Carb tuning - no change when turning idle mixture screw

timsch

Member
I rebuilt the carb on my '48 8N. I was trying to dial it in using the method of setting the main jet to 2 turns out, setting the idle mix screw to 1 turn out to start and then adjusting the idle mix screw to get fastest idle with engine warm and at idle.

The problem is that turning the idle mix screw has almost no effect. I get best idle with it turned FULLY in. Seems like I have a problem.

Would damaged threads in the carb body for the idle jet cause this? I did have an issue getting out the previous idle jet, resulting in maybe a 90 degree section of the threads getting damaged. The jet still seems to be held in the center, and I didn't notice any liquid (water) getting past the side of the jet when I checked it before reassembling it.
 
You can get that effect if the throttle plate is not closed enough to make the idle circuit work.
 
The engine has to running very slowly (400-450 rpms) for the idle screw to have an effect.
 
IIRC, I checked the throttle plate when I put it back together, and got it to seal as well as I could before tightening the plate screws.

I don't have a tach, so don't know the RPMs, but it was running very slowly. It ran slightly better with the idle screw all the way in than with it out at all.

That being said, it runs pretty well with the mixture screw all the way in. Just a bit rich smelling at the exhaust, but maybe that is the way it is with these old tractors.
 
(quoted from post at 16:09:31 08/04/16) IIRC, I checked the throttle plate when I put it back together, and got it to seal as well as I could before tightening the plate screws.

I don't have a tach, so don't know the RPMs, but it was running very slowly. It ran slightly better with the idle screw all the way in than with it out at all.

That being said, it runs pretty well with the mixture screw all the way in. Just a bit rich smelling at the exhaust, but maybe that is the way it is with these old tractors.
ll the way in is as rich as you can adjust idle mixture
 

That would explain the smell then. Apparently I haven't wrapped my mind around how all this works together yet. Thanks for the help.
 
(quoted from post at 17:25:07 08/04/16)
That would explain the smell then. Apparently I haven't wrapped my mind around how all this works together yet. Thanks for the help.
ust one of Marvel/Schiebler's little jokes......main power turned out makes richer, but idle adjust turned IN makes richer.
Main power is adjusting amount of liquid gasoline, whereas Idle adjust is adjusting amount of air, not gasoline.
 
(quoted from post at 16:45:42 08/04/16)
(quoted from post at 17:25:07 08/04/16)
That would explain the smell then. Apparently I haven't wrapped my mind around how all this works together yet. Thanks for the help.
ust one of Marvel/Schiebler's little jokes......main power turned out makes richer, but idle adjust turned IN makes richer.
Main power is adjusting amount of liquid gasoline, whereas Idle adjust is adjusting amount of air, not gasoline.
b:7c62d7813b][i:7c62d7813b]

DUH.......I remember a few days/nights/hrs/mins, of research that JMOR, and myself put on the phones & emails, learning everything that the Marvel Schebler carbs did and HOW they did it!!!

Lots of carb overhauls, got done.....just for having the knowledge!!!!
It was a BIG 'learning curve'!!

Gary[/i:7c62d7813b][/b:7c62d7813b]
 
(quoted from post at 20:14:53 08/04/16)
(quoted from post at 16:45:42 08/04/16)
(quoted from post at 17:25:07 08/04/16)
That would explain the smell then. Apparently I haven't wrapped my mind around how all this works together yet. Thanks for the help.
ust one of Marvel/Schiebler's little jokes......main power turned out makes richer, but idle adjust turned IN makes richer.
Main power is adjusting amount of liquid gasoline, whereas Idle adjust is adjusting amount of air, not gasoline.
b:627708a720][i:627708a720]

DUH.......I remember a few days/nights/hrs/mins, of research that JMOR, and myself put on the phones & emails, learning everything that the Marvel Schebler carbs did and HOW they did it!!!

Lots of carb overhauls, got done.....just for having the knowledge!!!!
It was a BIG 'learning curve'!!

Gary[/i:627708a720][/b:627708a720]
And here I thought you were simply enlightened by divine intervention! :lol:
We all learn along the way...........if we choose to.
 

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