Calling all Carb Experts-

[PCHunt]

Just bought an RV-6A, O-320-E2D, FP, which has not flown for about 4 years. Ran the engine, running VERY rich, so removed the MA-4SPA (10-5062) carb, and looked inside. Fairly clean, metal float, 2-piece venturi. Suspect the accel pump shaft packing might be leaking. Not sure about the rest.
Questions: Is metal float OK? Is 2-pc venturi OK? (Reading about "stumbling" or "hesitation" related to 1-pc venturi). Other suggestions on rich running?
Related questions: Love or hate Ellison TBI? Anyone using or tried AeroCarb http://www.aeroconversions.com/carb_index.html

"Learnin' by Lurkin'

[captainron]

Check float for leakage (gasoline inside) and needle and seat. Check float height also.

[Yukon]

What is your fuel pressure?

[PCHunt]

Originally Posted by captainron

Check float for leakage (gasoline inside) and needle and seat. Check float height also.

There was no fuel inside the float. Needle valve and seat seem to work OK, but I think I'll replace them anyway. Float was properly set at 7/32". Thanks.

[PCHunt]

Originally Posted by Yukon

What is your fuel pressure?

Unfortunately, I didn't record the fuel pressure during the runup, just noted that I had some.

[ron voss]

Had the same problem with mine. First start up was great, but next couple were very rich. Took carb apart and checked and cleaned it all out. No problems since. Carb had sat around an untold number of years before being used, and possibility of debris from building may have been factors.
Ron Voss N642R
O-320 angle valve
21.7 hours so far

[gmcjetpilot]

Questions: Is metal float OK? Is 2-pc venturi OK? (Reading about "stumbling" or "hesitation" related to 1-pc venturi). Other suggestions on rich running?

Give Precision Airmotive a call in Everett Wa. for the full scoop. They also have info on their web site you should check out, with trouble shooting symptoms and basic but fairly detailed overviews of their carbs. http://www.precisionairmotive.com/ (Click support)

Their trouble shooting manual list a bunch of items and remedies for rich running, caused by: bad primer, wrong venturi installed or incorrectly, float, Bad accelerator pump valve, Accelerator pump stem packing, Sticky float, Float valve not seating, Worn out needs overhaul.

Hesitation: Idle mixture adjusted to rich, Idle tube air-bleed blocked (torque 3-5 in/lb), Pump plunger linkage mis-adjusted (too much fuel on acceleration).


Floats over the years keep changing: metal to plastic, than back to metal and now a new improved composite. The last metal and new composite floats are still in play, at least the last iterations of them. If your metal float is not leaking, corroded and the needle seat and hinge is not worn, good to go. Its your money. Why fix if it ain't broke. A new float kit is not too bad money wise. I think about $135.

Two piece venturi? It's fine, if its not separating, loose as I have been told. The one piece had some problems due to poor field installations, as I understand it. Some think the two piece has better flow? See Precision's site for service bulletin MSA-2, rev 3, 10/95.


Despite all the groovy info, you might want a qualified shop with equip to test it and give it a good eye ball inspection (not even a rebuild just a once over).

As far as the alternatives to carbs, Ellison is fine but expensive and some have a hard time setting them up. It relies on a spray bar with tiny holes so fuel filtration is key. The other, AeroCarb looks like a toy. You really can't do better than the stock Carb. They are dirt simple and reliable. Unless you are going to do sustained inverted flight, the crab will serve you well. The alternative to a carb is drop $3,500 plus on a fuel injection conversion.

[PCHunt]

Thanks for the detailed reply. I have visited their web site, both before and after your post. Called them, as well, and was pleased with their response.
In the end, I decided to buy some parts, including a 1-piece venturi, and put the carb back together, and see what happens.

[whifof100ll]

Originally Posted by PCHunt

Thanks for the detailed reply. I have visited their web site, both before and after your post. Called them, as well, and was pleased with their response.

In the end, I decided to buy some parts, including a 1-piece venturi, and put the carb back together, and see what happens.

Are you sure that the carb is the cause of the richness issues? How are you determining that it is too rich? Is it too rich only at idle or at higher throttle settings? Does the mixture knob fix it. Are you sure problem is with Carb. Any hot air induction possibilities? What impact does carb heat have? Have you checked timing?

Surprised you opted for a one peice venturi. One peice is what they ship on new rebuilds now, but the two peice venturi is the better running one. There was an AD a while back because of possible part ingestion, but I think it has been recinded on certified A/C's. With proper assembly, the two peice is fine.

One peice typically has hesitation at transition off the idle jet (1400-1500RPM). You can live with them, but it is there. I have a new carb same model as yours on an E2D and it has a one peice venturi, and the hesitation that goes along with it.

I agree with above post that the MA is dirt simple and typically reliable. Stay with the carb and sort the problem. It will give you years of trouble free service once you sort it.

FYI I don't think accell pump packing is the issue even if it is leaking.

Also, are you running the engine with the cowl on or off? I had a very perplexing problem once that turned out to be how airflow off the prop was impacting airflow into the carb with no airbox or cowl on a test run. Ran like sh*& without the airbox and fine with it.

Good luck,

Dale Lambert
RV-6 finishing kit
Ugly green C177 with E2D

[PCHunt]

Originally Posted by whifof100ll

Are you sure that the carb is the cause of the richness issues? No, not sure. Could be something else, such as the primer. But the carb has been on the plane for 10 years, and has sat outdoors without flying for the last 4 years, so I wanted to look inside anyway. How are you determining that it is too rich? Is it too rich only at idle or at higher throttle settings? Does the mixture knob fix it.
Ran it on the ground only, at 1500 RPM leaned about 1" on mixture, and RPM went up to 1800. At full throttle, it took about 1.5" back on the mixture to get max RPM. DA was about 3500'. Any hot air induction possibilities? What impact does carb heat have? Was running the engine with the cowl off, but the induction filter box on. Carb heat had minimal effect on the mixture.
Have you checked timing?
Not yet.

Surprised you opted for a one peice venturi. One peice is what they ship on new rebuilds now, but the two peice venturi is the better running one. There was an AD a while back because of possible part ingestion, but I think it has been recinded on certified A/C's. With proper assembly, the two peice is fine.
Still not sure I'll use the one piece venturi. The center piece of the two piece venturi comes out easily, appears to be held in by the clips, which are held in by the bowl gasket???

One peice typically has hesitation at transition off the idle jet (1400-1500RPM). You can live with them, but it is there. I have a new carb same model as yours on an E2D and it has a one peice venturi, and the hesitation that goes along with it.

I agree with above post that the MA is dirt simple and typically reliable. Stay with the carb and sort the problem. It will give you years of trouble free service once you sort it.

FYI I don't think accell pump packing is the issue even if it is leaking.
One of the problems that can cause rich running that was listed on the Precision Airmotive web site was "leaking accelerator pump shaft packing", and it was cheap, so I thought I'd replace it now.

Also, are you running the engine with the cowl on or off? I had a very perplexing problem once that turned out to be how airflow off the prop was impacting airflow into the carb with no airbox or cowl on a test run. Ran like sh*& without the airbox and fine with it.

Thanks for your input, it all helps. Anyone else that can help with the Venturi Decision??