Engine quitting when I move throttle to idle on landing

[Aviator13]

1974 SuperCub with 150hp. Had the carb and both mags rebuilt a year ago. Replaced ignition harness and all the plugs about a year ago. Throttle cable was replaced with new about 18 months ago. About a month ago, went clamming and upon moving the throttle to idle on landing, the engine quit. My mechanic said has to be idle adjustment, he checked my throttle cable and it was clamped correctly. Adjusted it so the engine would idle when the throttle is all the way aft. Now, I am sheep hunting/landing at 6400' and it quits at idle on landing again. I was leaned correctly.

What should the idle RPM be at full aft throttle sea level during my eng runup checks? Can that spring under the idle adjustment screw get weak and allow vibrations to move idle screw?

I think I am gonna mark the idle screw with slippage paint and see if it moves.

Any other suggestions would be GREAT.

Thanks in advance.

 

[Fuel]

Look here for some info.

http://www.kellyaerospace.com/articles/Accessory_AMT.pdf

http://www.sacskyranch.com/eng135.htm

 

[jgerard]

I see you're in alaska... Maybe it's those pesky mag switches they are thinking about putting an AD out on. I read here there seems to be an issue up there with them

If you switch between av gas and auto fuel the carb bowl gasket swells and shrinks and you may have an air leak into the float bowl of your carb. Could need to adjust the float level on the carb too. What about carb ice? Just throwing ideas out


Jason

 

[Steve Pierce]

Fuel posted good articles on the carb and idle mixture adjustment. I would check that first.

 

[aktango58]

Did you get a sheep?

 

[cubamigo]

If primer is not locked, fuel being sucked in cylinders. Rich mixture could make engine run rough and possibly quit at idle.

 

[T.J.]

Sounds like one of the little fuel holes in the carb body is plugged. As you close the throttle, the butterfly closes the holes individually. I think there is three of them. ( I could be wrong about that.) If the last little hole is plugged, no fuel, engine quits.
I found a skeeter carcass in one once. Idle is usually 5 to 7 hundred, depending on how the engine runs. The Lycoming engine manual will have the correct idle RPM.
The carb doesn't need to be overhauled, it just needs to be cleaned.
Good luck.

 

[Bob Breeden]

You mention it quits at idle at 6,400. Does it quit back at sea level? At 6400 feet there is little atmospheric pressure pushing past the plate to put oxygen to the engine. This is as if the butterfly were closed further......same thing happens to me when on the ground at idle at higher altitudes if the idle rpm is set low to start with at sea level.
Bob

 

[Aviator13]

Thank you everyone. What is strange is back a year ago when the carb was rebuilt, the idle was set correctly. I fly all winter, spring bear hunting, and idle is still fine and then a month ago it quits at sea level during landing and needs adjusting. Adjust the idle per the instructions (www.sacskyranch.com/eng135.htm) and then it quits a month later at idle upon landing at 6400'? Idle is at 300-350 last night (sea level) after a good engine warm up, why it is so low again, I don't know. Set it at 750RPM and after 5 landings last night it is still at 750 and did not quit on landings. HMMM, what else is going on there that is moving the idle adjustment? No apparent blockage in the 3 holes TJ, thanks. Bob, I understand the altitude/less pressure effect on carb, that is why I like 750RPM cause I land high alot. Thanks though. Aktango58, I should have not mentioned sheep, all that sheep stuff in AK is top secret you know, ha.

Has anyone had that idle adjustment screw move that they know of? Does that spring get weak? I marked it with slippage paint so we shall see.

 

[Aviator13]

Cubamigo, I always am anal about ensuring the primer is locked after every prime and during before landing ck, thanks.


Jgerard, what pesky mag switches are you referring to? I have not heard about that?

 

[jgerard]

Jgerard, what pesky mag switches are you referring to? I have not heard about that?

Im just teasing about the mag switch I know it's not related

http://www.supercub.org/forum/showt...AD-Coming-for-PA-18s-FAA-Looking-for-comments