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MA-4SPA Intermittent Idle Flooding

FitzRX7

Well Known Member
First, thanks for any help.

I've got a O-320 w/ MA-4SPA carb that is giving me fits. Runs perfect, except for a nasty habit of flooding intermittently at idle. It's been thru 3 carb repair shops, some multiple times, and it still floods occasionally. Mechanical fuel pump was replaced about a year ago, 6 psi at idle, 2 psi at full throttle (.5-8 psi is the allowable range IIRC).

The main symptom of this is a slowly decaying idle while taxiiing. If I turn the fuel selector off the idle almost immediately recovers, back on and it starts decaying again. Hit a bump or taxi a bit and it stops misbehaving. Or starts doing it again. Pretty sure not getting needle seating well enough intermittently but nobody has been able to fix it.

The carb was upgraded to the new blue float (which is pretty much when all this $hit started), and alignment is perfect, needle/seat have been replaced 3 times I believe, still no relief. I can go complete flights without this behavior and then other flights it does it on taxi out and after landing.

Thoughts? Thanks again.

Jon
 
It does sound like the float needle and seat are suspect. Are you a tricycle or tailwheel? Do you see the same thing in flight? Maybe time to swap that core out with a new factory or rebuilt one . Yes, I know they are not cheap but I do like to see that propeller keep on turning during final.
 
RV-4/Tailwheel. Occasionally have noticed it in level or descending flight, not nearly as noticeable as on the ground. Float/needle have been replaced 2-3 times in the last 4 years. I understand refurb should solve the problem, but it's maddening to not be able to figure out what's wrong with this one.

Jon
 
Usually in the rebuild kits there is a guage template setting that float to valve setting. That might be a place to revisit. Without that guage its pretty much a guess. If carb is off on bench you might fill with some 100LL and even apply a little pressure to see if its running thru the seat and out the vent. Should have shut off even at couple pounds pressure. Look at it also at the tailwheel down position . Good luck
 
I would be looking for a crack in the main body of the carb housing, as well as a problem with seals or ball in the accel pump.

Have you properly adjusted the idle mixture with the engine hot? A fat idle mixture can cause your symptoms as the engine "loads up." Doesn't explain the resolution with bumps, however. That clearly points to a float or needle seat issue. Don't see how that happens with three new sets of parts. Also possilbe that the blue float is defective. You indicated that this all started when you installed it. A manufacturing defect could be creating a different level of bouyancy than normal and this would throw everything off.

Seems the float is the smoking gun, based upon your narrative. May be worth replacing it. This assumes it has never been replaced since it's initial installation.

It would be wise to verify the fuel pressure with a mechanical gauge, especially at idle. High pressure produces the same symptoms as a leaking needle/seat. It is not that uncommon for a pump to fail with high pressure. I would also be sure that you are not injesting debris into the carb from a failing hose.

Larry
 
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flooding

That this happened after float replacement indicates that the float is occasionally hanging up on the wall of the float cavity. I think there is a service bulletin on this and possibly a gage to measure side clearance on the float.
the fit of the float pivot is very sloppy and is also a factor as this allows a lot of sideward travel for the float. Put a little side pressure on the float and move it thru its full travel. Repeat in the opposite direction. If necessary bend the float bracket very carefully to obtain clearance.
 
THANKS!! Lots of questions/answers:

-Mechanical fuel pump replaced 100hrs ago, no change in intermittent behavior.

-Accel pump parts all rebuilt by most recent shop.

-Carb shop bench tested carb with 6' of head pressure, no flood. I resinstalled carb and with facet boost pump fuel poured out of vent. Turn pump off, made a phone call, turned it back on and zero leaking.

-Carb shop has cutaway fit guide, zero clearance problems indicated by guide, minor witness marks around float hinge pin clearanced at most recent tear down, zero change in behavior.

-cold/hot makes no difference, today misbehaved slightly on taxi out, slowdown on base to land, and once on ground idled perfectly 5 minutes to hanger. Completely random.

-all engine fuel hoses new in last 100hrs, no debris ever found.
 
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I resinstalled carb and with facet boost pump fuel poured out of vent. Turn pump off, made a phone call, turned it back on and zero leaking.

That's concerning. Clearly your facet pump is not properly regulating, at least not consistently. Can you link all of your issues to the boost pump running? If so, I would replace it.

Do you have a fuel pressure gauge? I believe you may need one to sort this out.

Larry
 
Thank Larry. No issues follow the boost pump, have a great Garmin G3X pressure gauge, never over 6psi even with the pump on. The fuel pouring out was just to indicated how random the seat not completely sealing is, put pressure on the carb and it flooded, 5 minutes later put pressure on it and it didn't. Just so strange that it can ever-so-slightly hang up and then work fine.
 
Thank Larry. No issues follow the boost pump, have a great Garmin G3X pressure gauge, never over 6psi even with the pump on. The fuel pouring out was just to indicated how random the seat not completely sealing is, put pressure on the carb and it flooded, 5 minutes later put pressure on it and it didn't. Just so strange that it can ever-so-slightly hang up and then work fine.

Issue then is likely with the float or it's linkage or the needle / seat. Intermittent issues could be debris in the needle seat interface or the float or it's linkage hanging up on something.

Larry
 
Needle/seat/float hang up only possible logical problem, just frustrating the carb shop(s) can't figure it out. Intermittent problems are the worst.
 
Maybe the float hinge is intermittently binding and not allowing the float to rise until it gets bumped or jarred loose again. I am thinking about the float hinge pin and how it is safetied. How about the cotter pin in the end of the float hinge pin? Is it installed correctly such that it wont cause the hinge to bind?
 
Marvell-Schebler would take your carb as a core and sell you another one that has been rebuilt. When I sent my old one in, that is what they recommended.
The carb should also be set up for RV use as well since we need more main jet size to compensate for the enhanced breathing of our intake system.
 
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