Backfiring

johnee26@hotmail.com · #1 06-19-2009, 11:38 PM

We acquired our RV7 a few months ago. Flies like a dream, engine v smooth (Superior XP-10-360-B1 B2 with VP prop ) but 'backfires' badly when idle selected in the air. We operate with CHT about 1350 by leaning the mixture, but the backfiring exists whatever position the mixture is in. Any ideas?

PCHunt · #2 06-20-2009, 12:19 AM

Assuming you mean EGT 1350, and not CHT. Looks like a typo

You might want to do a "search" on this forum for backfires and "popping" when the throttle is closed. Seems like it is a common occurrence, and may be normal. The higher HP engines seem to be more affected.

My little carbureted 150HP monster doesn't seem to have the problem.

frankh · #3 06-20-2009, 12:47 AM

its not backfiring..its popping in the exhaust ans its perfectly normal..If a little embarrassing..

Enjoy and don't close the throttle all the way ob final or the neighbours will start getting concerned.

Frank
7a ex Pat living in Oregon

Scott Hersha · #4 06-20-2009, 06:22 AM

You might want to make sure your intake tubes are tight. On my RV-6, I had a couple that weren't completely tight and that made a little difference. It still popped though, but not as bad. I used to think it was a carbureted version only thing, but recently, my friend - Joker - with a fuel injected O-360 was popping louldly on downwind at idle. My new RV-8 also pops at idle (carburetor). I think they all do. If I stay off the idle stop, it doesn't do it. I don't think if hurts anything - except noise abatement / 'neighborliness'.

Scott
RV-8 FB

67MK Larry · #5 06-20-2009, 08:27 AM

It is perfectly normal, the reason you dont hear it in spam cans is the mufflers hide the pops. Listen to spam cans that had augmenter tubes and the pop is much louder. I use the backfiring, "pops" as the point to stop pulling back the power on decents, so as to keep shock cooling to a minimum. just my two cents. Larry

johnee26@hotmail.com · #6 06-21-2009, 10:40 AM

Thank you everyone, you have all been most helpful....great forum

Captain Avgas · #7 06-21-2009, 06:55 PM

Quote:

Originally Posted by johnee26@hotmail.com

We acquired our RV7 a few months ago. Flies like a dream, engine v smooth (Superior XP-10-360-B1 B2 with VP prop ) but 'backfires' badly when idle selected in the air. We operate with CHT about 1350 by leaning the mixture, but the backfiring exists whatever position the mixture is in. Any ideas?

Excess popping in the exhaust may be a sign that you have minor (or major!!!) leakage in either the induction or exhaust system. It has been reported that even leakage at the EGT bayonets can exacerbate exhaust popping. Check everything carefully.

The other major cause of popping is poor idle mix setting. Often the idle mix needs to be re-adjusted after an engine is run in. A significant increase in RPM at idle when you lean the engine right out with the mix control is evidence that the idle mix setting is too rich. No increase is a sign that the idle mix is too lean. About a 50 RPM increase is optimum.

There are many well maintained and tuned RV engines out there that do not pop, or do not pop excessively.

dhammer · #8 06-22-2009, 02:58 AM

Quote:

Originally Posted by Captain Avgas

About a 50 degree increase is optimum.

Shouldn't that be 50 RPM increase?

Don

Captain Avgas · #9 06-25-2009, 08:26 AM

In 2007 Larry Vetterman who manufactures all of Vans RV exhaust systems (and is regarded as something of an exhaust guru) tested three different exhaust systems for relative performance on his own RV7A. He tested a four-into-one system, a standard RV crossover system, and a four pipe system. None had mufflers. The engine was a Lycoming IO360 M1B.

I will quote verbatim from a part of Larry Vetterman's report:

"One test that I also conducted was to bring the throttle to idle during approach and landing to see if any of the systems would backfire and pop. I could not get any of the systems to do it as the fuel injection system (idle mixture) is set properly and there are no induction leaks on this engine".

I conclude from this that Larry Vetterman does not believe that exhaust popping is inevitable or "normal" (with or without a muffler) on a well maintained and tuned Lycoming engine.

Radomir · #10 06-25-2009, 09:00 AM

Like a lot of RVs (but not all, I guess) mine now pops a little.. it started popping after I put the C/S prop on it.. it never popped w/ FP prop.. no other changes were made to the plane.. I'd doubt that changing the prop introduced intake leaks etc.. etc..

I guess it all means that YMMV

I'm not losing any sleep over it..