Carb stumble

[Ron Babos]

I have this C-85 driving me nuts. Had problem with stumble at 1400 rpm where it hangs and won't accelerate smoothly past it. Changed to single piece venturi, new pepper box atomizer, new shaft bushing and went through the rest of the carb and it does exactly the same thing. It's on a PA-11. No intake leaks were found. It will transition smoothly with the carb heat on, which is telling me it's slightly lean. With carb heat off, using mixture it will gain slightly on idle and high rpm. The problem area is just in the starting area of 1400 -1700. Any unusual things that I might look at? Mags are just redone and retimed which made no difference. Last thing I noticed is the bracket element is dry to the touch, and the carb heat flap is not going completely to full cold. Any ideas are welcome.
Ron

 

[citabrickr]

Gonna have to take it completely apart and clean all jets, orifices and passageways, you still have a clog somewhere. Had the same problem with an A65 and thought I had done everything too, till I did it again. :roll:

 

[two-12sfernow]

I have to agree with Citabrickr, on another PA-11 w/C-85, I had same problem and tried about everything except a new tail wheel tire to avoid what I new needed to be done. I got lucky and cleaned it out with an aerosol, Chemtool I think. Still have to tear it down though.

 

[Ron Babos]

I wish it was that easy. When it was apart the last time I really went through all jets and holes in the body. The kicker is that even with a change from 2 piece to 1 piece venturi and removing the 0-200 atomizer that was in there and putting in the new pepperbox for the c-85 it runs exactly the same.
On the carb heat box of this plane there is no draft tube at the bottom to such excess heat out when not needed. The slanted cowl piece on the bottom of the heat box has a hole that lines up to the hole in the carb box. Could the prop blast be drafting the carb somehow to cause this condition. As we know, it's fairly common for this carb engine config to exhibit a slight lean condition anyway but could this hole exagerate it?
I'm probably the third guy to work on this carb , with the first one a certified engine builder. The second one put the 0200 atomizier in it, I guess to try and fix it.
Ron

 

[bob turner]

Since I have never seen a 2-piece venturi in a small Stromberg, I take it you have a Marvel?

You sure you found all the fuel and air passages? Your float needle and float work smoothly? If Stromberg, look very carefully at the slot in the needle for roughness.

 

[Ron Babos]

Carb Stumble

bob turner: Yup Marvel, not Stromberg . Carb had been rebuilt when the engine was done, but I also went through the whole thing with jets, bleed hole size, and checked all passage ways. It is pristine in there.
Ron

 

[Dennis Blankenbaker]

I had a problem a lot like what you have that was caused by a leak in the gasket at the elbow where it attaches to the clyinder.

 

[JimC]

It sounds like an induction leak to me as well.
JimC

 

[OVEREASYGUY]

Carb Stumble

I had that same problem - I put some big 31 inch bush wheels on my airplane and it went away.
so order up those bushwheels right away and all your problems will go away.
cliff in Maine.

 

[superchamp]

Did this start before or after the new 1 piece venturi? on my lycoming o-235, the carb had an AD to go from the 2 piece to the 1 piece. Put in the new 1 piece and ran like crap-wouldn't turn full RPM. Went back to the 2 piece and ran great. Turns out a lot of the 1 piece venturi's had machining issues which screwed up air/fuel flow.

 

[Ron Babos]

carb stumble

superchamp: Runs exactly the same with two piece or one piece. I will do a complete recheck on the induction system. Kicker is it runs so smooth at idle. Mind you since this condition of lean in the 1400 rpm range exists on most of them to some extent a small leak may send it over the edge and make it extreme.
Ron

 

[PA-11 HORSEMAN]

Ron
What are you using for gas?
My PA-11 with a C-85 does the same thing with straight Auto Gas, but clears up using 100LL, I have been using a 75/25 blend of 100LL/Auto Gas and get way better response. Might be worth a try if you are using Auto gas only.

 

[Ron Babos]

carb stumble

He's using 100LL but unless the float is set way too low which it isn't you really should not be able to tell what gas is in there. Seem to me I read somewhere that fuel level will be higher at the same float setting with 100. Something to do with specific gravity or something.
Ron