Idle Stumble

Hello All -

I guess I have the "bug" to make everything perfect in my two months of owning this RV.

I have a -10, injected, one mechanical mag and one light speed.

Every flight is different. Different weather. Temperature. Altitude. Passengers...etc. So, I learn a little bit about the airplane with each flight.

On hot days, after a flight, while I am getting ready to shut down, the airplane will sometimes, but not always, stumble every 5-10 seconds at low RPM (1000 or maybe 1100).

This has happened twice. Once after a short fuel stop on a hot day, and once after a fairly long taxing to parking on a hot day.

What could this be? Fuel vaporization ? Failing mag ? Mag idle transition ?

I have an email in to LS to see if they can point me to a test.

At this point, I plan to switch between mags the next time it happens and see if the stumble changes/quits...if not, then perhaps mixture.

I tried the electric fuel pump last time, to see if that had an impact but it did not matter.

Ideas appreciated.

Scott

Scott, properly adjusting the idle mixture seems to help a lot with this.

Vic

vic syracuse said:

Scott, properly adjusting the idle mixture seems to help a lot with this.

Vic

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OK...good. I will try that. Pretty sure I was at full rich both times after landing...although my check list has a step for semi-leaning after landing, which I forgot to do.

istrumit said:

OK...good. I will try that. Pretty sure I was at full rich both times after landing...although my check list has a step for semi-leaning after landing, which I forgot to do.

Click to expand...

I think he meant to adjust the idle mixture on the FI servo. An RSA style servo has a toothed wheel for adjustment. There is an R with an arrow for Rich. You can find good instructions for doing this via search.

Larry

lr172 said:

I think he meant to adjust the idle mixture on the FI servo. An RSA style servo has a toothed wheel for adjustment. There is an R with an arrow for Rich. You can find good instructions for doing this via search.

Larry

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Ah...thank you very much for the clarification.

Exactly

Just got around to seeing your reply-- and yes, I was referring to the mixture adjustment on the servo. Be careful when doing it if you leave the engine running. It is safer, but takes longer, to start and stop it after each adjustment, but be sure to run it long enough to clear it.
Also, you mentioned you were taxiing with mixture full rich. Sounds like you just forgot to lean it from your comments. Good habits will come with time.

Vic

Idle mixtures are almost always "sloppy rich" and misses at idle or plug fowling are common symptoms. It's a bit of a pain to set, but once you do, it will start like a Harley from dead-idle (700rpm), be less prone to fowling plugs and run much nicer especially accelerating off idle into mid-range. That's actually part of how you set it. It's hard to set on a running engine, but if you have a brave friend, it can be done. The safest thing is to shut the engine down between tests.

Test procedure:
Let it idle for 10-20 seconds. If the RPM drops after 3 seconds, lean more... if it holds, goose it up to 1700, if it coughs, lean more. If it just jumps up, you're done.

You mentioned leaning after landing. As a principal, I prefer (encourage) taxi at an excessively lean mixture setting regardless. Don't half-*ss it, you want to set the mixture so low it stumbles and quits running above 1700rpm. That way you don't forget and take-off with a "just kinda low" fuel mixture that causes partial power and/or overheats the heads. Just one of those good habits to form.

Thanks Gents...Thats good advice...also, yes, I will start setting taxi lean to very low...so it will stumble on throttle up if I forget it (which I don't, but.....)

Based on when you get the stumble my money is on vapor lock.
G

istrumit said:

Hello All -

On hot days, after a flight, while I am getting ready to shut down, the airplane will sometimes, but not always, stumble every 5-10 seconds at low RPM (1000 or maybe 1100).

This has happened twice. Once after a short fuel stop on a hot day, and once after a fairly long taxing to parking on a hot day.

Scott

Click to expand...

I finally solved this problem using the replacement 4psi flow divider spring per Lycoming SI-1539. I bought mine from AFP.

This spring increases pressure in the FI lines enough to substantially reduce vaporization-based stumble.

Tim Lewis said:

I finally solved this problem using the replacement 4psi flow divider spring per Lycoming SI-1539. I bought mine from AFP.

This spring increases pressure in the FI lines enough to substantially reduce vaporization-based stumble.

Click to expand...

Thanks for sharing the outcome Tim. Very timely for me. I have been struggling mightily with this issue since installing my FI setup a month or so ago. Mine is rough immediately after turn off on the taxi way on a hot day. I was just doing some research on whether or not to install the 4lb spring (I already have it). It's going in tomorrow now and will hope for similar results.

Larry

lr172 said:

Thanks for sharing the outcome Tim. Very timely for me. I have been struggling mightily with this issue since installing my FI setup a month or so ago. Mine is rough immediately after turn off on the taxi way on a hot day. I was just doing some research on whether or not to install the 4lb spring (I already have it). It's going in tomorrow now and will hope for similar results.

Larry

Click to expand...

FWIW - I returned from a long trip on a hot day yesterday - prime candidate for the behavior I have been asking about.

But, this time, I remembered to lean it out during taxi back to the hangar.

Leaned out, it did not stumble. I experimented with full rich just before I shut down and it did stumble.