What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

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

Emery

Active Member
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
 
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.
 
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.
 
fuel leaking

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
 
Not resolved.

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.
 
Carb problem

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.
 
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
 
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
 
Carb Needle and seat test

Thanks .Good advice on testing the carb needle and seat. I will do.
 
Carb part# is 10-5135 on a converted 160 HP

Carb part# is 10-5135 the last number can't be read with certainty, but would make sense that it would be a 5. I'm searching for the engine log book. It is a converted 160 HP.
 
Float needle and seat test. Accelerator pump ball check,Vent hole, Breather

Today I have the carb needle and seat under a 6psi load of compressed air. The source is shut off. With just the weight of the float holding the needle to the seat, and after a couple of hours, the pressure is still at 6psi.

ACCELERATOR PUMP
I have visually checked to make sure the accelerator pump check ball is in place, and it is. as well, while plugging the intake holes for the accelerator pump, and using my 1psi lungs, I could not blow by the check ball or unseat it.

VENT HOLE
The vent hole is not obstructed with clear passage from the air intake side of the carb.

The carb is breathing outside cold unfiltered air.

BREATHER
When I remove the dipstick, I can blow back through the breather hose easily.

I will possibly reassemble the carb and try it again although at this point I doubt there will be a change as I have not found anything definitive.

Thanks for all of the ideas, and keep them coming.

If a rag had gotten into the intake somehow, I'm not sure how I would find out.
 
Carb

the shaft that the float pivots on is a very loose fit. There have been problems with the float rubbing on the side of the float cavity. This has the potential to prevent the needle from completely shutting off fuel.
I think there is a service bulletin for this and also there is a measuring tool for determining that the float is properly centered in the float cavity.
 
There have been problems with the float rubbing on the side of the float cavity

Hmm that gives me hope. I will investigate further. Thanks
 
While it is apart, you should replace the brass float with the newer unsinkable blue plastic float. Unfortunately, many $$$ but safer. Good luck.
 
So, I am running on the ground an O-320

Food for thought: Some of these carbs can be pretty rich in certain circumstances. Given that all of this testing is done on the ground and less than full power, it is possible that you have no problem and that your carb is just overly rich at 2200/29". However, you will rarely operate at that setting.

It might be best to do some experimentation in the air to determine if you really have a problem. You can always lean with the red knob during those flights if you are truly too rich.

Larry
 
Unfortunately, many $$$ but safer. Good luck.

The jury may still be out on that. Marvel has changed this float several times since leaving the brass floats and a few versions were much worse than the brass ones. While the solder joint on a brass float can leak, they typically don't break down as often over time as have some of the composite float experiments.

Larry
 
Float

The jury may still be out on that. Marvel has changed this float several times since leaving the brass floats and a few versions were much worse than the brass ones. While the solder joint on a brass float can leak, they typically don't break down as often over time as have some of the composite float experiments.

Larry
Plus one on that. Kelly makes an approved metal float. No plastic for me.
 
New results.

I will add one more note to close off this post. I have finally given the engine a good run with the carb reassembled with all original parts.

I did not adjust anything, and somehow I no longer have the way too rich condition and roughness that required so much leaning.

While the carb was apart, I blew out a few orrifices, reused the original float, and did not detect any wear in the float fulcrum pin to float area.

With the original problem, I did not have an air filter in the fab airbox.

I'm getting a static full throttle rpm of 2200 rpm and it increases by maybe 20 rpm if I lean.

All in all I am happy with my new results, but not sure what the original problem was.
Thanks for the suggestions.
 
I will add one more note to close off this post. I have finally given the engine a good run with the carb reassembled with all original parts.

I did not adjust anything, and somehow I no longer have the way too rich condition and roughness that required so much leaning.

While the carb was apart, I blew out a few orrifices, reused the original float, and did not detect any wear in the float fulcrum pin to float area.

With the original problem, I did not have an air filter in the fab airbox.

I'm getting a static full throttle rpm of 2200 rpm and it increases by maybe 20 rpm if I lean.

All in all I am happy with my new results, but not sure what the original problem was.
Thanks for the suggestions.

No air filter? I think that is a critical piece of the puzzle.
 
No air filter

OK. Enlighten me. It was the only thing that I added that was not in my first problem test.

I have run other engines without an airfilter while working on them seemingly without much difference with or without an air filter.

If this Lycoming is sensitive this way, than I have learned something.

Can anybody confirm this? Thanks
 
FAB not installed

I chased a bad idle and a rough running O-320 carb engine, and found that running it without the FAB and filter installed caused most of the issues. As soon as I installed the FAB and filter, the engine ran "sweet", and only needed minor idle mixture and idle speed adjustments.

The engine ran well at high power, but idled very poorly. I can only assume that the airflow past the carb was disrupted enough to cause the issues. I also had a left mag issue that contributed to the problem.

NB: I was running the engine cowl off, with a bare carb. No FAB installed, no filter installed. Reinstalling both fixed the issue, even with cowl off.

Not sure if this was your issue................
 
Last edited:
Back
Top