I recently sent my Farmall M Carb to Denny's Carb. Shop to be rebuilt as the tractor was not running real good when I bought it. Since I got the carb. back, it seems to run fine at fast idle and full throttle but seems to run rough at slow idle. I am running the Idle jet at about 1/2 to 3/4 turn open, and just can't seem to get it to idle as slow as I think it should, without it idling rough. I live at 5500ft. elevation. I don't have a tachometer to put on it but just set it by ear. Is it possible that they missed something when they rebuilt the carb, or does the "M" just not idle real slow? Thanks for any advise!
- Thread starter Jim R
- Start date
Bob M
Well-known Member
Jim - A well-tuned M should idle smoothly down to 400 RPM and even lower - ie. delightfully slow(!)
Keep in mind the carb idle MIXTURE jet screw does just that. You adjust it to where the engine runs smoothest at idle. You then adjust the idle RPM using the idle STOP screw, then go back and tweak the idle MIXTURE screw again if necessary to give the smoothest idle.
Now if turning the idle screw out (CCW) to the point it does not lower the idle RPMs and idle RPMs are still too high, it could be there's a vacuum leak between the carb and manifold flange or between the manifold and the head. More likely however is the governor to carb throttle shaft linkage is not properly synchronized. You'll need to re-synchronize it in order to achieve a nice, slow idle.
Do a search on this forum for a detailed Farmall H/M carb synchronizing procedure - it's been posted several times here.
Keep in mind the carb idle MIXTURE jet screw does just that. You adjust it to where the engine runs smoothest at idle. You then adjust the idle RPM using the idle STOP screw, then go back and tweak the idle MIXTURE screw again if necessary to give the smoothest idle.
Now if turning the idle screw out (CCW) to the point it does not lower the idle RPMs and idle RPMs are still too high, it could be there's a vacuum leak between the carb and manifold flange or between the manifold and the head. More likely however is the governor to carb throttle shaft linkage is not properly synchronized. You'll need to re-synchronize it in order to achieve a nice, slow idle.
Do a search on this forum for a detailed Farmall H/M carb synchronizing procedure - it's been posted several times here.
Janicholson
Well-known Member
Three items. The carb shop refered to has a reputation on this site that is not complementary.
If the M has a magneto ignition it will idle rougher than a distributor equipped M (no centrifugal advance) If your engine has a distributor it is good to check the centrifugal advance to assure it is working as designed.
If it is possible to smooth it out using the choke, it is lean. if it is rich, screwing out the idle needle (near the top of the carb) leans the mix. Jim
If the M has a magneto ignition it will idle rougher than a distributor equipped M (no centrifugal advance) If your engine has a distributor it is good to check the centrifugal advance to assure it is working as designed.
If it is possible to smooth it out using the choke, it is lean. if it is rich, screwing out the idle needle (near the top of the carb) leans the mix. Jim
Thanks for the comments and advise so far! I'm not a "mechanic" but I'm learning a lot here. As far as the reputation of the carb. shop that I used, I discovered that after I had already used them. I had another friend with a Farmall "C" that used them and he had to send the carb back to them the second time to get it done right. My Farmall "M" is a 1951 model and has the IH Distrubutor that is horizontal, with the coil on top of it. It is still a 6 volt Positive ground. When I first start the tractor up, it is very rough running when cold, but if I leave the choke pulled out a little, it smoothes right out but has to be good and warmed up before the engine really smoothes out without the choke out part way. I hope this helps you to advise me. I will check on the adjustment of the carb and govenor linkage. Any other advise would be appreciated!
It is pretty darn cold here in WI for them to run like a top right off the bat. I think we have come to expect the perfection of fuel injection. When the tractor is warm you should be able to take it from high idle to low idle in a hurry and not have it stumble at idle.
Matt Gall at MoTec engineering can open up your carb and get some more performance out of it for you. He even dynos them before they leave to set the jets to get the most power. I have sent my SMTA and H carbs there and would send every other carb there. IIRC like $75 shipped back to you. With that, the carb kit is less than $25 so there is plenty of labor on that.
BTW, DENNY'S SUCKS!!! Didn't you think that was a bad sign that you buddy had to send his carb back?
Matt Gall at MoTec engineering can open up your carb and get some more performance out of it for you. He even dynos them before they leave to set the jets to get the most power. I have sent my SMTA and H carbs there and would send every other carb there. IIRC like $75 shipped back to you. With that, the carb kit is less than $25 so there is plenty of labor on that.
BTW, DENNY'S SUCKS!!! Didn't you think that was a bad sign that you buddy had to send his carb back?
gene oslund
New User
I have an ihc H that always idles rough. It smooths out under load. One day I swiped the carb and friends H which Idled smooth. My tractor still ran the same. I think I need new valve springs.
John M
Well-known Member
- Location
- Nunyafn business
Turn it out some more!
rustyfarmall
Well-known Member
Simple?? ALL carburetors are actually very complex. If all you did was open it up and replace a few parts, yes, that is simple, and if that carburetor really didn't have anything wrong with it in the first place, then you probably WERE able to put it back together and have it be functional. Can you tell us what gauge of wire you used to make sure the idle metering circuit is open and free of varnish and rust?
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