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Idle cut off...engine still wants to run

F

FNG

Member
I have a RV-9A with a Lycoming O-320 carberated. When I cut the mixture, the fuel does not shut-off all the way, and the engine stumbles on at 200-400 rpm.

I have checked the control cable, and it is getting all the way to shutoff. I have disassembled the carburetor and and cleaned up the cut off valve rod inside the carburetor (this cured the problem on a C-172 I owned with the same symptom).

I have adjusted the idle mixture control with no affect.

The problem became worse when I installed a P-mag (I am sure that is because the P-Mag is better and igniting the mixture of fuel remaining..I still like the P-mag.)

I was thinking of trying a rebuilt carburetor. Marvel Schebuler MA4SPA.

Any suggestions would be appreciated. Has someone had a similar problem?

Thanks,

Tom

Dues paid.
 
Based on personal experience, I recommend that anyone with this type of problem stop flying the airplane. The reason being, that the same thing that can cause the valve to not close all the way can also cause it to not fully open when you move it to full rich which can cause problems on a takeoff.
 
There is a method for adjusting the rotation of that valve. You can find details in the overhaul manual for the MS carbs. I can't recall the procedure from memory, but believe it is just unclamp the tension from the arm, but must first identify a known closing point. I would guess it has crept a bit or something else in the linkage has changed and the valve is not fully closing. Another possibility is wear on the ID of the housing for that valve.

If you removed the spring shaft from the top to reseal that shaft, you probably didn't get it set right on reassembly.

Larry
 
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That's the problem

FNG said:
Thanks so much!

You may have fixed my problem... I have that bend,

Tom
Click to expand...

I've showed this to two other people that has had the same problem and replacing it fixed the problem. Don't forget to order new lock tab washers for the screws that hold the two half together and a new gasket, for reassembly.
 
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Wanted to read the solution... but the page will not load. Can you paste the relevant text to a reply here? Thanks!
 
My carb has started not shutting off completely when I pull the mixture back to cutoff, so I took it off and took it apart to see what was up. I found that the mixture metering valve was worm and had a bend that had set in the spring. Ordered new parts and will put it back together this weekend. Carb parts can be ordered directly from Marvel Schebler https://msacarbs.com/

You'll want to see the pictures also, go to my website and then My 2016 Flying Adventures page.
 
mystery worm

I have only found worms in my tequila. Never have seen one in the carb yet.
Maybe I could put one in the gascolator and see what happens.
(now seriously... Just had the carb rebuilt. A pro did the work. Mine still never quite quits on idle cutoff. I have been shutting the P mags down. Been that way several years.
 
I have installed the new mixture metering valve and the engine did shut off as it is supposed to do. However, I have not had a chance to test fly the aircraft yet, so I can?t be sure it is fixed. when the engine is up to full temperature it has been harder to shutoff.

The new valve was tighter fitting than the old one, so I am hopeful.

I won?t be able to fly for a few days, but I?ll post the results.

Thanks everyone for the replies.
 
I’ll start shutting down the P-mags if the new valve doesn’t work. Thanks for the reply, Imwas wondering if anybody was doing that.
 
FNG said:
I’ll start shutting down the P-mags if the new valve doesn’t work. Thanks for the reply, Imwas wondering if anybody was doing that.
Click to expand...

It is certainly an option. It is how your auto engine is shut down. However, a well built carburator should have no problems shutting down an engine with it's mixture valve. You can possibly accept the under performance, but I wouldn't. I would want to be sure that the problem does not also affect the valves ability to deliver full flow or was not a ticking bomb waiting to completely fail. The only reason that a carb would not affect a shut down is if the parts were worn/damaged or incorrect for the application, assembled incorrectly, or improperly adjusted. Regardless of how strong of a reputation the assembler had.

Keep in mind that P leads are notoriously unreliable and one of the reasons aviation engines still favor a fuel based shut down routine when mags are installed.

Larry
 
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Keep in mind that P leads are notoriously unreliable and one of the reasons aviation engines still favor a fuel based shut down routine when mags are installed.

Larry[/QUOTE]

So, if I shut off the mags, and the engine quits, I know the P leads are working. If I lean it, then all bets are off if someone moves the prop. (impulse equipped mag) On most planes I tend to alternate methods, just for data points that all is well. And, I really don't touch the prop unless preparing to hand start.
 
flightlogic said:
Keep in mind that P leads are notoriously unreliable and one of the reasons aviation engines still favor a fuel based shut down routine when mags are installed.

Larry

So, if I shut off the mags, and the engine quits, I know the P leads are working. If I lean it, then all bets are off if someone moves the prop. (impulse equipped mag) On most planes I tend to alternate methods, just for data points that all is well. And, I really don't touch the prop unless preparing to hand start.
Click to expand...

My warning with P leads was more related to a potential inability to shut the engine off in an emergency more than a hot prop.

Larry
 
If the new mixture valve doesn't fix it, you might want to check the accelerator pump check valve to see if it's allowing fuel to leak by.....
 
Thanks for the tip!

I replaced the mixture metering valve and the aircraft shuts down normally again..

I flew it twice and no more problem!
 
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