37chief and others with air cooled engines

Geo-TH,In

Well-known Member
What happens when the cylinder temp of multi cylinder air engines is different, one cylinder is warmer and one is cooler.

If I recall 37cheif has a 4 cylinder inline air cooled Indian. So my question to Chief is what was the draw back to the having 4 cylinders in a row? Did the back cylinder have overheating issues? If so, what kind of engine problems may have occurred?

What about in line V-twins air cooled motorcycles that used one carb? The front cylinder is operated at a cooler temp, back cylinder runs hotter. For combustion to be complete the temp of the cylinders have to be hot enough. So, if the front cylinder is cooler, what may happens to the combustion? Is it possible that front cylinder may have a little carbon build up and the rear cylinder may be warm enough to have complete combustion, no carbon build up? Keep in mind, we are only talking about one carb, so both cylinders are getting the same fuel to air ratio.

Now lets switch to small v-twin engines used on mowers that are cooled by the fins on flywheel. Which is the best, running engine at full speed or less than full speed? Some people think full speed means more air so engine runs cooler. However, full speed means engine burns more fuel which means more heat.

Other people think running at slower speed may mean you will over heat engine because you are lugging the engine, over working it and less air for cooling. So you will over heat the engine.

Which is best wide open or slower? Won't running an engine wide open cause more ring wear?

So, when it comes to small engines what is responsible for valve seats falling out? Valve seat recession? Valve failure? Running too fast or running to slow? Keep in mind most small engines today have EPA compliant carbs, no adjustments. So we have to say the carb isn't the issue related to engine failure.

My last question. Is it possible, that by design, the shroud on V-twins may force more air through one cylinder, cooling that cylinder more than the other? If that happens what happens to combustion? I have a reason for asking all these questions, will post back later.
 
I had a Yamaha Virago motorcycle, which is an air-cooled V-twin. Yamaha V-twins are offset, unlike Harleys where the cylinders are in the same plane.

As I recall, the Virago's front carb was jetted slightly richer than the rear, because the rear cylinder actually cooled better than the front.
 
I wish Indian would tool up and bring back that in-line four. It was a bear to accelerate with - engine torque would lean the bike over - but what a cool looking machine! Rear cylinder heat on a V twin is addressed on newer Harleys by a feature which cuts out the rear cylinder when at idle to keep engine temp uniform. The timing is electronically altered so the engine runs fairly smooth on one cylinder - smooth for a Harley anyway.
 
Can not say for sure on all but O know the Yamaha 750 special tended to burn out he center cylinder over time due to it running hotter. I would say some thing about HD but t if you want that info send me an e-mail
 
It was found on an FLH with the fat front dresser fender the wind bypassed the front cylinder and hit the rear with more wind leaving the front hotter. Harley also could sell you an air scoop that would direct more air onto the motor.Valve seat probably come loose because not enough interference fit was used to replace them.Seat recession is related to spring pressure amoung other factors.
 
I didn't know Yamaha addressed different cylinder temps with different jets.

Now what do you think would have happened if Yamaha had tried using just one carb?
 
A lot of questions there. I know the answers to some, not to others. Here goes....
Engines that are made to operate most outdoor equipment are DESIGNED to run wide open. That includes lawn mowers, rototillers, string trimmers, leaf blowers, and the like. The equipment is designed around its maximum efficiency, and the engine is governed accordingly.
As to valve seats....the primary cause of valve seats coming loose or out entirely is HEAT. Since most seats are made of different material than the head or block they are pressed in to, they have a different coefficient of thermal expansion. The block or head material (generally aluminum or alloy) expands more/faster than the seat material, and if the temperature gets too high, the seat loosens up and gets out of place.

On another note....how much variation of temperature would you think there is between the cylinders on a V-twin?? And don't you think the designers of such an engine did not account for that?? Seriously? Or do you just like splitting hairs?
 
I'm not just splitting hairs, I'm looking for a solution to a real problem I have. I'm thinking my problem might be because of different cylinder temps caused by shroud design. Plan to run some test soon.

Do you think running an engine wide open will increase ring wear? Shorten life of engine?
 
As I said, most small engines are designed to run at full rated output. This is usually because the equipment that they are powering needs the full available horsepower and torque that the engine can provide.
As to ring life, that is basically determined by total piston travel, so it would logically follow that more RPMs would mean faster wear. More cycles equals more wear. The service life of the engine, rings, bearings, etc. is determined by the intended use of that engine. Engine life is rated by number of hours at specified RPM.

I worked in fabrication shop for a few years, and learned a lot about equipment design in general. As a rule of thumb, moving parts are designed to last 1,000,000 cycles. This applies to latches, hinges, and most internal engine or compressor parts. They are also extensively tested to insure that under normal conditions, those parts will last their entire projected life.

Running an engine (use a generator engine as an example) at slower than design may cause unintended problems like excessive carbon buildup, poor heat dissipation, excessive fuel consumption, etc. Small engine carburetors do not have the versatility of something like an automotive carb. They are usually fixed jets, and "tuned" for maximum efficiency and engine life at their design speed. In any case, they all have a finite life expectancy, and that is about all that can be expected from them.
 
On the old VW bug there was a problem with the #3 and #4 cylinder running hotter then #1 and #2 due to the oil cooler being above the 3 and 4 cylinder and the fix was to install a oil cooler like a small radiator so as to give more air flow over the 3 and 4 cylinder and move the heat away from them. Maybe an external oil cooler of some sort would help if it has an oil pump that is
 
Just finished using engine. To my surprise, allowing the engine to idle while warming up the hydraulics proved my theory is correct. One cylinder is getting more air, running cooler. The other cylinder is very hot. While warming up I increased the RPM's a little at a time and the temps did drop and eventually at a particular speed the temps were almost equal.

I also noticed how rapidly the temps change. If I'm working the engine hard, the temps go up as expected, burning more fuel, hotter temps. Without changing engine speed, the time it takes me to get off the seat and measure one cylinder then go the other and back to the first, the temps drop significantly.

I need to install 2 temp gauges ranging from 200-600, one on each head. I also need a tack that uses the tractor battery or has a battery that I can replace. I've had bad luck with the type that go around a spark plug wire and have a built in battery that can't be replaced. Then I can figure out what speeds works best for what I'm doing.
Faster seems to be better. Not sure totally sure I need to run wide open.
 
Single carb Vtwin would maybe have different cam timing or compression on rear cylinder- what lots of Harley Gs in police trikes had and some sporties in desert patrol use. Shorter duration exhaust means more seat time and more heat transfer to cut down on valve erosion, burning- and results in a bit less intake charge, slightly less power and heat build up. FLH used for sidecar in police service- FL rear piston a point and half lower compression- 7.2 instead of 8.8 or so compression- this on Panheads used by Maywood that got sidecars in winter. Sporties had XL rear piston at 7.5:1 instead of 9:1 in front cylinder for the long run desert racing in early Mint 400. Some small engine Vs have internal deflector vanes in shroud to favor 'rear' cylinder head area- most are offset by width of con rod bearing instead of the inline with knife and fork rod like Harleys. Yamaha big Vtwin with single carb manifold kit says use the front carb and the manifold is slightly offset and slightly different length per cylinder from center mounting- make a Harley chopper look, suppose to be even temperature running with 'slight' loss of top end speed but easier tuned for idle and midrange general riding. 1100 conversion for a 1990(?) model had dyno reading 5 hp less with single carb but peak torque about same with the single carb getting it about 200 rpm lower and spread 500 rpm @90% of peak torque greater- so a better midrange, less peaky power curve. This with stock pipes with some miles on them and the pipes had a temp meter on them for dyno test- front and rear read about the same- 2010 Cycle accessories mag test of the manifold kit- Cycle Guide? the seller of the manifolds has the reprint online of article, aquaintence has a freebie for work 1987 1100 Yamaha with 'California carb problems' to play with come spring, asked me to look up possible problems and repairs. Engineers worried about heat have some workable drills-the cost accountants may make a decision slightly different. RN
 
I ordered a tach. It has a battery that I can replace. Plan to start out at an idle, measure each head temp. Then increase rpm's a little at a time until I find what speed gives me the same left and right head temp. Then that's the speed I will use. The cylinder that is the coolest at an idle seems to make a little carbon on the plug. Keeping an eye on the plugs will tell me what I really want to know. And not running engine any faster than necessary hopefully will extend the engine life.

Not to worry I don't have a problem with plugs, spark, dirty air filters, good compression on both cylinders. Only thing I can think of is operating temps making the carbon.
 
Well, George, considering all the corners Yamaha cut when they built that bike (plastic parts, cheap suspension), they must have figured that it needed dual carbs to run right. Otherwise they would have tried to get by with a single. Harley, on the other hand, never uses a scalpel when a stone axe will do. So they just made sure the mixture is rich enough so the hotter of the two cylinders won't burn up. Or at least did until emissions regs forced them to switch to EFI.
 
I am not convinced your test results verify an cooling system air flow difference between cylinders. Since you have carbon in the coolest running cylinder I would suspect a rich mixture. Rich mixtures reduce running temperature and form carbon in the cylinder. How do the spark plugs color in the engine, does the cool cylinder show a rich condition?
 
Other little things are set to compensate for different cooling. The VW Type 1 engines (Bug & early Bus) also had different ignition timing for the #4 Cylinder (the one tucked behind the oil cooler) to make it run a skosh cooler. Remember seeing that in a service manual as they were explaining why a stock VW distributor should ALWAYS be put back exactly where it was supposed to go and to make sure you're using the post on the cap VW intended for #4, or to put it another way no putting in the dizzy any old way and swapping wires around to make it work. Other tricks like intake runner length or diameter could be used to vary fuel or air flow to a particular cylinder.
 
Old, to my surprise, the filter temp and lower engine temp was arounnd 125-150. I had to measure head temp, heat does rise. The block temp was much less.
 
How can one cylinder be richer than the other when there is only one carb for both cylinders? I've switched ingition modules, switched spark plugs. No change. Both cylinders have same good compression. There is no timing adjustments, timing is determined by the same 2 magnets on flywheel. Both share the same clean air filter. Engine doesn't burn any oil. No carb adjustments either, fixed jets.

I even measured the left side runs much cooler at lower speeds, feels like more air too.

What else can it be if it isn't air flow/lower temperature?
 
You may be correct, it may be air flow however, your test has not actually measured air flow. I could ask the same question, how can the air flow change with speed, where by, one cylinder runs cold at low speed and the flow equalizes at higher speed. Perhaps the engine has been apart in the past and a baffle was omitted...?

A check of the spark plugs will indicate the mixture, do they color the same? If not I would look at the inlet system. Look for a gasket that may have been installed wrong and is obstructing the flow. Since it is speed related, pay special attention for an obstruction near or down stream of the idle discharge port. Look for an air leak. Is it possible this engine has a casting crack which leaks air into one cylinder. The leak could have been masked by enriching the idle mixture. Now the leaking cylinder is fine at idle and the non-leaking cylinder is rich. At higher speed/power the carb no longer meters from the idle circuit. In addition, due to the higher manifold pressure the leakage rate decreases. Combined with the higher air flow in the intake system the leak is now non-significant.

My log splitter is a air cooled 5HP B&S, it starts and runs in warm and very cold weather and does not carbon up. My experience is that engine carbon results from excess fuel rather than over cooling. I am not saying you are wrong, rather need convincing you are correct.

Keep us posted....eh.
 
My boy has a snow blower, 5 hp briggs 2 cycle. The carb is set up for cold weather only. Think about, not really a need to blow snow in the summer.

Temperature effects the fuel to air ratio more than most think. In the winter I have to crank the main jets out 2 full turns in the winter on my Jubilee and Farmall C to make power. Or the engines will stall under load.

The kohler works well in winter, plug still shows carbon on left plug way more than right.

Engine has never had any work done to it. I did remove shroud to change out coils. The coil on the right was defective, wouldn't stop making a spark when shorted out. Both the left side has a new coil too.

All I can say is at an idle, the left side is a much lower head temp, the right side is very high, almost 500 degrees, danger of blowing a head gasket. There is no obstructions blocking cooling fins either.

To my surprise, the block and crankcase temps are very low compared to head temps.
 
(quoted from post at 21:05:03 02/01/15) How can one cylinder be richer than the other when there is only one carb for both cylinders? I've switched ingition modules, switched spark plugs. No change. Both cylinders have same good compression. There is no timing adjustments, timing is determined by the same 2 magnets on flywheel. Both share the same clean air filter. Engine doesn't burn any oil. No carb adjustments either, fixed jets.

I even measured the left side runs much cooler at lower speeds, feels like more air too.

What else can it be if it isn't air flow/lower temperature?
a single carb supplying an odd fire engine will always run one cylinder leaner than the other . It's the nature of the beast.
 
All of the early in-line 4 bikes had problems with overheating at the rear cyls. Some used bits of tin to direct airflow with questionable sucess. When Indian made the 841 for the North African Campaign, they turned a V-twin Scout engine sideways to keep it cool like BMW and NSU did. Watercooling cured the problem, as long as the distribution tube didn't rust away as has happened to many antique in-line engines.
 
a single carb supplying an odd fire engine will
always run one cylinder leaner than the other .
It's the nature of the beast.

I'm am convinced one cylinder is leaner than the
other. It's hard to convince others there is a
problem. I would like to know why is it the
nature of the beast? I've been scratching my head
for 6 years to come up with the solution. The
timing is fixed, jets on carb are fixed. Valve
clearance is fixed, hyd valve lifters. I've
switched out ign modules, withed spark plugs.
Went to next hotter plug. Doesn't use a drop of
oil. Has good compression, both the same too. Did
a leak down test, all is good.

So what's the cause and the solution?
 
George, it is typical for the heads to run hot on an air cooled engine. After all, that is where the combustion is. Years ago, when I had Corvairs, one of them had a cylinder head temperature gauge in it. Head temps ran higher than the rest of the engine. 500 is high, but not necessarily excessive. I would expect it to go down with increase in RPMs, and to rise with increased load. And, under load with higher RPMs, I would expect to see temps in the same range on either cylinder. More than 10% variation could mean air flow blockage or mixture imbalance. There are a host of things that can cause a cylinder imbalance. Intake leaks can top the list. Restricted airflow at the exit point could be a problem. Sharing a single carb does not insure an equal mixture to each cylinder. In theory it should, but in reality, a dent in a manifold can make a drastic difference.
Keep plugging at it. Sooner or later the solution will appear.
 

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