Adjusting Marvel Schebler carb.

I put a rebuild kit in my 8N9510 carb. I reviewed all the posts I could find on how to re-adjust the carburetor.

I bench-set the adjustments as follows:
1. Idle speed screw, three turns inward after making contact with the post.
2. Idle mixture screw, 1 turn out from bottom.
3. Main mixture, 1 turn out from bottom (I know that some suggest two turns out, and leave it).

With the above settings, I needed to play with the choke (in/out), and the throttle lever to keep the engine running. When it warmed to operating temperature, the engine pulsed (sped-up, slowed-down constantly) by the governor. I could not get it to run at a constant RPM by adjusting the main mixture or the idle mixture.

At this point, I bought a new carb on Ebay. I set the adjustment screws/jets same as above with the very same result. No matter what I do with the idle mixture and the main mixture, I cannot get it to run at a constant speed.

Note that the governor was not causing the engine to pulse when the old carb was taken off (the old carb's needle valve was sticking causing the carb to flood and dump bas out the air intake, so the carb had to be dealt with...or I would have left it alone in the first place.

What am I doing wrong?

Put the old carb back on the tractor. Chances are 7 out of 10 you have a new Chinese clone from eBay and it will never run right.

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 a M/S carb ) say to set both the side-pointing idlemix and the down-pointing mainjet to 1-turn as a starting
point. I set the down-pointing mainjet to 1-1/2 turns and don't fool w/ it until the final step.

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

Do it like this:

1. Adjust idle mix jet until RPM increases

2. Adjust 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 - 3 three times.

Remember the side-pointing idlemix 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 rpms.

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 ½ turn for max power (remember, you
already had it 1 ½ 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 hrs in a bucket of caustic carb cleaner &
blowing out every orifice w/ a rubber tipped air gun, you didn't do a rebuild, IMHO. You just replaced parts.
75 Tips

I don't care where the idle is adjusted to, I never run at an idle anyway. For the high speed adjustment, when warm I start out at 2 1/2 turns open, then with it running at full speed, I adjust the needle in until the engine falters and then back it out 1/4 turn. usually pretty good at that point but may need a bit more out if power is lacking or not running right under a load.

"1. Adjust idle mix jet until RPM increases
2. Adjust 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 - 3 three times."

??? two steps but you want me to repeat steps 1-3 three times?

yes
75 Tips

I could do that IF there were THREE steps listed...you only list TWO steps and then you tell me to repeat three steps. One of us is not understanding the other.

"2. Adjust 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)"

Do you mean to adjust the idle-mix "adjustment" screw? Or is there a "set screw" involved?

(quoted from post at 14:37:04 08/24/20) "2. Adjust 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)"

Do you mean to adjust the idle-mix "adjustment" screw? Or is there a "set screw" involved?

Click to expand...

here are two adjustments in the idle circuit. One is the needle valve and the other is a stop set screw on the linkage on the back of the carb. It sets the idle SPEED while the mixture screw sets the idle fuel air mixture.

If you want, send me your OEM M/S carb and I will rebuild it, test it, and send it back guaranteed to run. 3 Turns??? Where did you get that from? Was your kit the TISCO one? There's a ton of original literature on the M/S carbs on the other Ford N-Tractor web site -email is open. All the aftermarket, Cheena made parts today are NOT preset to any specs. Carb settings must be made when engine is hot, and make 1/8 to 1/4 adjustments at a time, AND wait til engine speed catches up. Often it takes a few times to get it right. Forget the gov -you need to get carb right first. It helps to have the Essential Manuals. None of the OEM manuals says anything about 3-Turns out.

MARVEL-SCHEBLER TSX CARBURETEORS –FORD TRACTORS –HOW TO ADJUST
When adjusting the carb, turn main jet out 1-1/2 turns and leave it. Then adjust side pointing idle mixture screw to fastest idle, not smoothest. This procedure has to be done when the engine is really warmed up and often takes 2 or 3 tries. When you turn a screw in or out, watch how may turns you go, best to only go 1 turn at a time, then WAIT for the engine RPMS to catch up. The proper method is to make adjustments while under load and at certain rpms, but early 8Ns have no Proofmeter to show R's so best guess scenario takes over. Both Ford and Marvel/Schebler say to set both the side-pointing idlemix {Idle Mixture Adjustment Needle} and the down-pointing main jet {Main Power Mixture adjustment Needle} to 1-turn as a starting point. I set the down-pointing main jet {Main Power Mixture adjustment Needle} to 1-1/2 turns and don't fool with it anymore.

Then adjust the side-pointing idlemix {Idle Air Mixture Adjustment Needle} for fastest idle; not the smoothest idle. Next, adjust the behind the carb idle-speed (throttle) set-screw for very slow 400-rpms idle. Do that idle mix {Idle Air Mixture Adjustment Needle} adjust for maximum idle at least 3-times. Do it like this:

1. Adjust idle mix jet {Idle Mixture Adjustment Needle} until RPM increases

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

3. Grasp the rod between the carb & governor & twist/shake it slightly; the slop in these rods often stops the adjustment cam from moving.

Repeat steps 1 - 3 three times with about 10-15 minutes between tries.

Remember the side-pointing idlemix {Idle Mixture Adjustment Needle} is out for lean, in for rich.

I don't let my N's run dry. The M/S carb has a cast iron bowl & it will rust when it gets dry. Running aluminum carbs dry is ok.

The Main Power Adjust needle is CW for reducing fuel (leaner) and if fully functional will shut off all carb fuel (even idle) when screwed in fully CW to it's seat. This leans it to point that engine dies. You say that this results in backfire. Is that correct? Just before all the way in (so lean it just barely runs, I don't see that you should observe a rich-condition-black-smoke at this point. Do you see that at this point? The Idle Mixture Adjust needle is just the opposite, that is CCW leans the idle mixture. That is because the Idle Mixture Adjust needle controls AIR, unlike the Main Power, which controls fuel. More AIR (same fuel)=leaner idle mixture. CCW=more air=leaner.
Main Power needle is generally not very sensitive except near fully closed (full CW), whereas the Idle Mixture needle is more sensitive.

FORD N-SERIES CARB & ADJUSTMENTS:


FORD 9N/2N ESSENTIAL OWNER/OPERATOR/PARTS/SERVICE MANUALS:

FORD 8N TRACTOR ESSENTIAL OWNER/OPERATOR/PARTS/SERVICE MANUALS:




Tim Daley(MI)