o-320 rpms climb by 250rpm when leaned @WOT/check float next/Have I overlooked anythi

[Emery]

So, I am running on the ground an O-320 that has been pickled for about 10 years. I have installed it on my RV-9. It has a decent idle smoothness. It has decent mag drops at all rpm ranges. I have unhooked the fuel primer line to eliminate that fuel source.

So far I have not spent any money on it.

When experimenting with different throttle RPM settings above 900rpm, the engine is rough until given major leaning (I'd say about an inch and a half of mixture knob movement) at which point the engine smooths out and I gain about 200rpm. At wide open it is closer to 250rpm.

I have not had the carburetor apart yet (Marvel schebler MA-4SPA) so that will be my next step. For the experienced out there, is this a classic Marvel case? I am also aware of marvels float problems and updates. I am comfortable with working on carburetors.

My best leaned WOT so far is 2150 rpm. Hopefully that will improve with work.

Any comments or similar stories will be appreciated. Thanks

[BobTurner]

When you say you ‘unhooked’ the primer line, did you cap it off? You can’t leave a large air leak into the intake manifold.

[Emery]

Yes. No vacuum leak.

[BobTurner]

With engine at idle, move mixture to cut off. You should see a small, 50 rpm rise just before it dies. Adjust idle mixture screw if necessary.

[Emery]

I'm about to pull the carb off and realized that there is a bunch of fuel leaking out the airhorn. So yes, I'm going for the float and needle and seat. Thanks

[Emery]

Well, I took the carb apart today. The brass float was dry inside. I submersed it in 212 degree water to check for bubbles....nothing. Float level is 7/32 and float moves freely. Carb is very clean. Problem Could be the needle and seat. The main jet does not appear to have been drilled out as a #40 drill will not pass through and a .092-.093 bit will.

I'm going to sleep on it, but I might just put the carb back together and try it again.

A leaking needle and seat is bit abstract. Difficult to see and be definitive about. Usually you just change them. Too bad they cost $75-$100.

[DavidAhrens]

You can and should check the needle seal with air pressure while on the bench before reinstall. Common practice after carburetor work. Goggle it , if needed, I can give you more of an explanation.

[MiserBird]

Is your engine a 150HP with a part #10-3678-32 carb?

[mahlon_r]

Some things to check if the carb is OK. But raw fuel collecting in the airbox would indicate that it isn't or there is a rag or something in the intake manifold.
Assuming you have a 10-5135 or similar carb, on the bottom of the carb there is a vent hole cast into the lower bowl. it is located just inside the airbox flange. Make sure this hole is not obstructed in anyway by a home made air box gasket or any part of the airbox. If that vent hole is blocked, the engine will not make power. Also make sure you have no heat leak into the airbox and that the air is free to move from ambient to the carb through it. Also, I would make sure your fuel pressure is in limits and that your engine breather is free and unobstructed.
Good Luck,
Mahlon

[lr172]

Originally Posted by DavidAhrens

You can and should check the needle seal with air pressure while on the bench before reinstall. Common practice after carburetor work. Goggle it , if needed, I can give you more of an explanation.

+1

I would not put the carb back on without confirming the integrity of the needle/seat interface, given the symptoms you describe. Also give a look at the check ball in the accel pump chamber.

Larry