Carter Yf 938sd Running Rich.

I'm having trouble with my carter YF 938SD when I turn the idle mixture screw all the way in till it stops the it sill runs. When I turn the screw out two and a half turns it runs rough. Could I have a bad accelerator pump?

 

Call Jon at THE CARBURETOR SHOP / Company History. He will help. He has all the original Cater inventory and technical books, lathes for making new metering rods, etc. When Carter was closed out, they called him to take EVERYTHING. HE KNOWS HIS STUFF. In fact he's making me a high altitude rod for when we take Willie to Colorado.

 

Check for a vacuum leak at the throttle shaft.

 

That would cause a lean issue not a rich one. I fixed the throttle shaft a few years ago.

 

If you close the idle jet with the idle jet screw and the engine keeps running, the carburetor is running on one of the other circuits instead. If everything is right, you should be able to stall the engine by closing the idle jet. You should also be able to stall the engine by closing the throttle flap completely (via the idle stop screw). If you close the throttle completely and the engine keeps running, you have a vacuum leak that effectively holds the throttle open via the leak.

What I would do - I would look at the FSM description of the carburetor, and imagine what circuits could pass fuel at idle speed if the idle circuit were shut off. I don't know specifically what circuit this could be. The carburetor is designed to pass fuel control from the idle to the main circuit at some low speed above usual idle speed, like 800-1000 RPM. Since the mixture seems rich when the idle jets are open, you'd think there is fuel going through somewhere else - maybe the accelerator pump? I don't know the YF that well. Seems possible.

 

In my experience, a stuck float sends gas streaming out of the bowl vent as fast as the fuel pump can deliver fuel. The level of the fuel in the float bowl is controlled by the float adjustment. Again in my experience, the float level changes the mixture of the main circuit by changing the static pressure of the fuel at the jet(s), typically at the bottom of the float bowl. Dunno, maybe the YF is different.

 

You're going to make me crack the books and study the YF! ... Pretty sure the air above the bowl has to be at the same pressure as the air horn, so there must be a vent in some form.

 

A stuck float on a YF manifests as fuel spilling from underneath the bowl - that's where the "vent" dumps, around the accelerator pump activating strut.

 

Wouldn't it just spill onto the intake manifold where it would ge obvious?

 

Yes, it does spill into the carburetor throat. On carbs where the vents are above the air horn (like Holleys or the Motorcraft carbs) all that gas dribbles down the on the top of th carb and out from under the edge of the air cleaner. Maybe not with the YF. However, in my experience, never mind rich - it's so much gas that the engine won't run.

 

It is as Petel says in extreme cases. Bottom of the carb. It can also be seen in lesser cases by looking down the throat of the carburetor. You will see excess fuel coming in there.
Try a fuel pressure regulator.

 

Exactly what it does do. If by manifold you mean the F-head head.

 

Well I fixed it. I let it run while looking down the carb and saw gas flowing out of the high speed/primer circuit with the mixture needle all the way in. I replaced the accelerator pump and and reseated the pump check ball and the problem went away.