After Fire

[Eagle1671]

My very first post to the group.  

This problem is bugging me crazy.   Right after I start my 1966 M20 C Mooney I am getting a backfire (after fire to be exact) .    Not able to figure out what is causing it?

I replaced the left mag with an overhauled one.  Replaced all spark plugs.  The ignition switch and starter vibrator checks out.  All ignition leads look good.  After engine starts runs perfectly!

Has anyone exsperienced after fire after start up?

[Guest]

How does one “over prime” a C model?  It only has an accelerator pump on the carburetor not a manual primer pump.

Clarence

[carusoam]

Gravity controls the overflow.... excess liquid fuel drains and finds its way to the front tire... a well set-up carb only takes a pump or two, or three if really cold out....

Getting excess fuel to not burn before it gets to the muffler has got to be a challenge...

• Helps to have an engine monitor.
• helps to know about the spark plugs.
• knowing a wire harness looks good probably doesn’t tell very much.

The time where fuel burns in the muffler is usually in the traffic pattern during landing.  There are mixture settings that can mask the real issue.  There are threads around here that cover this as well...

PP thoughts only, and welcome aboard,

-a-

[Mooneymite]

As part of your trouble shooting, have you tried starting the engine with the mixture control less than full forward?

If this stops the afterfire, it would point toward the carburetor idle mixture setting.

[carusoam]

2 hours ago, Freemasm said:

@carusoam  I see your posts a lot and tend to agree with them and appreciate your shared knowledge. A point of differentiation here, in my mind at least. A flooded carb bowl and an over-primed cylinder/engine are different situations with different (one being potentially costly/destructive) consequences. We don't know @Eagle1671s start methodology.  If you've even witnessed someone over-working a throttle during a start or between failed starts you'll understand. Thankfully, they are usually compensating for a bad acceleration pump so their actions are typically benign. I knew of a guy, every field has one,  who had gotten into that habit in a loaner aircraft before acquiring his own 152. He often got a significant "pop"  after engine catch.  The damage was limited to the muffler section but those failed baffle sections could have blocked the exhaust path with bad consequences. Just a thought. I'll respect your opinion. Obviously the thread originator needs to determine the source/methodology of any unburned fuel at start-up to prevent any exhaust system damage. I'd still strongly consider a borescope inspection if that were my aircraft.  @Eagle1671, let us know what you find.

Cheers boys.  

Freemasm... thanks for your feedback.  I appreciate it.

 

This is from my private pilot experience owning an aged and very worn M20C...  prior to having MS available...

Starting the O360 in temps above 20°F without pre-heat... over-priming was a scary norm.

Then There is my poor ability to write very clearly...

1) I was trying to define what happens at the front of the process.... with over pumping the throttle...

2) then leaning towards what happens in the middle of the process when spark plugs may not be working properly... Champion High resistance and failure to spark kind of things... I left out ignition timing altogether...

3) Leading into other conditions and times where fuel is known to ignite/pop inside the exhaust/muffler...

4) Essentially the fuel has to come from somewhere, avoid getting burned in the cylinders, then ignite later on in the exhaust.

5) That is a lot of theory without much data to go on...

6) I didn’t get to ignition of fuel /popping in the exhaust...

7) the video camera inspection of the exhaust is a great idea.  I just don’t have that experience to write from...

 

I left the conversation with as much detail I could supply... hoping to spark some ideas from the OP.

Otherwise it becomes a recommendation to follow the POH and get an engine monitor...and send data to the good folks at Savvy... 

 

PP thoughts only, not a mechanic...

Best regards,

-a-

[Hank]

17 hours ago, Eagle1671 said:

My very first post to the group.  

This problem is bugging me crazy.   Right after I start my 1966 M20 C Mooney I am getting a backfire (after fire to be exact) .    Not able to figure out what is causing it?

I replaced the left mag with an overhauled one.  Replaced all spark plugs.  The ignition switch and starter vibrator checks out.  All ignition leads look good.  After engine starts runs perfectly!

Has anyone exsperienced after fire after start up?

It would help us all if you posted your exact starting procedure. I've never had this problem in 11 years of C-model ownership, except for a single backfire when I was experiencing chronic starting problems.

[orionflt]

check your ignition switch, when you start you are starting on the retard breaker points, when you release the key you swap back to the normal points. if there is a moment during the swap where the mag grounds it is the same thing as turning off and on the ignition really quick. that is why when doing a mag check if you accidentally go to off you do not turn the key back on. at lower RPMs your not dumping as much fuel so the bang is smaller then at a higher RPM, both can cause damage.

Brian

[Oldguy]

I will also throw in my suggestion to get your exhaust system (esp. muffler) checked out. I believe we had a presentation at the last Mooney Summit from someone who almost died from CO poisoning following a backfire splitting a seam in their muffler.

Don't be that next guy.

[Eagle1671]

Ok....this has been long over due.  I had three mechanics on this and spent $2000.00 chasing the problem to find out it was not after fire at all. The problem was a cracked engine mount.

The lower right engine mount was cracked all the way through.  When the engine would start and the torque would rotate engine it would sound like backfire noise but it was actually the engine mount hitting metal to metal.  So the fix was a newer style engine mount with welded  gussets .

I borrowed a friends hangar and took engine off and replaced engine mount with the newer style one.  Took me a month to do during spare time.  Glad to have that job over with.

[PT20J]

Thanks for letting us know the outcome. 

[carusoam]

Great follow up, Eagle!

got any pics to go with that?

Best regards,

-a-

[Eagle1671]

I think I do. I will go back and pull some.  The crack was so small could not see it on inspection.  One mechanic just happened to have his eye in the right spot during engine start while trying to diagnose the problem and saw that part of engine mount separate about a quarter inch rapidly producing the sound that had three separate mechanics scratching their heads.

[BKlott]

It would be interesting to know if the aircraft logs indicate whether or not the engine mount had ever been pulled and sent out for inspection during the fifty three years it has been in service? This is something that really should take place at the time of an engine overhaul.

[kmyfm20s]

That seems crazy a small crack could make that much noise?  

[Eagle1671]

Believe me.  I thought the same thing.  Three mechanics thought it was something to do with after fire......carb issue?.. it sounded like a small backfire believe it or not.

[Eagle1671]

9 hours ago, BKlott said:

It would be interesting to know if the aircraft logs indicate whether or not the engine mount had ever been pulled and sent out for inspection during the fifty three years it has been in service? This is something that really should take place at the time of an engine overhaul.

There is an AD for the older Mooney engine mounts that they must be inspected every 100 hrs of service ....if it is not the new style with reinforced gussets.  If you find a crack it must be replaced with the new style of engine mount per the AD.

[Eagle1671]

Pics of crack.   Was hit with hammer so was not flush so could see crack location.

[Eagle1671]

[carusoam]

That’s where my M20C lost 1/4 of its bolts connecting the mount to the firewall.....

What hid what was going on was gravity, and the tube was sitting on the block, in the picture...

with the tube on the block... the engine didn’t sag much...

If the tube slides off the block... the engine sags parts of an inch...

My experience... was a new level of vibration was happening, but not enough to ring a bell...

One day I saw the added sag, the next day it was not there....   THAT rang a bell!

I knew... Broken Planes don’t fix themselves...

The added sag was questionable, is it really sagging more?

But when the added sag was not there, I had proof, it really was sagging more!

Thanks for sharing the pic!

Best regards,

-a-

[Greg Ellis]

On 9/21/2019 at 6:53 PM, Eagle1671 said:

Ok....this has been long over due.  I had three mechanics on this and spent $2000.00 chasing the problem to find out it was not after fire at all. The problem was a cracked engine mount.

The lower right engine mount was cracked all the way through.  When the engine would start and the torque would rotate engine it would sound like backfire noise but it was actually the engine mount hitting metal to metal.  So the fix was a newer style engine mount with welded  gussets .

I borrowed a friends hangar and took engine off and replaced engine mount with the newer style one.  Took me a month to do during spare time.  Glad to have that job over with.

My plane is sitting in the shop right now for the same problem.  Although I did not hear any noise or experience any issues other than multiple exhaust leaks over the last year.  Thanks to advice from this group I had my mechanic check for anything that could cause vibration that would cause the exhaust leaks and turns out my lower right engine mount was broken all the way through.  Then, once again thanks to this group, SBM20-175-1 and SBM20-192A-1 and AD 75-09-08 were pointed out to me. They address this very issue of the lower right engine mount breaking.  I looked at my logs and the AD had been complied with every year.  But I have now an engine mount with the gussets already in place so the AD will be removed.

Good that you were able to find this out.  Not so good you had to spend 2 grand to trouble shoot it.

This is a photo of mine after removal from the plane.