M37 Hesitation

copperline

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I FINALLY got the truck running and running well. What a mission this has been.

One issue I have now is pretty bad hesitation right out of idle. She idles well but when I initially press the gas she bogs down bad then catches up and runs well after she revs up.

I have adjusted timing and idle mix to highest vacuum, approx 18.

Its a brand new rebuilt carb from Midwest. Very impressed with the rebuild.

Any ideas as to what I can do to fix that hesitation?
 

jeffhuey1n

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I had a problem in which the mechanical fuel pump wasn’t working. I installed a inline electric fuel pump before the mechanical fuel pump. The truck runs well now. Just have to fix my other problems. I’ve got a couple ideas why it’s necessary to add the electric one. One: cam lobe which activates the fuel pump is worn. It’ll pump fuel but not in the quantity nor pressure it needs. The other might be because the mechanical fuel pump is in bad shape. Regardless, the fuel pressure is insufficient to maintain engine power. You might need to look and see if there’s a fuel filter inline somewhere upstream from the carburetor.
 

CMPPhil

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How fresh is the gas? I have this same problem with all three of my gas MVs in the spring goes away with first full tank of fresh gas.

If the hesitation persist with fresh gas then the other suggested problems, accelerator pump, or low fuel pressure at idle are likely causes.

Cheers Phil
 

Bill Nutting

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I have given up on cas with ethanal. I found a station near me that sells “marine fuel”. It may also go by the name of RV fuel. It doesn’t have ethanol in it. My M37 even smells better using this fuel! It costs about 3.00 a gallon, but it’s worth it. This stuff even stores longer. That being said, you got some good advice trying to drive it with the chock pulled out a bit. That might tell you if the mix is a little lean.
 

M35A2-AZ

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I have given up on cas with ethanal. I found a station near me that sells “marine fuel”. It may also go by the name of RV fuel. It doesn’t have ethanol in it. My M37 even smells better using this fuel! It costs about 3.00 a gallon, but it’s worth it. This stuff even stores longer. That being said, you got some good advice trying to drive it with the chock pulled out a bit. That might tell you if the mix is a little lean.
Bill, I have been using Ethanal free fuel in my MV's and other small motors, stores a lot better. I have to go up to Prescott, AZ about 100 miles away to get it but it is worth it. We go see my bother in law in Prescott so I just take my cans.
 

54M37

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Stock M-37 carb has a vacuum operated accelerator pump not a mechanical one so you won't see gas flowing into the carb by hit the gas petal. Seen this many times usually the problem is the accelerator pump piston leather seal or the cylinder it rides in is oversize or worn. Unfortunately you have the take the carb apart to check these things.
 

copperline

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Pulling out the choke did the trick. Its about 3/4" out and what a difference. This is the best she has driven ever.

So what does that mean? Running lean? What can I do to adjust that. I have already adjusted what I think is the only adjustment on these carbs which is the idle mixture screw. I did it to max vacuum but honestly that adjustment was hard to do as a full turn would make no difference in either direction. It was extremes that I would see and hear a difference so I just put it in the middle of those extremes. I'm guessing about 2 turns out from bottom.
 

NDT

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Pulling out the choke did the trick. Its about 3/4" out and what a difference. This is the best she has driven ever.

So what does that mean? Running lean? What can I do to adjust that. I have already adjusted what I think is the only adjustment on these carbs which is the idle mixture screw. I did it to max vacuum but honestly that adjustment was hard to do as a full turn would make no difference in either direction. It was extremes that I would see and hear a difference so I just put it in the middle of those extremes. I'm guessing about 2 turns out from bottom.
Send it back to Midwest!!! Not rebuilt properly. Not run tested.
 

NDT

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I agree. The first thing I would check is the accelerator pump. Just because it is a newly rebuilt carburetor doesn't mean it might not have issues. You should be able to look down the barrel of the carb and see gas squirt out when the throttle linkage is moved.
How you gonna look down the carb?
 

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GopherHill

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You will have to take the top off the carb. Check the cup on the accelerator pump that it is not undersized for the hole it fits in. The accelerator pump is vacuum operated as I remember, that was long ago. There may be different holes for the linkage to obtain more volume, this is all inside the carburetor. You can't see the fuel squirt with the design... Maybe with a mirror , engine running.
 

Bill Nutting

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When we restored our M151a2 I sent the carb to Brooks Engine Service. He makes modifications to the carb that makes the 151 purr. Most 151’s run best with the choke pulled out about a quarter inch. Our MUTT was no exception. After Brooks got done with it pulling the choke is no longer needed, there is no hesitation either. I don’t know if he does M37 carbs. Might be worth looking in to. PM me if you need his number.
 

Bill Nutting

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Pulling out the choke did the trick. Its about 3/4" out and what a difference. This is the best she has driven ever.

So what does that mean? Running lean? What can I do to adjust that. I have already adjusted what I think is the only adjustment on these carbs which is the idle mixture screw. I did it to max vacuum but honestly that adjustment was hard to do as a full turn would make no difference in either direction. It was extremes that I would see and hear a difference so I just put it in the middle of those extremes. I'm guessing about 2 turns out from bottom.
In my mind it sounds like it is running way too lean. There are some problems running for extended times with the choke pulled out too far. If you run too rich you will foul the spark plugs. Another possible problem is unburnt fuel “washing” the oil off of the cylinder walls. I did this to a boat I once owned and destroyed the engine. Drive it for a few miles with the choke pulled to a spot that has the engine running good. Then pull the plugs and see if they are black and sooty. If they are, you are too rich. You may want to check compression as well. Sometimes running rich will hide a low compression issue. I’m thinking the carb is not set up right.
 

harptheblues

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In my mind it sounds like it is running way too lean. There are some problems running for extended times with the choke pulled out too far. If you run too rich you will foul the spark plugs. Another possible problem is unburnt fuel “washing” the oil off of the cylinder walls. I did this to a boat I once owned and destroyed the engine. Drive it for a few miles with the choke pulled to a spot that has the engine running good. Then pull the plugs and see if they are black and sooty. If they are, you are too rich. You may want to check compression as well. Sometimes running rich will hide a low compression issue. I’m thinking the carb is not set up right.
I had the same problem once on my M37. I found a vacuum leak. Fixed the leak and it ran fine afterward.
 

John Mc

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Monkton, VT
Doesn't the stock carb have a separate idle mixture adjustment? If so, is the idle mixture set too lean?

I'm not very familiar with these engines, but hesitancy coming up from idle in my chainsaws (once they are warmed up) is almost always from the idle mixture set too lean. I realize that a chainsaw is quite different beast, but thought I'd mention the possibility. If there is any similarity, then it's possible to be running well under power, but too lean at idle. You might not see any indicatioon on the plug, if you don't idle much.

Just a random thought which may not apply to thse engines.
 

Floridianson

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Stock M-37 carb has a vacuum operated accelerator pump not a mechanical one so you won't see gas flowing into the carb by hit the gas petal. Seen this many times usually the problem is the accelerator pump piston leather seal or the cylinder it rides in is oversize or worn. Unfortunately you have the take the carb apart to check these things.
I am trying to understand and looking through the repair manual on the carb. If it is vacuum operated does not the vacuum drop off when the throttle opens up? So does the vacuum keep the plunger pulled back and the spring pushes it down when we loose vacuum? I have not had one apart and just as said trying to understand. My manual is not real clear on how it works just how to repair and test. Tanks
 
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