Need advice about backfiring

decathlon737 · #1 09-28-2008, 07:50 PM

Guys,
I've own several airplanes with FI and am aware of backfiring issues if throttle is pulled rapidly on descent. Here's my scenario, I'm running a recently overhauled Bendix fuel servo, flow divider and injectors. I can pull throttle slow and get it down to only an occasional backfire but the problem is that it's backfiring through the induction system and deforming my air filter. I've heard this was a lean condition and have tried to richen it up a bit. I've adjusted the idle mixture to the rich extremes and still cannot get an rpm rise when ICO is pulled. I'm striving for a 15 - 25 rpm surge but am unable to get it to rise any when I pull mixture. Can I adjust the mixture to get more travel toward the rich side? I've checked for induction tube and exhaust leakage, anything I'm missing or forgot to check? Any comments or help welcomed.
Thanks

rvbuilder2002 · #2 09-28-2008, 11:05 PM

Quote:

Originally Posted by decathlon737

I've adjusted the idle mixture to the rich extremes and still cannot get an rpm rise when ICO is pulled. I'm striving for a 15 - 25 rpm surge but am unable to get it to rise any when I pull mixture.

If that is the case, then it sounds like there might be a problem with the fuel servo. You should be able to adjust it rich enough that it has a rather stumbly idle and will give you 100RPM or more rise if mixture is pulled very slowly from full rich to ICO.

AlexPeterson · #3 09-29-2008, 06:28 AM

Bill, the more typical "pop, pop" heard on an FI engine when rapidly throttled back in flight is afterfiring, or explosions in the exhaust. What you are describing is genuine backfiring if you believe it is happening in the induction system (based on your air cleaner observations). I don't know if that can be heard in the cockpit or not.

On the ground at idle, where in the range of mixture position does the engine finally quit as you lean it? Also, what are full throttle takeoff fuel flows?

kiwipete · #4 09-29-2008, 08:45 AM

One thing to watch is you say that" I've adjusted the idle mixture to the rich extremes and still cannot get an rpm rise when ICO is pulled."
You may only be able to detect the rise if you pull the mixture very slowly.
I thought I had too lean an idle untill I wound the mixture out a little at a time and discovered that I could indeed get a rise. If I pulled it back in a couple of seconds it appeared to just die.

Peter

decathlon737 · #5 09-29-2008, 01:27 PM

Alex, ICO kills the engine just fine, I don't have a fuel flow monitoring system yet, one of the items I will be adding later if the residential construction market ever returns

. kiwipete, I have tried pulling the mixture very slow and fast, I have a cable driven tachometer and have not been able to detect any rise in RPM when ICO is pulled. I'm thinking something is not quite right with the servo or the mixture linkage. Any more thoughts?

AlexPeterson · #6 09-29-2008, 02:58 PM

Bill, where in the range of mixture between full rich and ICO does the engine die at idle? 3/4? 1/2? I would think you need for it to just barely kill the engine at ICO. If it is indeed too lean, it may not idle at all if you find yourself in cold weather.

Mark Burns · #7 09-29-2008, 03:18 PM

Bill,
You say it's a true backfire and deforming your air filter.....while the engine is running.

This sounds like something more than just a mixture problem to me.
What's the status of your magneto's? Age, hours etc.

It's sounds to me like you could be getting a crossfire in the magneto distributor.

I hope Mahlon will see this thread and give us his thoughts.

Mark

decathlon737 · #8 09-29-2008, 04:23 PM

Alex, good question, I'll check tomorrow. Mark, you have a good point, I've rechecked mag timing but could have a problem in one of the mags. I get frequent exhaust after firing if I pull throttle to fast which I feel is pretty normal and can be controlled by adding a little throttle. My air filter and alternate air door on the FAB was deformed at the last oil change arousing suspicion that true backfiring was occurring. Could have happened only once or several times as I'm sure i couldn't tell if it was exhaust or induction firing while operating the airplane. Repaired the filter and alt. air door about 6 hrs ago and all seem fine now. Could have happened on start up or possibly a hot start? Just returned from the airport, while there I turned mixture adjustment to full rich (adjuster bottomed out to rich side) and was able to pull ICO very slowly (maybe 4 second pull) and see about a 10-15 RPM rise. Can this linkage be recentered more closer to neutral? It looks to have a fine thread - coarse thread arrangement for adjustment. Thanks for the comments, keep them coming.

SteinAir · #9 09-30-2008, 05:38 PM

Hi Bill,

As you well know the afterfiring is a moot point on an RV and normal. The backfiring is not, especially if it is responsible for deformation of your intake components. It sounds like you've got a good handle on it. Can only be one of two things (or a combination thereof), timing or mixture. More likely to be timing than mixture given what you've described so far, but then again it's hard to tell. If you do indeed have the mixture at full rich during idle, then I'd be a little bit suspect there. Most of these Bendix Servo's shouldn't need to be anywhere close to that - and if they are they usually require you to actually lean a bit on the ground to keep it from flooding the engine at idle. I can't remember any of them I've seen or flown behind needing to be anywhere that radically adjusted (even my 1,000 hr+ old RV6 with the RSA system on it). Are you 100% sure the mixture stop is all the way in full rich?

The fuel thing is a little suspect just because of that. Also timing could be a bit suspect as well. What is your idle RPM...is it down aroun 500 RPM or so?

Sorry I don't have much to add other than it sounds like your nose is leading you in the right troubleshooting direction. Things just don't sound quite right....

Cheers,
Stein