Fuel Mixture

Tue Jun 18, 2013 7:49 pm

How can I lean out the fuel mixture on my cub?
It seems to be running rich.
Black on the tip of the stack and blows black crap all over the hood.

Re: Fuel Mixture

Tue Jun 18, 2013 8:03 pm

Check and see if the air intake is plugged or the oil bath filter. Diagnosing Engine Troubles.

Re: Fuel Mixture

Wed Jun 19, 2013 7:16 am

Thanks for the link!

Re: Fuel Mixture

Wed Jun 19, 2013 7:39 am

vwfish wrote:How can I lean out the fuel mixture on my cub? It seems to be running rich. Black on the tip of the stack and blows black crap all over the hood.
Describe the black crap; sooty, oily, wet. Engine running fine, good power, or faultering? Check spark plug color/condition for hint to problem.

Carburetion, could be some one enlarged the main jet. That's in addition to the other potential problems listed in the link posted by Rudi.

Problem might also be mechanical. Suggest doing a compression tests, dry then wet.

Re: Fuel Mixture

Wed Jun 19, 2013 9:06 am

Most engines leave a black coating in the exhaust pipe (soot) unless they are really worked hard for long periods. The jets are preset for handling a load, which requires a richer mixture than just piddling around. The spots on the hood are probably droplets of water from condensation in the exhaust as it warms up when first started carrying specs of carbon with them.

Re: Fuel Mixture

Wed Jun 19, 2013 10:05 am

Yes, in a nutshell there is no fuel mixture adjustment on a Cub carburetor.

About the only thing that could be causing an overly-rich condition is the float being set too high.

Re: Fuel Mixture

Sun Jun 30, 2013 2:53 pm

I adjusted the idle screw and throttle stop per the operators manual and it seems to be running much better.
Looks like the PO backed the throttle stop all the way out and just used the idle screw to adjust (it was backed out about 5-6 turns).
The manual says to turn the idle screw all the way in, back it out one turn, set the throttles stop for desired idle speed and adjust the idle screw out 1/2 to 1 turns for smooth running.

Re: Fuel Mixture

Sun Jun 30, 2013 3:37 pm

What John said, plus the IH carb had 4 jet/Venturi tube combinations. Remove plug in manifold above carb and replace with your finger to play with fuel/air mix. If adding more air makes it run better/smoother then you need a smaller jet.