the question that comes to mind is, are the welds as bad as they visually appear, since the car has lasted since 1971? Though I am not a weld engineer, I do know that sometimes a weld may appear to have poor penetration, uneven coverage and other undesirable characteristics, when in reality, it is actually more than strong enough for the task. With that said, let me examine the practice of "beefing up" the welds.

The first thing I suggest doing is getting the car on a frame machine and making sure that it is straight and not tweaked. The vehicle may have "taken a set" over the years and adjusted to the poor welds. Following the parable of the willow tree that bends but does not break, if the car is currently compensating for a tweaked structure, it may be possible that reinforcing that could cause a problem without the unit-body being pulled straight, too. Any good body and frame shop with the proper equipment can determine this, and if necessary, pull the Plymouth straight. Then you can weld to your heart's content.

Since you are taking on this effort, I might suggest that you try to "box in" with additional material any area that looks weak or that could provide added support. This effort can also include triangulation, when applicable, yielding an extremely strong structure. If you want to really go the extra mile, you can contact Bonal Technologies in Southfield, Michigan (www.meta-lax.com) to see if you can rent or find a local job shop that uses their Meta-Lax device, and/or the company's weld-conditioning equipment. It relies on a very proven theory that is used extensively in many industries and works on sub-harmonic vibration. Let me explain.

It has been discovered that if the harmonic peak of a work piece is identified and a resonance in the lower one-third of the harmonic curve is induced, not only are the residual thermal stresses eliminated, but the weld is much more effective, the penetration is better and the stress induced in the heat-affected zone is neutralized. This theory also works great on engine parts and any other metal that has residual thermal stress. When I had my shop, I owned a Meta-Lax and used it on all my engines and components. As an aside, the unit I owned was purchased pre-owned from Bonal and it was first employed to do some parts on the Galileo space probe. Have fun restoring your Mopar!

It appears that you are the "go-to guy" when it comes to answers that are based in fact, instead of being an educated guess. Most people who live where it is really cold won't have much sympathy for me, but I grew up in upstate New York, so I have done my time in seriously cold weather. That said, my problem is that I have a 1969 Road Runner that is strictly a drag car. The local tracks do not permit antifreeze in the race cars because of the difficulty cleaning it off the racing surface.

As a result, I am wondering if there is a hard-and-fast rule about how cold it can get outside and in my unheated garage before the block is in danger of freezing?

The engine has an oil retention device (an engine diaper) that makes it impractical to drain the block after every race in the winter. When I found my 1969 GTX in a junkyard in 1978 in Massachusetts, the block was split at the freeze plug, so I have been paranoid ever since then about this subject.

The coldest it gets here is 25 degrees F at night. I am sure there are others who worry about this same problem. Thanks for your help, and keep doing what you're doing!
Jeff Shaw
Madera, California

In theory, at any temperature below freezing, water will solidify and expand, thus having the potential to crack metal. This is especially true with something that stays in a dormant state, such as a drag race car. But in actual practice, the risk is not as great as the textbooks may state. The key is going to be determining how much water is in the engine and where it resides. Even a block that has the drains opened has the potential to crack if there is any water lying in a small area of the internal webbing. Thus, there is never a guarantee that there won't be any damage. Through looking at empirical data, however, I doubt very much that the block will be damaged if the majority of water is drained from it and the temperature only gets down to 25 degrees on a few nights.

The key to keeping the engine safe will be removing as much water as possible, so that if expansion via freezing occurs, it has plenty of room to grow without stressing the metal. Factory engine blocks, especially older ones, are usually quite robust and can take more abuse than you might think. I have seen quite a few problems with lightweight aftermarket race blocks, since the material is much thinner in the water-jacket region in order to save weight. My only concern here is, if you ever experienced a deeper and more extended freeze, then you might end up hurting the engine unless you provide some external heat source or remove the engine diaper. The fact remains, though, that if you haven't hurt the engine by now, it will most likely be fine.


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