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-   -   Engine Stumble Help Requested (https://vansairforce.net/community/showthread.php?t=143125)

brad walton 10-27-2016 04:15 PM

My experience is similar to Jon Jay. My #2 exhaust would stick open randomly/periodically. But it only ever lasted a few seconds, so anything I did seemed to cure it. Cut power, cured. Richen mixture, cured. But always accompanied by loss of EGT. This went on for a long time because it was so intermittent. When I removed the valve cover, it was clearly a valve tight in its valve guide. Reamed it out and no more symptoms. In retrospect, I could hear the valve occasionally not closing fully when at high rpms. Just an almost imperceptible miss.

Aggie78 11-15-2016 05:29 PM

The End of The Story!
 
So, to circle back to the beginning of this thread and wind things up...while the aircraft was at Saint Aviation for an avionics upgrade, I asked Jesse if he would mind adding popping the #2 cylinder head cover off and checking the exhaust valve for freedom of movement after the way it had been running and the info learned on this thread.

He agreed, and we added it to the bill of work.

I arrived today (as the project is reaching its end) and Jesse informed me that the mechanic he'd brought over to check this out for him opened things up, and they discovered a verrrry difficult to move exhaust valve-so much so, that it took a socket extension and a bit of persuasion with a hammer to move it at all.

The mechanic disassembled the associated valve train and then cleaned out the guide with a hone (removing a fair amount of gunk) and the valve moves silky smooth now. Test flight tomorrow, but I gotta believe this is going to resolve the engine stumble.

Wouldn't have guessed an exhaust valve issue without this website! Thanks to all who contributed, and I'll PIREP how she runs (especially lean of peak) after the flight home.

Thanks again, all!

Rob

humptybump 11-16-2016 06:11 PM

As noted above, it's not injection nozzle or similar since I have a carburetor.

I would like to persue the intake tube theory. Is there a test for an individual intake tube?

As for the sticking valve, I'll ask my mechanic. I do occasionally run 100LL but as noted, I burn very little of it and a recent inspection of the plugs were very clean. That wouldn't completely discount a valve.

JonJay 11-16-2016 09:45 PM

Quote:

Originally Posted by JonJay (Post 1122243)
If you search through the posts, the last several instances describing exactly the same symptoms all proved out as stuck exhaust valves. .

So, we can add one more to our list. Our engine monitoring tools are great, but as Bart LaBlonde previously stated to me, things happen very fast during the combustion cycle.

Very glad you found the issue and it was an easy fix. Yay!

Snowflake 11-16-2016 10:10 PM

Quote:

Originally Posted by JonJay (Post 1127371)
So, we can add one more to our list. Our engine monitoring tools are great, but as Bart LaBlonde previously stated to me, things happen very fast during the combustion cycle.

Hmm... Wonder if there's a way to detect a sticking valve somehow with instrumentation that we don't already have in our planes... I see an untapped market... :)

BillL 11-17-2016 09:57 AM

Easier is better . . .
 
Quote:

Originally Posted by Snowflake (Post 1127376)
Hmm... Wonder if there's a way to detect a sticking valve somehow with instrumentation that we don't already have in our planes... I see an untapped market... :)

There is a way. The acceleration/decel of the crankshaft creates an rpm fluctuation during a revolution. This is easily detected with instantaneous speed sensor (with adequate number of teeth) and then algorithms for detection can directly identify an event and offending cylinder. This is used on large 12-16-18-20 cylinder engines to detect combustion anomalies. It takes a pretty high speed processor, but those are widely available these days. I don't know if these are commercially available for smaller engines. Maybe Ross will be interested, I know the people who have done this.

Example Publication $$: HERE

Taking time to remove the VC and check each valve annually would not seem like much work. It seems a lot easier to notice abnormal engine operation and then do the check.

lr172 11-20-2016 10:36 AM

Quote:

Originally Posted by humptybump (Post 1127331)
As noted above, it's not injection nozzle or similar since I have a carburetor.

I would like to persue the intake tube theory. Is there a test for an individual intake tube?

As for the sticking valve, I'll ask my mechanic. I do occasionally run 100LL but as noted, I burn very little of it and a recent inspection of the plugs were very clean. That wouldn't completely discount a valve.

I could be wrong, but I doubt the build up in the exh valve guide is lead. It is likely carbon and produced by the oil carbonizing (i.e. burning) due to the excessive heat that Lyc's see on the exh valve stem due to very limited oil cooling in the rocker box. The exh valve stem has very little exposure to lead in it's pre-heated state. I would expect to see the same problems in a lead free environment.

Aggie78 11-20-2016 02:25 PM

Flight Home PIREP...
 
3 legs, no problems!

Pushed it too, running it very LOP and looking for any issues...

Ran like a top! :D

Now I need to research the suggestion about running Marvel Mystery Oil as a preventative.

Also chasing down a lead that we might be able to get Swift Fuels 94UL at my home base, too, the use of which I understand would help as well.

Thanks again to all for the help!!

Rob

humptybump 11-20-2016 06:17 PM

Quote:

Originally Posted by lr172 (Post 1128140)
I could be wrong, but I doubt the build up in the exh valve guide is lead. It is likely carbon and produced by the oil carbonizing (i.e. burning) due to the excessive heat that Lyc's see on the exh valve stem due to very limited oil cooling in the rocker box. The exh valve stem has very little exposure to lead in it's pre-heated state. I would expect to see the same problems in a lead free environment.

Yes, this was explained to me today as we checked various possibilities. We did the "wobble test" on the exhaust valve and it was "nearly perfect" with zero sign of sticking or buildup.


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