A little engine help

[Ftlausa]

I have been getting intermittent backfire during taxi, both pre and post flight.  My mechanic has done several things to try to address the problem, but it is still occurring.  So, I am hoping that someone on MS might have an idea. 

 

The backfire started almost immediately after I acquired the plane a couple of months ago.  Since then, we have repaired a cracked exhaust riser, cleaned the injectors, cleaned and re-gaped the plugs, and adjusted the mixture.  The backfire was horrible before the exhaust riser was repaired (and I had assumed the cracked riser was the source), but it continued at a much lesser rate after each of the subsequent measures, disappearing briefly after we enrichened the mixture, but it started back today with a few little pops.  

 

I have a 96 MSE. 

 

Any ideas? 

 

   

[carusoam]

I would expect the crack in the exhaust may be caused by excess fuel getting to the exhaust and popping in there...

Do you have an engine monitor and FF gauge?

I'm thinking on the lines of higher than expected FF, or a spark plug that is not working as expected.

Pulling plugs and measuring resistance may be of help.

Keep in mind, I am a private pilot, not a mechanic.

Best regards,

-a-

[Ftlausa]

I do have a JPI engine monitor and fuel flow monitor.  The cylinders are all pretty equal in temps, with the EGT variation being generally less than 100 degrees, and CHT variation being around 25.   

[carusoam]

What FF do you see while the popping is occurring?

Do you have a maintenance record of spark plug resistance?

Which plugs are you using?

Best regards,

-a-

[Guest]

Check the anti flood or sniffle valve on the bottom of the intake manifold it may be stuck,or broken. Also check that the O rings on the four intake tube are not leaking.

Clarence

[Ftlausa]

I have not made note of the FF when its happening, but its generally during near idle, so FF will be on the low end, and either full rich, or leaned slightly for taxi.  I don't know the type or about the last resistance check on the plugs, but I will check. 

[Hank]

I generally lean a lot on the ground, meaning that I move it at least 50% toward Shut Off. The other day I had to park in the grass, and when I gave it some throttle to move it started to die until I advanced the mixture some.

 

You may try leaning more on the ground. If you lean too much, the engine won't run enough to taxi without richening up some. I don't go to Full Rich until running up the engine, and it's lean enough that if I try to take off, it won't run, so there's no danger there.

[marky_24]

I would check for proper idle mixture, and mag timing. My bet is your idle mixture is way rich. Why I think that is because you said it went away and is now coming back. I assume it's getting warmer where you are and that is resulting in a higher DA giving you a richer mixture than before.

A quick way to check is to pull the throttle all the way to idle, and slowly kill the engine with the mixture Knob, as you get close to the cut off point the engine rpms should rise 50 rpm before the engine dies.

[Piloto]

Try running the engine on the ground on one magneto at a time to see if you can isolate the problem to a magneto. Backfiring can also be caused by faulty plugs, ignition wires or a magneto. Sometimes is the magneto switch itself or a P lead chaffing on ground. When switching magnetos wiggle the key to see if it is the switch. If the plugs are over 500hrs I would suspect a bad plug.

 

José 

[Bob - S50]

Not a mechanic. Backfire would indicate to me incmplete combustion prior to the exhaust valve opening. Anything that causes a late ignition or slow burn would contribute to that. I would guess late mag timing as the primary cause. Leaner or richer than peak EGT would cause a slow burn. Bad plug(s) could cause late combustion too.

Good luck.

Bob

[Bob - S50]

Thought of something else. How about a sticky exhaust valve or weak return spring? If your problems occur mostly after start and go away after the engine is warm, I would think this could be the cause.

Best of luck.

Bob

[MooneyBob]

My J backfired while taxiing. It wasn't too bad but very annoying. All the time. I swapped the plugs for tempest fine wire. No problems since then.

[Hank]

Are you sure it is back-firing? When I come in to land and pull the throttle to idle, she will often burble, which I always thought of as the sound of power ready to apply. It generally stops when I add power to taxi at 1000RPM and leaned way out, but can come back when throttling to idle again to make turns. Even large-block cars make this noise, but not my little imported 4-cylinders. Our Lycomings are 4-cylinder engines, but there is nothing "little" about 360 in³!

[N33GG]

I'm not a mechanic either. I would find one that could help me get to the bottom of this quickly. Backfiring is not good. Your engine is trying to tell you something. There are a lot of good suggestions here. Move quickly. That is my recommendation.

[DAVIDWH]

Similar situation with IO550.

 

Mechanic pulled plugs and rotated prop. I was instructed to listen carefully as the mags went click click, click click, click click.

 

Mechanic asked if I could hear mags. I said yes I could hear each click and they sounded fine to me. 

 

He explained, to my naiveity, that mags are not supposed to go click, click, you should only hear one click if they are firing correctly.

 

Mags timed from two clicks to one click and engine performed as normal.

 

Not sure timing is your problem, but easy to check.

[Ftlausa]

UPDATE:  After working our way through the list of possible causes, and none correcting the problem, my mechanic narrowed the issue down to either the fuel servo or the PowerFlow exhaust having a crack.  So we pulled both and sent them off for bench/pressure testing.  As luck would have it, both were defective.  Fuel servo being overhauled and replacing exhaust collector due to an unrepairable crack.  I'm not happy about the cost, but I am glad to (hopefully) finally have solved the problem. 

[AESpecialists]

UPDATE:  After working our way through the list of possible causes, and none correcting the problem, my mechanic narrowed the issue down to either the fuel servo or the PowerFlow exhaust having a crack.  So we pulled both and sent them off for bench/pressure testing.  As luck would have it, both were defective.  Fuel servo being overhauled and replacing exhaust collector due to an unrepairable crack.  I'm not happy about the cost, but I am glad to (hopefully) finally have solved the problem. 

  Simultaneous failure of things with similar symptoms is frustrating at best. Just as bad as an intermittent electrical problem.

Congrats of finding the problem(s)