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I have been struggling with proper setup and adjustment of Marvel Schebler DD13 780 and DD14 790 Carburetors on an Onan B48.


Engine has been running and firing for a few weeks but cant seem to get the carb to run it.

I have a plentith of different seats, gaskets, idle jets, main jets, fuel pumps and fuel inlet plates from the two carburetors and generic rebuild kits.

Both carbs have foam floats and neither carb had an assist spring installed on the float. Main needle orientation as pictured below is another variable I am wondering about, from which direction should the two ears mount onto the float from?


I have found both carbs to be set at almost .25" float gap upon dissassembly rather than .125" that is called for by service manual. Have set back to .125" float gap making the float almost parallel w the gasket face.


Internal fuel passages have been cleaned and verified using thin wire and carb cleaner.

Fuel is being sent in from the bottom of the inlet orfice via fuel plate and externally mounted mikuni vacuum pulse pump.


.020" points gap, NGK B-6L spark plugs

Lack of fuel does not seem to me to be the issue, the motor can be started daily and has been running steady at both idle and high rpm while sitting for extended periods of time in the garage- far beyond warm up, but cannot maintain consistency in air fuel ratio or power level under load while in operation .

The only way my setup has been able to run is with .125 - .25 of a turn on the main jet; rather than the 1.25 on main and 1 on idle that the service manual call for as a starting point. Anymore out on the main jet and it needs the choke to run.


Have found some very helpful threads and info among these forums. Some progress has been made, but it's not quite a tractor yet.

Any input or ideas would be appreciated



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