Engine run-on after ICO

[LimaFox]

I recently changed the fuel pump on my O320-D3G (installed in a RV6). After completing a check flight and attempting to shut down the engine with the mixture, the engine ran on a bit roughly at about 500 rpm. Advancing the throttle slowly with the mixture in ICO finally allowed the engine to stop.

I am trying to understand how a change of fuel pump would cause this problem. I am currently investigating the primer circuit as a source of fuel, but failing that I will look to the carburetor.

Has anyone had such an experience?

In the event that the troubleshooting leads to the carburetor, does anyone know where I can find an overhaul manual for a Marvel-Schebler MA-4SPA carburetor?

Lee Fasken
RV6
Ottawa Ontario

[GeneL]

Sounds like you might have a float getting heavy.This is very common, and you need the latest float type (plastic I think). With the new pump or sometimes boost pump it overwhelms the float/needle and seat, then fuel floods into the carb. I would do the needle and seat if not a complete rebuild, parts are pricey but worth the peace of mind. Gene

[aerhed]

Your mixt cable turns the step valve in the bowl inlet. Make sure your mixt cable pulls the arm to it's stop. If it does, and you still have run-on, then fuel is getting past the step valve. It does this if worn or scored. Usually you can just set the RPM at 1000-1200 and pull mixture to ICO from there and it will kill briskly. If it doesn't, then you've probably got a real bad step valve. The valve shaft is actually a spring like a tach cable. I've seen the valve rusted or burred up to where the shaft flexes without really closing the valve fully.

[LimaFox]

The float had been changed, but that was some time ago (14 years according to the records). We had checked the mixture control at the same time we changed the fuel pump: it was going to full idle cut-off at the carburetor.

Thanks for the suggestions. Its probably time for an overhaul.

Lee