Piper Warrior large engine vibration between 900-1200 RPM

Steve Job

Steve Job

Pre-takeoff checklist
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Steve Job
This started right after our last annual. Above 1400RPM, everything seems smooth. It acts the same on either mag. The RPM drop is also about the same on either mag at 1500RPM and 2000RPM if only running on one. No issues before the annual.

The mechanic thinks the mags are bad (both of them). He didn’t adjust - but he checked - the timing during the annual. All compressions are good. Also, we can see the EGT rise on each cylinder while doing the mag check.

Any ideas from the collective wisdom of POA? Multiple spark plug issues? Carb issue? The mags are at about 400 hours, so they are due for the 500 hour check soon anyway. Just trying to wrap my head around what electrical problem could cause this issue.
 
I always hate it when there was no problem before annual but there is suddenly one after annual. Last one I had was A&P induced. At least he fixed it for free...
 
I’ve been thinking that a flight should be taken with the AP who’s gonna do the annual just before it so he can see what works/doesn’t work.
 
Prop balance or tracking can do that. Blade might have been bent in the shop, or one blade had a lot of metal dressed off it. Or the engine shockmounts might be worn/loose/sagging. Would be interesting to see the snag sheets to see what was "fixed" or disassembled.
 
Dan Thomas said:
Prop balance or tracking can do that. Blade might have been bent in the shop, or one blade had a lot of metal dressed off it. Or the engine shockmounts might be worn/loose/sagging. Would be interesting to see the snag sheets to see what was "fixed" or disassembled.
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I was there for almost the entire annual, and he didn’t mess with prop. At least while I was there. Interesting idea about the engine mounts though. I will take a better look at that. Thanks!
 
Steve Job said:
This started right after our last annual. Above 1400RPM, everything seems smooth. It acts the same on either mag. The RPM drop is also about the same on either mag at 1500RPM and 2000RPM if only running on one. No issues before the annual.

The mechanic thinks the mags are bad (both of them). He didn’t adjust - but he checked - the timing during the annual. All compressions are good. Also, we can see the EGT rise on each cylinder while doing the mag check.

Any ideas from the collective wisdom of POA? Multiple spark plug issues? Carb issue? The mags are at about 400 hours, so they are due for the 500 hour check soon anyway. Just trying to wrap my head around what electrical problem could cause this issue.
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You're asking for posters to guess how to fix a problem that could go from running rough to not running at all.

It might be a good idea to have your mechanic investigate further instead.
 
3393RP said:
You're asking for posters to guess how to fix a problem that could go from running rough to not running at all.

It might be a good idea to have your mechanic investigate further instead.
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He did investigate, and wants us to get the magnetos checked - to the tune of about $800. I’m simply looking for some other opinions here. It runs rough at low rpm, but much smoother at a higher rpm, as stated in the original post.
 
But all you'll get here are posts about how someone had a problem with carburetors, magnetos, propellers, engine mounts, exhaust systems, and several other maladies.

You'll still need a mechanic to check which one you choose.
 
3393RP said:
But all you'll get here are posts about how someone had a problem with carburetors, magnetos, propellers, engine mounts, exhaust systems, and several other maladies.

You'll still need a mechanic to check which one you choose.
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That’s true, but we are very active participants in the maintenance of our plane. I would never blindly trust any mechanic’s opinion. If what he said didn’t make sense to me, I might even post on this forum. If by doing that, I discover a nugget of wisdom he (we) didn’t consider, then that’s a win. He’s basing his opinion of the mags on a hunch, not science. So why not open up the discussion to hear other hunches? He would listen to a good counter argument.
 
One of our club planes ran horrid between 800 and 1100 RPM.. smooth as silk elsewhere.. owner balanced the prop and that made a big difference.. owner just recently got new mags, and the problem seems just about gone. Note, mag checks were fine.. and I've often wondered how "legit" a runup is when it really barely gets the engine to may 25% power. Would a full power lean mag check be more telling?

Also, we're used to high tolerance precision in our cars. Plane engines are a different, completely analog breast. I'm surprised they run as smooth as they do. Really incredible piece of tech
 
A. Bad mag
B. Bad spark plug lead.
C. Bad plugs
D. Induction leak
E. Bad muffler
F. Sticking valve.

The mechanic removed plugs for compression check, I would start with leads and plugs.

Steve Job said:
This started right after our last annual. Above 1400RPM, everything seems smooth. It acts the same on either mag. The RPM drop is also about the same on either mag at 1500RPM and 2000RPM if only running on one. No issues before the annual.

The mechanic thinks the mags are bad (both of them). He didn’t adjust - but he checked - the timing during the annual. All compressions are good. Also, we can see the EGT rise on each cylinder while doing the mag check.

Any ideas from the collective wisdom of POA? Multiple spark plug issues? Carb issue? The mags are at about 400 hours, so they are due for the 500 hour check soon anyway. Just trying to wrap my head around what electrical problem could cause this issue.
Click to expand...
 
Clip4 said:
A. Bad mag
B. Bad spark plug lead.
C. Bad plugs
D. Induction leak
E. Bad muffler
F. Sticking valve.

The mechanic removed plugs for compression check, I would start with leads and plugs.
Click to expand...

Thank you. We ordered two new plugs to cycle through the engine. It definitely happened as a result of the annual. It’s hard for me to believe that both mags went bad.

Interestingly, we did find a muffler leak -the hard way. While running up the engine on the ramp working on the rough engine issue, the scat tubing from the exhaust shroud to the carb heat box burnt up. It was so hot, it melted the carb heat cable lining, and we had to replace it (actually, the mechanic came to our hangar and replaced it). But no joy on the rough running engine.

I hadn’t thought about a sticky valve.
 
Some of the better prop balances can detect different types of vibrations and could help narrow it down, all else fails a good prop balance is always good anyways.
 
Steve Job said:
Thank you. We ordered two new plugs to cycle through the engine. It definitely happened as a result of the annual. It’s hard for me to believe that both mags went bad.

Interestingly, we did find a muffler leak -the hard way. While running up the engine on the ramp working on the rough engine issue, the scat tubing from the exhaust shroud to the carb heat box burnt up. It was so hot, it melted the carb heat cable lining, and we had to replace it (actually, the mechanic came to our hangar and replaced it). But no joy on the rough running engine.

I hadn’t thought about a sticky valve.
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I hope you mean you replaced the muffler not just the tube and the carb heat cable. If not, there is your problem.
 
+1 on the above. If the scat tube burned up there was "fire" going into the carburetor. The venturi may have been damaged by the "fire". Wit regard to the prop an out of track condition will cause vibration that increases with RPM but it will NOT go away above 1400. An out of balance prop vibrating in the 900-1200 rpm range will have a repeatable harmonic at twice the rpm. Does it run rough and sound bad or is it just vibration. Mags and plugs are a stretch. Charlie Melot 40+ years of troubleshooting.....
 
charlie said:
+1 on the above. If the scat tube burned up there was "fire" going into the carburetor. The venturi may have been damaged by the "fire". Wit regard to the prop an out of track condition will cause vibration that increases with RPM but it will NOT go away above 1400. An out of balance prop vibrating in the 900-1200 rpm range will have a repeatable harmonic at twice the rpm. Does it run rough and sound bad or is it just vibration. Mags and plugs are a stretch. Charlie Melot 40+ years of troubleshooting.....
Click to expand...
Now that makes a lot more sense! Thank you! The muffler-tailpipe had a hairline crack, not visible to the eye. It took a smoke test to find it, then a TIG weld repaired it. Maybe it cracked when the mechanic took off the shroud during the annual to check for cracks (irony)? The scat tube didn’t burn until we repeatedly ran it up on the ramp, but I’m sure the hot exhaust was introduced to the carb during the first carb heat test, or probably leaked around the butterfly valve all the time. I will let everyone know what we find, and thank you for the great ideas!
 
Were the old plugs checked for high resistance? One or more dropped plugs (during annual) could have damaged resistors, which in turn, can damage the magneto coil (over time). My first thought was induction leak, which is noticeable at low RPM but not high, or sticky valve(s).
 
Photo%20Jul%2011%2C%205%2045%2030%20PM.jpg
Just an update that the mystery is now solved. We explored carb issues, spark plug issues, and potential mag issues, but then had a different A&P take a look at it. Turns out the intake manifold gasket on cylinder 4 was partly missing. New gasket, and now it runs perfectly. We were one day away from pulling the mags to be sent it for the 500 hour inspection (about 100 hours early). Thanks for all the helpful advice!

https://www.dropbox.com/s/4peppcmb9qdkfa2/Photo Jul 11, 5 45 30 PM.jpg?dl=0
 
Had the same thing happen to me once, thought it might have been cause by a backfire and slightly loose intake
 
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