Occasional "engine popping" noise

[jackshulerdds]

When I bought my Champ 7AC last year it had a Stromberg Carb in the 65hp engine. The engine has 300hrs smoh and only a few hours since a TOH. The airplane had been totally rebuilt in l996. and the idea was that I wanted to try not to have any major expenses during the first few years of my retirement. With that in mind, and with the fact that she would occasionally "cough" whenever I had to apply sudden throttle, I had a rebuilt Marvel-Schrieber carb installed. EVER SINCE, the engine wants to "pop (a backfire sound) at low throttle settings.(Up to about 1300 RPM) It NEVER does this cruise. It is most noticable at taxi or on base /final. When I power back below 1300 on final, I get the occasional pop. I am tired of having nearby buddies tell me that I might have something wrong with the engine. My mechanic has heard it, but says that Marvel Schrieber carbs have TWO jets, one for low RPM and one for high RPM. If you get "between them", you get the engine pop.
Has anyone else had this happen. Like I say, it never happened until I got the rebuilt carb. I , by the way, am living proof that a DENTIST doesn't necessarily have to have mechanical ability.

Thanks to a great site.
Doc

[Ron Babos]

Doc: It's mixture related and would not worry about the engine if it runs normally otherwise. Sound like you have the classic lean popping conditon. Normally all you need to do is open the idle mixture screw about 1/2 to 1 turn (counter clock wise), after the engine has warmed up to operating temp. Too much and the engine will run rough and lumpy at idle.The engine rpm may drop slightly and the idle speed screw may have to be turned in to set the idle rpm back to where you like it. If none of this pans out, go looking for intake leaks which could be making it leaner than normal. The popping sound is caused from fuel entering the exhaust from the lean mixture that will not burn completely. There is an excess of raw fuel in the pipe and the next cylinder in that bank will light it up with it's exhaust flame. Esentually , it's a wimpy back fire.
Ron

[Paul Agaliotis]

Check your exhaust gaskets and mufflers for cracking. This will cause the engine to do what you are describing.
Paul

[jackshulerdds]

I thank those of you who replied to my "occasional poppin noise" of a year ago. To put an end (I hope) to this, I thought I would let you know what it was.

At annual, they found

1. A crack in number one cylinder.

2. An incorrect gasket placed on number three cylinder (used an 85hp gasket on a 65hp engine) when the cylinder was rebuilt.

3. A valve which was not fully seating. Also this cylinder (number three) required a new piston.

Runs great now, although I must save up some money in my "Aeronca flying fund" before I can fly it much.

again, thanks
doc

[Ron Babos]

A few hours since top and 300 since major and you had that much wrong with it? I'd be hunting down the rebuilder to cough up some cash. What happend to the piston that it had to be changed ?
Ron

[joea]

Ron,

Take another look at his first post. 300 hours since major and "A FEW SINCE TOP"...

I would really be talking with whoever did the last work on the engine!

Joe A

[MikeB]

I'm guessing that the intake gasket was leaking on number 3 and causing it to run lean to the point where it burned the piston and warped the valve. However, I agree with the others, that this should not happen with only a few hours (even 300 since major). As I've posted on this site before, I bought a engine that was supposedly "rebuilt to zero time specs" and looked good in the log book. Among other things, there were numerous oil leaks at the push tubes (clamps on wrong) and wrong size piston rings installed. Also, a rebuilt carbureator that had a large 'flat spot' in it. When I took it apart it looked as though it had never been cleaned internally. This by an AI, by the way. I have the problems sorted out now and have a couple hundred hours on the engine, but it does get you wondering about everything else.

Problems as you described should not happen, nor should the engine require a top overhaul if only 300 since major overhaul. One thing I have noticed is that if the cylinders are reused (honed, etc.) not enough attention is paid to the condition of the valve guides. With any time, most are pretty worn which will, of course, shorten valve life.

Mike

[Ron Babos]

joea: That's what happens when you read from right to left Same thing , just backwards.
Ron

[joea]

Ron,

After a margarita or three I can read from either direction!

joe