Engine stumble

[DanH]

Bill, that works for me, if it was an exhaust valve sticking...but the sticky one appears to be an intake.

Does low dynamic compression result in a long combustion time? Looks like coffee with Mr. Taylor in the AM

[Doug]

Quite a mystery, #2 seems lean (or less rich) to me compared with the other cylinders - perhaps intermittently.
Look for the obvious first - you mentioned replacing fuel lines - check you are not sucking air somewhere.
Check the carby mounting bolts, check inlet tubes, clamps and flange gaskets. Check the carby's finger strainer and fuel flow.

[DanH]

Bill, your supposition about partial sticking at the beginning of the event (yeah, I know, maybe) resulting in increased EGT would work for an intake. There is plenty of data linking decreased compression to increased EGT. Apparently has nothing to do with combustion time, but rather work vs enthalpy.

Example snip borrowed from an engineering school paper:
http://rescomp.stanford.edu/~efroeh/...erformance.pdf



An intake that started sticking just off its seat would lower dynamic compression, and the EGT indication would shoot up. The valve itself would heat very quickly with limited or no heat transfer to the seat. Stem diameter would increase, stick the valve at a point further open, and EGT would tank with no combustion.

[N427EF]

Great in depth analysis
I read all of Dan Horton's post with great interest.

Only one thing is bugging me about the whole intake valve sticking.
I cannot refute any of the details or come up with a more likely cause than what Dan has described.
Generally speaking though, exhaust valves are almost exclusively the only valves sticking in aircraft engines.
Rarely is it an intake valve and then only due to improper machining and assembly of cylinder head components.

You'll know soon enough when you remove the rocker valve cover and check you valves.

[Mike S]

I had the valve seat come out of the head on an intake valve in my Franklin engine----------wonder if the Lyc is subject to the same issue??

[N427EF]

All my valve troubles were with a Franklin on a Stinson 108
The description of the Op's engine troubles exactly matches my own experience.
But as Dan pointed out a graph analysis of EM data always beats a pilot description.
Mike,
Having an intake valve seat come out is probably more common on a Franklin since there is almost no one left who knows anything about properly assembling a Franklin cylinder. In any case, it is precisely the cause if improper cylinder assembly and could happen in either a Franklin or a Lycoming.

[DanH]

Originally Posted by N427EF

Generally speaking though, exhaust valves are almost exclusively the only valves sticking in aircraft engines. Rarely is it an intake valve and then only due to improper machining and assembly of cylinder head components.

Just for fun, I typed "stuck intake valve" into the word string search block on the NTSB site. Only got two hits, both interesting. CEN12LA384 was a 40 hour airplane, and NYC99LA173 was a freshly "repaired" valve stick.

Repeating the search with "stuck exhaust valve" netted 32 returns.

I guess we could say a stuck exhaust is 16 times more likely, but a stuck intake cannot be discounted.

You'll know soon enough when you remove the rocker valve cover and check you valves.

Review NYC99LA173

[crabandy]

I'm still trying to educate myself on the reading the graphs, here's a previous flight before sending the carb in:

https://www.savvyanalysis.com/flight...f-185d49cfb305

Am I seeing LOP at about 1:10 and then ROP about 2:10?

[crabandy]

"Review NYC99LA173 "

So the mechanic unstuck the valve but did not fix why it stuck in the first place, When it stuck again later it stuck closed and the rocker arm ended up being the weak link?

[crabandy]

I am a little worried that it was improper machining and or assembly, I was the 3rd owner (far as I know) of the engine after it had been rebuilt.

What is the possibility of my operation close to peak EGT causing the sticking valve? I was always less than 65% power and 380 CHT's, but what about 91 octane and 32 degrees of timing in the mix?