farmall m runs poorly once engine gets warm

[huzzah man]

I have a 1949 Farmall M converted to 12 volt.Recently it began running rough. I cleaned the fuel filters, drained the gas tank,checked the plugs, replaced the distributor cap. After all this it started and ran smoothly at idle and for a while under a load. After an hour of brush hogging it started missing again, but still idled smoothly under no load at low RPMS.
Any suggestions?

 

[Weaselfoot]

Look for a crack in the fuel lines which allows air to enter. You won't see fuel leaking out but air can get sucked in. Is there an air vent on the filler cap which is plugged? (Just quick ideas which might not apply.)

 

[jenkinsph]

I would check the ignition system again, rotor, linkage movement on distributor, plug wires and other things like plugs you checked previously. When it is missing can you pull the wires on the plugs one at a time to determine which one is the problem?

Since an M has a gravity flow for fuel (with the tank up top under the hood) I doubt air entering the system is the culprit. As long as you have a good fuel flow to the carb that should not be an issue.

My best guess is a plug or plug wire is the problem.

 

[DJ54]

Might be an idea to give the choke a quick on & off pull next time it does that. Could be a speck of something in the main jet. A quick tug on the choke under load has a tendency to pull it on though. If it straightens up if for just a bit, you know it's fuel related.

I've also in the past had a speck of carbon foul a plug too. I tend to by the mid grade gas when I'm working the C's or the 656 pretty hard. Seems to eliminate some predetonation I seem to get at times with this ethanol laced gas.

 

[Birdhunter1]

Might be an intake manifold or gasket sucking air once it warms up. Take a can of WD-40 or carb cleaner and when it is running rough spray around manifold gaskets and the manifold itself. If it is sucking air it will run great for a few seconds when it inhales a puff of something flammable vs. air and you will have pinpointed your leak.

 

[SHORT KID]

Might be an intake manifold or gasket sucking air once it warms up. Take a can of WD-40 or carb cleaner and when it is running rough spray around manifold gaskets and the manifold itself. If it is sucking air it will run great for a few seconds when it inhales a puff of something flammable vs. air and you will have pinpointed your leak.

I would fill the fuel tank with fresh fuel and add a quart of auto trans fluid with it, this new fuel really sucks on older engines. I have seen this work a number of times. Good luck.

 

[Mysfyt]

This happened on an H that I used to own and it turned out to be a bad coil. Just start with the simple and no expense stuff first like cleaning the electrical connections, checking all connections are tight, etc.

 

[jaotguy]

second the thought of the coil / points / condenser .... leaning more towards the points first , then the condenser then coil

 

[gghb]

ditto on the coil my 49 cub was giving me trouble and a new coil solved it. touch the coil when it.s running rough and see how hot it is if very warm replace

Greg