Backfire and burble trouble.

[AlexPeterson]

Originally Posted by Gary 40274

Excessive lean mixture at idle can cause popping on the ground. To check for proper mixture slowly lean the mixture at idle and you should get a 50 to 75 RPM rise just before idle cut off. If you don't I suspect you are running lean and need to enrichen the idle setting. Is this Carb or injected?

Gary Specketer

I agree that it probably can be minimized (but will still do it on rapid closing of the throttle) by messing with idle mixture. The AFP can be a bit finicky on idle mixture, and may even need to be changed winter to summer.

When I do simulated engine out landings, I go full rpm, mixture pulled all the way. I keep my hand on the mixture, and periodically during the glide run it in and clear the engine. If I just pull the throttle to idle, it will after fire a lot on the glide. When at 100', I push mixture in and move to the throttle.

[Don at Airflow]

TOAD:

Try this for setting idle mixture. 50-75 RPM rise is too much, but I have had a lot of customers come in here with the mixture set way too lean, plus then they lean the mixture on the ground with the mixture control. If you have a manifold pressure gauge we use this for setting the idle mixture. First run the engine up on the ground to get the oil temp up to operating temperature, then throttle back to idle. At 750 RPM, slowly pull the mixture control and watch the MAP gauge. If it starts to go up while leaning the mixture the mixture is too lean. If the MAP goes down while leaning the mixture is too rich. Usually we set the idle mixture so that the MAP only goes down none at all or maybe only 0.1? or 0.2?. In any case you good throttle response off idle (no hesitation on the acell). If you get the mixture set to the lowest MAP and there?s a little hesitation on a brisk throttle advance, richen the mixture 1 flat. Then you set. On most 4 cylinder Lycoming?s in good health the MAP at 750 RPM is around 9.5? to 10? Hg and the idle flow is around 1.2 to 1.5 GPH.

Obviously make sure you don?t have a manifold leak (intake pipe connections, manifold drain sticking, etc.). Always run the engine up (1800 to 2000) for a bit to clear out the engine between mixture adjustments. We always set the mixture with the engine running, but DON?T DO THIS UNLESS YOU KNOW WHAT YOUR DOING AROUND SPINNING PROPELLERS.

If you still have issues with the idle performance call us directly. We can walk you through the procedure again. Or bring the plane here and we?ll set it for you.

Don

[TOAD]

Well, Thanks Don I am very grateful for the instructions on Airflow performance setting the idle mixture. I always fool with this stuff when I have instructions
The invitation to fly from Seattle to Florida to have you set the mixture for me is most appealing .

I trust. Your response was a little unexpected and GREATLY appreciated. I am thinking that the only thing I may need would be a gasket seal if I get that far, or there looks to be an induction problem. As I say I had been focusing on the exhaust and still think that there is too much leakage around the slip joints but I am broadening my search.

I need to get some pictures posted here soon. I have the plane painted up and my son is doing some nose art for me. The plane is looking awesome.

[TOAD]

Saturday I set the mixture a little richer (about a flat and a half). This did help a lot. It is now set so that I get zero increase in RPMs as I lean it out. I will try it just a little richer tonight. I am still popping but it is not near as bad as it had been. I had an explanation comparing the lean idle setting... to the pop you get as you turn the gas off on a torch. I get it now. I think I truly understand the mixture setting for the first time. I am excited to think that I can finally mitigate this popping to a reasonable level for me. Wish me luck, and the kid is painting the nose art as I write this....