What’s causing popping noises at low RPM?

I have a high-wing homebuilt aircraft into which I have installed an IO-360-A1B 200-hp Lycoming salvaged from a Lake Amphiban, with 660 TT. My problem is that it makes loud popping noises at low RPM, such as when landing or taxiing. I have a cross-over type exhaust, and have had a mechanic check both the exhaust and the intake for leaks, but this is not the problem. Do you have any idea what else might cause this problem?

KEITH PIERSON

Hibbing, Minn.

From what you mentioned, it sounds as if the idle mixture may be running a little lean. I have heard engines do this in the past. As a matter of fact, if my memory serves me well, this was not unusual in some of the TCM IO-520 series and certainly caused no ill affects that I can recall. This popping could be heard on landing when the power was pulled back about the point of touchdown and sometimes even during taxiing the aircraft.

If you read my recent article about how to adjust idle mixture (How can I fine-tune idle mixture, June 22 issue, or online at GeneralAviationNews.com), you’ll remember I mentioned using a manifold pressure gauge to set the idle RPM while observing the tachometer. Just a quick review and maybe you’ll say “”hey, I think that may be worth checking out.””

You may not have a manifold pressure gauge on your installation, but I’ll bet you could round one up to run this check. What you’re looking for is about 650–700 RPM and about 10 to 11 inches of manifold pressure at normal engine operating temperature. My guess is that you’ll find a higher manifold pressure than that.

At this point all you need to do is enrich the idle mixture by turning the star wheel on the side of the fuel injector throttle body located on the opposite side from the idle mixture screw. If you look closely, you’ll see the letter “”R”” stamped on one of the blocks that the star wheel is located between. I’d suggest while you are running the engine and observing the tach and manifold gauge you have a maintenance technician adjust the star wheel a couple of clicks in the rich direction.

If the idle mixture was, in fact, on the lean side and causing the “”popping”” sound at low RPM, once the technician turns the star wheel towards rich, the manifold pressure should begin to come down. He may have to adjust the idle screw at this point to bring it to the range mentioned earlier.

Keith, I’d give this a try and hopefully it will correct your problem.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: [email protected].