I have a 1952 8N (8N 500585) that has a popping/sputtering sound when it idles (450 RPM). I tuned up the ignition system (points .025", Autolite 437 spark plugs .025-.028") The markings on the flywheel are non existent so I rotate the distributor clockwise/counter clockwise to set the timing. I replaced the carburetor with a fully restored Marvel-Schebler (idle adjustment needle 1/2 turn; main jet adjustment needle 1 turn). The results of the compression test: 1-115, 2-115, 3-120, 4-115. I also used a propane torch looking for an air leak involving the intake manifold with none observed. Any advise/guidance would be appreciated concerning this popping/sputtering sound at idle.

I thought this was in our How-To's library, but did not see it (we will add it ASAP). Bruce Haynes (aka Bruce (VA)) wrote a nice tutorial on carb adjustments. I'll copy/paste it here for you, and we'll post it in our library for future reference:

Adjusting the M-S Carb
by Bruce Haynes

Make sure the tractor is at operating temp; that usually takes 10 – 15 minutes at idle, depending on ambient temp.

Both Ford and Marvel/Schebler (assuming you have an M/S carb) say to set both the side-pointing idle mix and the down-pointing main jet to 1-turn as a starting point. I set the down-pointing main jet to 1-1/2 turns and do not fool with it until the final step.

Then adjust the side-pointing idle mixture for the fastest idle, not the smoothest idle. Next, adjust the behind-the-carb idle-speed setscrew for a very slow 400-rpm idle. Do that idle mix adjustment for maximum idle at least 3 times. Make sure that you turn the screws slowly, like 1/8 of a turn at a time, and wait a second or two for the engine to catch up. Take your time!

Do it like this:

1. Adjust the idle mix jet until RPM increases

2. Adjust the idle-mix set screw until the engine nearly stops (as slow as you can get it, unless you have a tach that tells you 400 rpm)

Repeat steps 1-2 three times.

Remember the side-pointing idle mix is out for lean, in for rich.

If you do not have any problems inside the carb, it is easy to get the idle down to 350 - 400 rpm.

Your last step is to go back to the main jet. Remember, in for lean, out for rich. If you end up turning it OUT more than 1/2 turn for max power (remember, you already had it 1-1/2 turns out), then stop right there because you have a dirty carb or a fuel problem.

Lastly, each of us has his own definition of a carb rebuild. If your rebuild did not include soaking the carb 24 hours in a bucket of caustic carb cleaner and blowing out every orifice with/ a rubber-tipped air gun, you didn't do a rebuild, IMHO. You just replaced parts.

These tractors run at their best with Champion H-12 (512) spark plugs.