M idles poorly

Dr. Bert · Nov 11, 2008

Help. Cannot get my M to idle smoothly. New plugs- hot blue spark thru each. Totally rebuilt carburetor-all passages open, soaked in carburetor cleaner twice (overnight ea. time),fuel flow through the bowl is a steady pencil-sized stream-valves adjusted-compression test showed # three 10 pounds lower than the others. They were 110#s. At 1/3 throttle and up the engine runs very smoothly, but at an idle it runs rough and "lopes" slightly. With the idle adjustment turned out about 4 turns it runs a little better, but not well. Where do I go next? Thanks for any suggestions.

MIKE FRENCH · Nov 11, 2008

Does it help if you close the choke some, probaly needs a manifold gasket or manifold as it is sucking air.

Fixer · Nov 11, 2008

Dr Bert. touch your hand on the intake between the carb and exhaust manifold. When I first got My M back together I couldn't get it to idle either. It was icing up. the manifold was really cold. If it is put a coulple cans of heat in the gas tank. Bernie Steffen

Wayne in MN · Nov 11, 2008

Remember turning out the idle screw will lean the mixture on M & H carbs. It is an air screw, not fuel screw.

Dr. Bert · Nov 11, 2008

Mike: closing the choke seems to make it worse. Have checked the manifold gasket -ie- squirted shots of ether around it when cold--no change. However, will try again tomorrow. Thanks

Dr. Bert · Nov 11, 2008

Wayne-- I'm aware the turning out the idle screw leans the mixture, but doing that seems to improve the idle a little.
Bernie-- thanks for the tip--will check it tomorrow.

Bob M · Nov 11, 2008

Dr. Bert - The advice offered below is all good - especially Bernie's tip about carb icing. If the temperature is between about 35 and 50 and it's humid internal carb ice is likely the problem...

I'd also check the ignition timing and adjust if necessary (should be TDC with the engine stopped).

Dr. Bert · Nov 11, 2008

Bob M. Timing set so the magneto trips with the pointer right on the second mark on the crankshaft pulley. With all the helpful suggestions I am going to start all over and re-check everything, especially the manifold gasket. Icing may be a problem, but the tractor is in a heated shop--70 degrees--could icing still occur? Thanks.

Bob M · Nov 11, 2008

Dr Bert - At 70 degrees icing shouldn't be a problem. (Icing typically doesn't become an issue until below about 45 deg and very high humidity).

On the timing try idling the engine then loosening/turning the mag back and forth a few degrees each way to see if there's a spot where the idle improves.

NE NEB FARMALL · Nov 11, 2008

What about your governor? Have you taken off the cover and adjusted that? What about the governor spring if is wore or stretched out it could cause that.

Dr. Bert · Nov 11, 2008

ne neb farmall: Had considered the governor, however, I had gone through it and adjusted and checked everything about a month ago. Will take a new look at that too. Thanks for the tip.

rustyfarmall · Nov 12, 2008

Possibly due to excessive wear in the distributor.

Brownie 450 · Nov 12, 2008

Could your air intake be blocked at some point? You might try idling the engine with the air cleaner cup off & see if any improvement is noted. Hoses could be having the inside layer coming loose & partially blocking air flow. Is the choke open fully? Is the float level set correctly? Too high of a float level would give a slightly richer mixture.

Anonymous-0 · Nov 12, 2008

The magneto should be set to trip on the FIRST of the double marks. You are timed too slow.

RJ-Wi. · Nov 12, 2008

Sounds like you are not getting enough air. Your idle screw should only be open 1 to 1 1/2 turns. Take the carb. intake hose off and see if that helps. There may be some obstruction on the air supply.

teddy52food · Nov 12, 2008

First of the marks is for running timing.

rustyfarmall · Nov 12, 2008

A defective or out of adjustment governor will not cause rough idling problems. Governor issues usually do not present themselves until the throttle handle is pulled back.

rustyfarmall · Nov 12, 2008

How much play is in the carb throttle shaft? Too much slop will result in a serious vaccuum leak and will also affect governor throttle response.

Anonymous-0 · Nov 12, 2008

Nonsense! The FIRST mark is when the impulse should trip. the mag has a 35 degree lag angle. The marks are much closer than 35 degrees. This is from an original Farmall manual. That being said, his problem seems to be overly rich mixture. Timing on the second mark rather than the first probably wouldn't affect running as much as creating sluggish starting.

Anonymous-0 · Nov 12, 2008

Take the carb bowl back off, remove the small idle stem, then take a small drill bit and run it down the passage. If it does not go all the way down, its plugged. Always lots of sediment in this area. If the throttle body is clear, then this will be the only other place to look for a clog. Hope this helps, ChadS

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