Why did it do that?

Steve Brown · #1 05-17-2008, 10:47 PM

Today at SCK I was trying to kill the motor in the usual way with the mixture. The motor kept running, although roughly.

I enriched it and it ran smoothly again. Then I really pulled the mixture firmly, plus screwed it out.

It reluctantly ceased operation.

This has never happened before. When I killed the motor at PAO when I got home, everything was normal.

It was very hot at Stockton. Very cool Palo alto by the time I got these.

I mention the heat because I suspect it has something to do with it, but I don't have a clue what.

szicree · #2 05-17-2008, 10:52 PM

I'm going with the simple stuff. I think perhaps your mixture cable is not allowing the arm on the carb to go all the way to the stop. I'm sure that close to the stop is enough to shut things off most of the time, but perhaps the heat made enough of a difference to prevent shut down this time.

Geico266 · #3 05-18-2008, 05:00 AM

On my old RV-3 O-235 I had to replace the MA-3A carb due to it being "worn" out". The mixture control did not funtion all the way in the lean position due to internal wear. This was allowing fuel to leak past the mixture control I guess. I just replaced it with a rebuilt one. Works great now.

videobobk · #4 05-18-2008, 06:35 AM

If I am at a higher than normal density altitude, I have to add some to the rpm when I pull mixture or I have that problem. You might try that next time. That would be an easy fix!

Bob Kelly

Steve Brown · #5 05-18-2008, 07:13 AM

Thanks guys!

Higher RPM next flight. Check the cable/arm travel next oil change. Hope those work so I don't need to replace the carb.

High density altitude may be the problem. Even though the airport was virtually sea level it was wicked hot. Plus one thing I forgot to mention is that I was taxiing around for a while with the carb heat on. I may have even left it on when I went to kill the motor. My overcooked brain cells can't remember if I noticed the carb heat on before or after I pulled the mixture.

Noel Simmons · #6 05-18-2008, 08:30 AM

I just replaced the carburettor on the RV-9A. along with a loose throttle shaft and worn out internal parts, I think the accelerator pump was leaking by too. After I did this the engine runs much better. I should have done this much sooner.

osxuser · #7 05-18-2008, 10:07 AM

Classic symptom of not having a full cut-off of fuel. In normal conditions (Denser air) the mixture gets lean enough to be incombustible anyway. But with the high temps (Or altitudes) comes lower density of air, so the mixture is richer, and remains in the combustible range.

Adding RPM should do the trick, if you are hitting the ICO stop and it still does it, look into the carb.

Kyle Boatright · #8 05-18-2008, 10:44 AM

I had a similar problem a few years ago. A new mixture shaft in the carb (about $100) and everything was fine...

dhammer · #9 05-19-2008, 04:36 AM

The mixture of an aircraft carb is changed by varying the air pressure applied to the float chamber. It is a balance between the air pressure at the venturi and inside the float bowl. That's why the symptoms are different with different density altitudes. If there are any internal air or vacuum leaks you may be unable to get the mixture lean enough to shut it down under some or all conditions. If that is the case, advancing the throttle at cutoff will kill it, but doesn't solve the underlying problem.

Check and make sure the mixture arm is hitting its stop. If so, you have the classic signs of a worn carb. How long since it's been overhauled by an approved shop? That might indicate what you need to do; repair or overhaul.

Don