JD M on one cylinder when cold

I have my 420 set pretty lean and without choke when it starts it will run on one cyl. Also when you try to put it into a hard pull before it is up to operating temp it will drop down to one cylinder. Once up to operating temp it will lug down and runs great. I have progressively found this mixture setting through time in an effort to get lean as possible - it has solved my plug fouling problem. I would recommend you start with the last suggestion first and when it cuts down to one cylinder start pulling the choke out, it it pulls back in on two, you probably have identified a fuel problem. I can live with the way mine runs for the rest of my days. I only start it up to work on it or work it.

~You might want to try a jumper wire from the starter big battery post to the coil [ign switch side] to eliminate a resistance problem in the ignition switch or wiring, [common on the M-40]. A weak spark can sometimes show up more on one cylinder.

~If you don't find anything obvious in the ignition system, try running it with the air inlet hose removed. If that helps see if the air inlet strainer [in the bottom of the carb intake elbow] is plugged or painted over. That will allow fuel to puddle in the air intake elbow until it is pulled in & burnt.

~Take a look at the throttle shaft for worn bushings or poor sealing [an air leak there can cause poor running cold]

~Another possibility is a leaking intake manifold gasket allowing a vacuum leak when cold.

~It wouldn't be a bad idea to run a cold compression test [carb & choke open] just to eliminate a sticking valve or worn rings when cold..... ..

I would remove the plugs and with the wires still attached but their case grounded, observe if each has about an equal good blue spark. If not, swap sides and wires etc to isolate a bad wire or plug.

I would inspect the points and see if the gap is about the same on each high cam lobe and that they are NOT pitted, burned, or oxidized. If any, replace them. You might even first try and open their gap a bit (like 0.002 or more than they were) (maybe bearings or lobes are worn uneven) to see if that helps. 0.016 to 0.017 if mag and 0.020 to 0.024 if distributor.

I would see if the side that fouls plugs has about equal compression with the other.

If either side has a flat black sooty looking plug or a wet shiny residue more than the other, I would first at least install a new plug there.

If they havent been replaced in a long while, I would try a fresh set of plugs and clean and re gap or just install new points and condensor, assuming the wires are in good shape, and clean and inspect teh cap and see if that helps and if not, post back with what you found and what you did so far. One side fires stronger at idle anyway, but it shouldnt miss out as you described. Another problem may be a condensor, but I would suspect a plug or wire or points first.

You may also want to check if the choke is workign properly (butterfly plate really closes) and if richening the power or idle jets helps matters. Does choking durign that period help things????? ????? ?? If so, post back.

Good Luck n God Bless, let us know Ol John T Nordhoff in Indiana