Balky carb throttle

[Paul Eastham]

With the onset of winter temps (all of 40 degrees in california ) I noticed my throttle control was feeling a bit less smooth. Then one cold morning it was hard to move at all. It did loosen up after a few strokes...I figured it was the control cable giving up on me, as many others on this forum have encountered, and ordered a new one.

Once I had the new part in hand, I disconnected the cable, and found that it was actually the carb itself that was causing the stiffness. Hmm, lubrication? I sprayed two tiny squirts of LPS-2 on either side of the carb's throttle shaft (the shaft that connects to the control horn), and it is moving smoothly again.
Reassembled and everything feels as good as new.

Never heard of anyone else having to do this...Cause for concern, or normal?

 

[MrNomad]

Please address this soon

If you freed up the carb by simply lubricating the shaft, it sounds like the throttle bushings are shot.

I would not delay addressing this problem. It's unlikely it will gall and stick in one position but what is more likely the cable will croak, or the cable secure will release, and/or it will leak at the shaft causing a lean condition as air enters along the worn shaft.

Either way, none of these are happy events @ 8,500'.

My car nut friends and I typically rebuild old carbs for our classic Chevys. Here is a picture of the tools, replacement bushings, and grinder we use in the drill press. While I haven't done an airplane carb, I assume the process is the same.

Call Ted at 405- 413-4972 & send the carb to him for repairs.

 

[Low Pass]

As mentioned above, you got problems. I personally wouldn't recommend flying again until you resolve the binding/resistance. Assuming you're using a Marvel-Shebler type, they're generally similar to the old auto carbs but simpler. I've rebuilt several of them - not terribly difficult. Just do your research and use good parts. The carb is a single point of failure item, or worse (it fails, you land. It leaks you burn).

 

[MrNomad]

Parts source request.....

As mentioned above, you got problems. I personally wouldn't recommend flying again until you resolve the binding/resistance. Assuming you're using a Marvel-Shebler type, they're generally similar to the old auto carbs but simpler. I've rebuilt several of them - not terribly difficult. Just do your research and use good parts. The carb is a single point of failure item, or worse (it fails, you land. It leaks you burn).

If you have a source/description for the shaft bushings, please send it to

barryd57 @ gmail.com

Sooner or later I'm gonna need the parts and I'd just as soon have them on hand.

Thanks.
Barry

 

[Low Pass]

If you have a source/description for the shaft bushings, please send it to

barryd57 @ gmail.com

Sooner or later I'm gonna need the parts and I'd just as soon have them on hand.

Thanks.
Barry

I've had good luck the last 5-6+ yrs with Chief Aircraft. Bought a used bowl once from EnParts in Lancaster, Texas.

 

[Paul Eastham]

Thanks for the advice. This is a stock MA-4SPA on a O-320 with only 200 hours on it. Are there bearings on the shaft in that model?

Another thought is maybe the accelerator pump is what's binding. Some oil probably dripped down onto the top of the pump when I sprayed it.

Looks like a swapped-out overhauled carb is about $750 at AC$, would it be significantly cheaper to have it repaired?

 

[Low Pass]

Thanks for the advice. This is a stock MA-4SPA on a O-320 with only 200 hours on it. Are there bearings on the shaft in that model?

Another thought is maybe the accelerator pump is what's binding. Some oil probably dripped down onto the top of the pump when I sprayed it.

Looks like a swapped-out overhauled carb is about $750 at AC$, would it be significantly cheaper to have it repaired?

To answer that question depends on your ability/desire to do the work yourself, availability of funds and the quality of the overhauled carb. I recall hearing concerns about the quality of rebuilt carbs on the street over the last few years, but I have very little direct evidence. Maybe someone will offer up their experience with rebuilt carbs and their sources.

 

[Paul Eastham]

I have no experience working on carbs so I'd rather not do this job myself. I have heard that my local shop also does not want to work on carbs.

 

[Sam Buchanan]

Thanks for the advice. This is a stock MA-4SPA on a O-320 with only 200 hours on it. Are there bearings on the shaft in that model?

Another thought is maybe the accelerator pump is what's binding. Some oil probably dripped down onto the top of the pump when I sprayed it.

Looks like a swapped-out overhauled carb is about $750 at AC$, would it be significantly cheaper to have it repaired?

Paul, in the process of clearing up an intermittent power loss on my O-320, I recently installed a new fuel pump and Aircraft Spruce overhauled MA-4SPA. The carb is rebuilt by Kelly Aerospace in Montgomery, AL and looks like a brand new carb. I considered rebuilding my carb myself, but after pricing out a new float set, throttle shaft, and all the other stuff that gets replaced in a good rebuild, I was over half the cost of the Kelly overhaul.

So far, the new carb runs nicely--it seems a rough spot at 1800 rpm with the old carb is now gone. It gives me a warm fuzzy feeling to have the yellow tag in the engine logs.

 

[LarryT]

Shop required to have current manual...

and the carb manufacturer has raised the price of the manual into the Stratosphere per one of my local repair shops. They just don't do enough work to justify the cost of the manual. I'm going from memory but seems like I was told that the manual price was in the range of $700 - $900! Great stategy to discourage competition.

Marvel-Schebler carburetors were originally used on John Deere tractors.

LarryT

S

I have no experience working on carbs so I'd rather not do this job myself. I have heard that my local shop also does not want to work on carbs.

 

[Leland]

Excessively stiff throttle control

In the cold weather, my throttle was very hard to push in but loosened once the plane took off. Disconnecting the throttle showed there to be no friction in the cable but the carb lever was hard to push for open throttle. We assumed the problem was the pump leather, and bought a new one from ACS for $150, but removing the old pump showed that it was fine. My mechanic found that the check valve was frozen, probably with fuel varnish. Cleaning the valve freed up the throttle control but I have not yet reinstalled the carb. The thing I don't inderstand is why the control loosened up in flight.
Leland
RV9A
430 hours on a new O320 from Lycoming

 

[Paul Eastham]

In the cold weather, my throttle was very hard to push in but loosened once the plane took off. Disconnecting the throttle showed there to be no friction in the cable but the carb lever was hard to push for open throttle. We assumed the problem was the pump leather, and bought a new one from ACS for $150, but removing the old pump showed that it was fine. My mechanic found that the check valve was frozen, probably with fuel varnish. Cleaning the valve freed up the throttle control but I have not yet reinstalled the carb. The thing I don't inderstand is why the control loosened up in flight.
Leland
RV9A
430 hours on a new O320 from Lycoming

That sounds like a match to my symptoms. Not sure how the stuck check valve would hamper movement of the throttle though...the valve normally prevents flow from the pump back into the fuel line, I assume?

At any rate, I found some other areas of concern on the carb so I have a replacement unit ready to install.