Backfiring problem.

[Lood]

After it's previous annual, my Mooney was delivered in excellent shape. I dropped off the ferry pilot at a friends farm to pick up a J3 Cub there and when I started up, all of a sudden, there was a lot of misfiring and backfiring going on.

We did a lot of fault finding missions and eventually, a spec of dirt in the fuel divider seemed to be the problem. I had it cleaned and all was well except that since then,  I've had a backfiring problem when I close the throttle in the flare just before landing. I took the Mooney in and when we removed the cowls, we immediately found one of the flange ears broken where the one exhaust manifold connects to the muffler resulting in a gap. All of us agreed that cold air entering  the exhaust when the throttle is closed caused the backfiring. I had the part replaced with a new one but unfortunately, this does not seem to have been the problem, because the backfiring when closing the throttle is pretty healthy and going strongly. The only way I can avoid the backfiring is to keep a slight amount of power throughout the landing. When using shorter runways however, I have to cut power a bit sooner and this again results in 3 - 4 very loud backfires.

We've once again checked all inlet- and outlet manifoldes for leakes and could'nt find any. So, where to now? Could it be plugs or maybe mag timing or idle mixture setting or where should we start looking? What bothers me most, is the fact that the engine did not backfire at all before the problem with the fuel divider.

I would appreciate any advice here.

[KSMooniac]

Where do you have your mixture set when landing?  If you go full rich, you might consider leaving it leaned out from cruise, and see if that stops the backfiring.  (I cruise, descend, and land LOP and don't touch the mixture until it is time to shut the engine down, or unless I have to go-around in which case the mixture goes forward with the throttle.)

[Piloto]

The idle mixture setting is too lean. On idle you should be able to see a slight increase in RPM when pulling the mixture. If there is no increase the idle mixture is too lean. It is possible that the the fuel control unit needs cleaning.

José

  

[KSMooniac]

Too lean?  I thought a backfire was due to un-burned fuel passing through the cylinders and igniting in the exhaust?

[Lood]

Quote: KSMooniac

Where do you have your mixture set when landing?  If you go full rich, you might consider leaving it leaned out from cruise, and see if that stops the backfiring.  (I cruise, descend, and land LOP and don't touch the mixture until it is time to shut the engine down, or unless I have to go-around in which case the mixture goes forward with the throttle.)

[KSMooniac]

I would try LOP cruise, and leaving the mixture alone for landing, unless you need to do a go-around.  Perhaps you mixture setting is fouled-up somehow, and you end up much richer than you think you are, but I'm not sure.  I also leave my electric fuel pump off except for takeoff.

[The-sky-captain]

I had the same issue a few times too Jim. I adjusted the time I had the boost on and played with the throttle position a little and it worked itself out.

[eaglebkh]

A random backfire here and there is common in my plane when idling with a hot engine.  But it is not loud at all - only slightly louder than the engine/prop.  I have not pursued getting it fixed because it is not that bothersome.

[Seth]

This is a more common phenomenon than many relize.  The early F and man E Mooney's have for some reason a problem with "popping" when on the ground.  It often occurs when changing power setting during taxi or randomly during idle.  The problem does not reproduce in the air.  Aggressive leaning does help, but some just pop at random times - as does mine.  The shop (a very good MSC) has assured me that this is not an issue, as some engines just do this.  I have done a lot of research and have actually found this to be true.

However, I can't help but to think that I'm destroying the long term life of my engine every time it "pop's" on the ground.  The factory rebuilt engine has about 220 hours on it at this point.  Does anyone else have this problem, or any suggestions despite what I have learned?

[Lood]

Thanks to all for your replies!

However, my engine does not backfire at all when taxiing, or idling or during any of the normal ground operations. It only backfires when I close the throttle completely, from it's final approach setting to nul, whenever it is time to do so during landing. Even keeping the mixture at cruise setting and not enrichening it makes no difference. The only way I've found to prevent the backfiring is not to close the throttle completely during landing. If I set the throttle to keep the engine RPM around 1000 - 1200 RPM, there's no bakfiring. This does unfortunately result in a bit of a float, which is no problem at long strips, but not adviseable when using shorter fields.

The thing that bothers me most, is the fact that for the first year that I've owned the Mooney, it never backfired once. It only started after the problem I had with the pollution in the fuel divider?

[airkraft]

Seth, our MSC in Denver agrees with yours.  Apparently it is not uncommon for ground backfires especially when the engine is hot.  Scott at Arapahoe Aero went so far as to temporarily replace certain fuel lines with clear tubing and was able to see the vapor blocks which I guess contribute to the backfires.  I've been assured that the phenomenon is not harmful, just annoying...GK

[Seth]

GK-

That is so refreshing to hear.  I trust Tommy and the guys at Freeway Aviation and the MSC based there (at W00 in Maryland), but to hear an outside MSC, just not word of mouth state this is a load off my shoulders.  Every time I hear it ground backfire, I cringe thinking I'm kocking the TBO down.  I usually try to be gentle to the engine, and I've put close to 200 on it since I purchased it (it has about 320 hours now since a factory reman), but this is good news.  When I hit 2000 hours on it, I'll post that here. 

Speaking of that, I'm going to start a new thread about TBO.  Watch for it and respond.

[Alan Fox]

Had the same problem , we found one of the intake flange bolts came loose and it was sucking air ...Also had a high egt on that cylinder ..

[Lood]

My Mooney is in for it's anual at the moment and I had an e-mail from my AME to say that the muffler is damaged and needs to be repaired - he feels that this might be a contributing factor to the backfiring. I'm also having a JPI EDM700 installed, so if we can't find the problem, I surely hope that the JPI can point it out.

[Seth]

I overhauled my entire exaust system and it still backfires.  Also, I'm also looking at installing a JPI EMD700.  There is a great deal down in Sarasota, FL right now on them.  I'm in MD however, so a local shop will most likely be my choice as the flight down and back would negate any savings.  Any good avionics shops in MD? VA?  DE?  PA? 

[TonyPynes]

Old thread I know.

I have had my engine torn down and inspected and then an annual by a thorough A&P.

Still pops on idle while taxiing.

[PilotDerek]

Mine will pop after I land and am taxiing back on hot days. It makes me cringe everytime I hear it. I have found nothing to stop it. It makes me feel better knowing other's have the same issue but I still hate hearing it.

[M20E-1964]

My 64 E model pops on the ground ever so often.... usually if I lean fairly well during taxi, it doesnt.

[AmigOne]

Quote: " I usually cruise at around 8500ft with the mixture set at 50 deg F ROP"

Not exactly on the topic that we are discussing but according to Mike Busch this is the worst place to be (do a search and you'll find the article) One should be at 100F ROP. Myself I can't believe that 50F make that much difference.

[TonyPynes]

Mine will pop after I land and am taxiing back on hot days. It makes me cringe everytime I hear it. I have found nothing to stop it. It makes me feel better knowing other's have the same issue but I still hate hearing it.

I feel the same way...if an engine pops something ain't right....even if by my count (from quotes on threads) two MSCs have said it is normal.

[butchgilbert]

Check to make sure all the slip joints in the exhaust system are tight and have NO GAPS. this will cause popping in the flare.

Been there, done that, good luck.

[Lood]

Indeed an old topic and much has happened since. I have a new engine that ran in well and has no snags, nor showed any funny signs after 60 hrs. I don't fly at 50 deg ROP anymore and I have tried LOP once. It worked very well, but for some unknown reason I still feel intimidated by it.

Regarding the backfiring problem, I can't really recall what solved the problem in the end. I think it had something to do with exhaust damage.

[Hank]

Backfiring is a bad thing, and can lead to exhaust damage.

Rumbling, bumbling and burping on short final with the throttle all the way out is neither "backfiring" nor harmful. When idling at ~1000 RPM, she is smooth regardless of red knob position. On short final when I pull the throttle all the way out, she makes 360 cubic inches of deep, gurgly, throaty rumble. Rather like an old Ford ¾-ton truck I drove one summer in college whenever I took my foot off the gas to slow down.

I've always thought of it as the sound of power ready to be applied . . . which is why my Honda's have never done it!

[jetdriven]

Our engine pops and misfires often while taxiing. I hear its due to the flow divider triying to even out the fuel flow between the injectors. I dont worry about it. Its a 361 cubic inch 4-cylinder. Now a high power mag check where you switch off the key and switch it back on will get a loud bang, and that can damage something.

[Shadrach]

I went for a little joy ride yesterday after work. For the first time ever in any airplane that I have ever flown, I got a single backfire during start up. I primed with the fuel pump for the same length of time as always, but I think that I might have had the throttle pushed in a little bit further than usual. My 201 appears to be pretty sensitive to throttle position during start up. If the throttle isn't opened enough it takes a lot of blades to crank. Too much and it will apparently backfire. When you get it right, it'll crank on the first couple of blades and go right into a nice idle. 1/2 inch of throttle seems to work for me.

Prime the engine with the throttle open. It takes less time to prime, is more effective and ensures consistency. Just don't forget to return it to1/8" open for start up.

If you look at a digram of the RSA-5 fuel servo, you can see that it's best way to prime (Byron explained this to me years ago).