Popping/after fire/backfire
 

I have a Lycoming IO360 a3b6d that has a bad backfire problem at idle. The engine has over 500 hrs since overhaul and has been doing this since being rebuilt. It is a new engine and airplane to me so I am trying to fix it. It has an airflow performance injection system, dual electronic EFii ignition and constant speed prop. It seems to be worse after flying, but if I lean it out it does not seem to pop as bad or as loud. I have looked and the intake system and have found no leaks. I was thinking maybe a sticky valve but compressions are in the high 70's and CHT's are in the green across the board. I have read on this forum before that RV's have a tendency to do that due to the exhaust system. I was not to sure about that. Thanks for any advice

Under what condition are you getting the popping? Is this a coming to idle in the air? If you come slightly off the stop does it go away?

If you?re just sitting on the ground idling then that?s not right, if you?re pulling it to idle on downwind then that might be normal.

It will pop in the air when pulling to idle especially if you pull it too fast which I try not to do. At idle taxing around it will pop but only after it warms up. It seems to pop more after flying and hardly ever when I am taxing out for takeoff when the engine is still warming up. The engine operates perfect at all other RPM's and never misses a beat. As I stated before the engine builder told me it has done this since the first start.

It does go away if you come off the stop. It is intermittent and there is no pattern in the popping except at idle all the way to the stop or pulling to idle in the air.

I have pretty close to the same symptoms with my O-320. Keep the rpms up after start up and all is well, no pow's, but that's not the way it should operate. Engine runs great otherwise, starts easy hot or cold, no hesitations, plenty of power. I've checked and rechecked everything that I know to look for, intake leaks, timing, idle speed, etc., and I think I've pretty much narrowed it down to the carb idle mixture needle not allowing a rich enough idle mixture. I've backed it out as much as possible, and it acts much better, but it's still not as it should be. One thing about it though, the gunshot explosions do tend to move the deer away from the runway when you're taxiing out. I've been watching this discussion hoping to learn if there is a fix, or if there's something I'm missing.

I believe you may be going thru exactly what I went thru with my new RV7 with a IO360. I test ran the engine and after it got hot, it would pop and backfire some at low RPM's and Idle. This drove me crazy! I thought my plugs were fouling and wound up buying fine wire plugs :eek:! Still didn't cure it! Then I started noticing, it never happened when the engine was cold, only when it was hot and especially when taxing after landing. I have come to realize that it is fuel boiling in the injector lines. At Idle, fuel flow is very small and it has time to get hot in those tiny injector lines sitting right on top of those HOT cylinders, especially in that tight cowl on a hot day after landing. Having straight pipes with no muffler amplifies the popping! I was having lunch recently at 52F sitting outside and saw two RV's taxi by after landing and guess what, they were popping just like mine!

Ok yes this is probably what you have going on and is normal, mine not so much this time of year but summer time after a flight it runs Pretty erratically and snorts and pops some taxiing in after a flight.

Something is wrong here. I'd find a knowledgable A&P and get this taken care of before it turns into something more serious.

One thing to check would be confirm proper idle mixture.
I have seen an overly rich idle mixture cause this.