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.

Thanks Ed. Your post was informative concerning Bruce Haynes' carburetor adjustment algorithm. I performed the procedures as described in the post. I can actually close (all the way in) the idle mixture adjustment needle without any noticeable change in the popping/sputtering/roughness/RPM. If I move the main adjustment needle to 1/2 turn open, as the last step, the RPM drops and if left at that setting the motor will die. I left the main adjustment needle setting 1-1 1/2 turns open. I have considered that there may be an air leak within the intake system altering the air:fuel ratio yielding a lean air:fuel mixture. I am considering replacing the 2 intake manifold gaskets as my next step. If you have further advice/guidance that would be extremely helpful and greatly appreciated.

Have it running and take a can of carb cleaner and spray around sealing surfaces on the carb and manifold. If the RPMs increase, you have found an air leak.

Thanks for your response. In my initial post (18 May 2026) I utilized a propane torch looking for an air leak. I repeated the procedure utilizing a spray can of Gumout carburetor cleaner. I cannot find an observable air leak.
I repeated Bruch Haynes’ algorithm for the adjustment of the Marvel/Schebler carburetor. The fasted idle, not the smoothest idle was obtained with the idle mixture adjustment needle closed to 1/8-1/4 turn open (400 RPM). I feathered the choke and with the choke pulled halfway out the idle RPM dropped (interpreted as a rich mixture) but the roughness was still present. As I slowly pushed the choke in the RPM initially slightly increased and the roughness decreased but was not resolved. As I continued to push the choke in the initial roughness returned when pushed all the way in. There is no hesitation when I “jerk/yank” the throttle from 400 RPM – 1500 RPM. I can kill the engine with the choke all the way out.
Do I need to move from the carburetor to the ignition system?
Any comments would be greatly appreciated.

When you tuned it, did you replace the condenser? If so, did you keep the old one?

I adjusted my Duralast spark tester to 5/16” which set it for the conventional automotive ignition (20,000 volts). The tractor easily started on 3 cylinders and all 4 cylinders were tested producing a white spark. I did not appreciate a blue and/or yellow hue.
I also rechecked the carburetor adjustments. The idle needle yields a maximum RPM at 1/8 turn open and the main needle at 1 turn open.
The carburetor is a Marvel-Schebler that was rebuilt by a person who does only tractor carburetors. Every orifice was cleaned, new seals, gaskets, screws, float valve, bronze throttle bushings installed and precision sized to prevent air leaks. He then put it on a running engine for fine tuning.
I feathered the choke again and as I pulled the choke out about ½ way, the RPM dropped. As I slowly pushed the choke in the RPM increased, but I still cannot get rid of the roughness (sputtering/popping).
Sorry for imposing upon your time, but your comments and guidance are greatly appreciated.

Can you read the part# on your M-S carb? It is located on the boss on the right-hand side of the carburetor body. It should read something like TSX 241x (x=A, B, or C).

Carb number is TSX 241B.
Additionally, there is a 59 and an 8 (B?)

The 241B is supposed to be the best carb for the 8N. There was a 241C that was dumped by Ford and they fell back to the B version. I was reading some tech support bulletins and found this:
The operation of tractors equipped with the TSX 241 carburetor, in which the engine tends to "stumble" either while idling or when placed under load, can be improved by installing the economizer jet, Part No. 8N 9914. The installation of this jet, along with proper carburetor adjustment, will, in most cases, correct this "stumbling" condition.

Did I understand you correctly that your idle jet is only turned out 1/8 of a turn?

In an earlier post I mentioned that I had followed Bruce Haynes’ carburetor adjustment algorithm and currently have the idle mixture adjustment needle turned out 1/8 turn. I can actually close (all the way in) the idle mixture adjustment needle without any noticeable change in the popping/sputtering/roughness/RPM.
I read with interest your post concerning the economizer jet. I read some additional information on the Marvel-Schebler economizer jet. As I understand it, the economizer jet works by resolving a rich fuel mixture by introducing additional air into the fuel mixture creating a leaner mixture. There was a reference to Service Bulletin 118 but I was unable to locate it.
As I see it, I have too much air in the system because I can actually close the idle mixture adjustment needle. I have searched for an air leak using a propane torch and also a can of carburetor cleaner without finding an observable air leak either time.
Please advise what additional measures I can explore.
As always, your comments are appreciated.

I still have the old condenser. I'll put it back in and see if that makes a difference. I'm also going to test with a spark tester. Thanks for your help.

Thanks, Ed. I'll try the carb cleaner and let you know the result.

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

Thanks, Farmer Brown. I appreciate the information about Champion H-12 spark plugs.