Popping / backfiring at low power settings

[brianm]

I've got a C-145, about 1200 SMOH. It likes to pop near idle, on the ground and in the air. The primer is good, I just replaced it because I couldn't get the old one to stop leaking down the plunger. The line from the primer to the cylinder was also checked and isn't leaking. I visually inspected the manifolds and didn't notice anything amiss. No cold cylinders. Pulled the prop through and didn't find any "easy" blades. At idle when I back off the mixture from full rich I get the expected 50-ish RPM rise before it drops off and dies. Runup, mag checks, and carb heat checks are as expected. Static RPM is about 2250 (7655 cruise prop). RPM is stable, as is manifold pressure. Makes plenty of power. The only unusual thing that has happened since the purchase (where it ran fine, of course) is for the trip home it was just around freezing and I didn't have a preheat available. The combination of cold and my not knowing the cold starting tricks for this engine caused me to take about 5 tries to get it fired up.

My next steps are going to be checking the spark plugs and maybe try running it without the air filter. Beyond that I think I'll have to call in the mechanic. Any ideas about what I might be missing?

[canav8]

Idle mixture circuit. Does it run fine at all other power settings? pull the mixture screw and look for FOD.

[brianm]

It runs fine taxiing around at 1000-1200 rpm, it's fine in cruise at 2450, and it's good in the pattern at 1800 rpm. The popping tends to happen on final when I have the power all the way back. It also happens on the ground below 1000 rpm, usually pretty near idle. I also suspect the idle mixture, but I see the expected rise and drop when I lean the mixture at idle. One thing I noticed yesterday was that it was fine on the ground after the initial startup and after run-up. It was a cold day, about 25F. After a couple of trips around the pattern (getting popping on final) I also started getting it at idle on the ground. This weekend I'll drain the carb and check for crud and maybe play around with the idle mixture setting. Do you mean that I should also take the idle mixture screw all of the way out and check it?

[DWood]

Intake air leak(s) can cause popping.
Might also search the forum as I believe it has been discussed before

[GAHorn]

Most popping at idle is due to a rich mixture. Unless it's excessive I wouldn't be concerned with it. Next time it's popping...try leaning it out and see if it goes away.