M Farmall Idles Rough

I have a 49 model M Farmall that idled rough and smoked a little before a complete rebuild. I went back with aluminum pistons that are oversized by using thinner sleeves, valve job, bearings etc.. When I got it back togther the smoking was gone but it still ran rough at idle, so I sent the carb to Denny's Carb Shop for a rebuild. Put it back on and it still idles rough. It runs perfect under load with lots of power but does not idle correctly. What is wrong with this tractor? Could it be a governor problem as I have not messed with it?

My first thought is that you have timing problem I would try retarding time a little and see what happens. I would not suspect govener to be problem.

My first thought is that you have timing problem I would try retarding time a little and see what happens. I would not suspect govener to be problem.

Denny's has not had a good following on this forum. Several issues. Don't count out that potential.
If it has a magneto, it will not have timing that is adjusted to the engine speed. It is full advance all the running time, retarded by the impulse coupling at start.
They do run a bit rougher and are harder to tune smooth. Does the idle mix screw have any effect?
JimN

Have you tried Autolite spark plugs. I had the same problem with my f-20 for years. Then a guy at a local show told me to try autolite plugs. i bought a set at the show and couldn"t believe the improvement. I will never buy another champion spark plug.

I would agree that you have a timing problem. Like Jim said , if you have magneto you have full advance once impulse has released. If you have battery ignition, it sounds like you are scattering the spark around due to worn auto advance mechanism. The timing should be near tdc at low idle and gradually advance to (depending on what shaft you have in distributor) full advance of eiether 22 degrees for fire crater, 30 degrees for std flat head aluminum or you may have a 40 degree shaft which was for cast iron regular bore gas engine. Anyway, if you have a lot of advance at idle , engine will have a rough sounding exhaust but will run fine at full throttle under load.

your first mistake was taking it to dennys carb shop, I have bought some things from them and are very rude people and the ladies that work there have no clue what they are talking about. They dont know anything about their own products. I bought oversized pistons from there and they didnt even know what the clearance needed to be for sleeve. I will never do business with them again. EVER!

The tractor originally had a mag, but I changed it over to a distributor off another M Farmall in a effort to correct the idling problem. Do I just twist the distributor to set the timing or is there more to it?

First make sure the centrifugal advance is working. It is checked either by using a timing light, or by rotating the rotor CW. It should turn about 1/4 to 3/8 inch at the tip, then spring back when released. (or the breaker plate can be removed to look at the springs and pivot weights).
Time as follows: Set the point gap to .020" on the very top of a distributor cam lobe.

Put #1 cylinder on TDC compression. (hold your thumb in the hole and turn it with the fan until pressure is detected coming out of the hole.

Then match up the timing mark on the front pulley. Putting a soft piece of wire in the cylinder through the spark plug hole also indicates when you are very close to TDC.

Then use a test light on the wire going to the points from the coil. Loosen the distributor. Turn on the ignition. (in neutral of course) and then turn the distributor clock wise(retard direction)until the light goes out. Then turn it back CCW slowly until the light just comes on. If it is out without turning, turn it CCW until the light just comes on. Tighten it down. Static timing is now set, and if the Centrifugal advance is working, it will be in time. JimN

On the automatic advance. About 90 percent of them old distributors need attention in that department. Worn out weights, pins they pivot on, weak or broken springs or rusted parts. You need to put a timing light on to check the actual advance and retard. Like I said , it should be at tdc at low idle ideally and gradually advance with increasing engine speed. You need some reference marks on pulley to see what it is doing. On an M, 2 inches on pulley is equivelent to about 30 degrees. There fore, it should be in that vicinity at full throttle and be steady, not changing back and forth. And again, there are several different shafts in these distributors with different advance amounts. I's betting your's is probably advancing several degrees at low idle there fore causing rough idle. So many variations over the years in these old tractors, gas heads, kerosene heads etc, different domed or flat pistons and if you don't have the combination right you have problems. On my own M that I am ressurecting from the long dead, it had a 22 degree shaft which is for fire crater pistons, but it has flat head pistons so I modified the advance and will have to calibrate it when I get it running. I made marks on pulley to reference different degrees. To do so you take pulley diameter times 3.l4159, divide that by 360 and multiply that by the degrees you want to make your mark. This gives you a measurment in inches on circumference of pulley to make your mark. Never had math after 8th grade but as a mechanic you use it every day.