Why Valves Stick

[Colonel]

An hour out of your life, and worth it. Mike Busch doesn't know everything, and
his advice is sometimes lacking, but his theoretical knowledge is very strong.

[anofly]

ok i listened and watched.
Keep cyl heads on the warm side but in spec
lean a lot on ground
I think i "heard" dont run low power settings,in flight, even though they should have said it loud and clear!! they said keep combustion temps up. I say run them 65-75% by far the majority of the time... maybe even higher?
I think I "heard " lean in flight at all power settings less than 75%, but they should have said that loud and clear, and he said sticking was more of a 'cyl head temp" thing than exhaust gas temp? thing.

Ok Colonel I will bite, what else?
and or what if anything might I have wrong above?
please and thanks

[Colonel]

Ok. The lesson here is that while you will develop lead and carbon deposits
on the ground (and during approach) you should lean the mixture for max RPM
on the ground and idle at 1100-1200 RPM after the engine is warm, to try to
minimize them. No surprise here.

The bigger lesson here is that in flight, lower CHT is not always better CHT.

Everyone knows that Lycoming's 500F CHT limit is insane, and that 400F is better
upper limit if longevity is desired.

I used to think that 300F CHT was fine. It is not. You want 350F to keep the lead
from depositing in flight in your Lycoming, which has cooler exhaust valves than
TCM, because they are hollow sodium-cooled unlike TCM which are solid and are
hotter on the bottom of the stem, above the face.

Keep in mind that the front cylinders of your Lycoming - esp cyl #1 - will run
substantially cooler than the rest of the cylinders. So, if you have a single CHT
(on your hottest cylinder) keep in mind that 350F on it translates to 300F on cyl #1,
and it will develop lead deposits.

[anofly]

Thanks
So legacy CHT probe and gauge , on my twin comanche,( read looks like a little spark plug) rarely reads above 300, more like 275 on one engine, and a smidge over 300 on the other.
7500-9500 feet. WOT, lean to peak and smooth, 2350 rpm, its canukistan so about zero deg C outside yesterday.
i do lean bigtime on ground, it is noticeable how much more brake it takes while taxiing, so i know its working.

[Colonel]

I've never liked those CHT probes that replaced the copper crush spark
plug gasket. Low values, IIRC.

Far better are the probes that screw into the bottom of the cylinder
head. I know in cold places people like to put heaters in there ....

CHT 275/300 (not sure I believe that) is too low, esp since your cyl #1
is going to be substantially cooler. That's ok on auto gas, but on leaded
fuel with a Lycoming, you're going to get sticking valves after the deposits
have a chance to accumulate.

Be alert for morning sickness, when one cylinder runs cold for a while. If
you see that, it's important to identify the cold cylinder - just shut down
as Mike recommends, and put your hand on the rocker cover, and identify
the cold cylinder - and have your AME do the rope trick aka Lyc SI 1425A
on that cylinder, to mechanically clean the deposits from the guide and
exhaust valve stem. Doesn't cost much, no parts required, just a few
hours of labor. Worst part is pulling the exhaust. I really hate working
on old exhaust that hasn't been touched in decades. What a cluster.

Friend of mine with a 172 across the taxiway at my old airport ignored
the morning sickness, and one day got an oil leak.

Turned out one of his pushrod shroud tubes had split. And the reason
that happened, was that when the valve sticking got bad enough, it
bent the pushrod - the weakest link - and split the tube.

His AME didn't have a clue about the rope trick. Kids these days.

[anofly]

I have a great mechanic and we (royal we?) have been doing the rope trick and checking how sticky the valves are in the guides, about every 300 hours since my last valve stick in 1999. It was costly on the 0540. it broke the lifter body, that meant split the case, that became overhaul.... of the bottom end.

[Colonel]

If you run your CHT over 350F (and below 400F) you might see a drastic
reduction in your valve sticking.

[Slick Goodlin]

I always thought it was because I forgot to put Marvel Mystery Oil in the gas, oil, and maybe coolant and a little in the tires just to be safe.

[Colonel]

I think MMO is a solvent, like AvBlend. Both can be effective at softening carbon,
if that's your problem.

I have found that adding AvBlend will help a little with valve sticking - probably
because the gorp on the valve and guide are a mixture of carbon and lead deposits -
but Mike is right that lead deposits on the guide and valve can only be mechanically
removed.

Lycoming says you are absolutely to never, ever sand the valve stems, and I suspect
not everyone pays attention to that. I have found that valve stems are best cleaned
wearing a rubber glove with a rag tied around your wrist, with a cut strip of Scotch-Brite
soaked in MEK. The MEK will soften the deposits and the Scotch-Brite will remove them
and it's not sandpaper. Ahem.

Now onto the valve guides. Mike Busch is a smart guy, but I don't think he's spent a
lot of time cleaning lead deposits from exhaust valve guides, which appear as a grey
paint coating on the top of the inner portion of the exhaust valve guide. Lycoming
recommends using reamers in SI 1425A and I've probably spent about $1000 purchasing
every exotic size of reamer (hint: .500 is easy to find) and they are almost completely
useless at removing lead from the inside of a valve guide. All they do is try to gouge
the inside of the guide, and you will never have the right size, guaranteed.

Because I'm so fucking stupid, I use different grades of SC flex-hones to very quickly
clean the lead out of Lycoming exhaust valve guides. Soak them in engine oil and
spin them for 10 seconds with an electric drill, moving them in and out, just like
honing a cylinder with dingleballs. Lead will be completely removed with no damage
to the guide. Beautiful shiny surface.



Use the reamers to measure the exhaust valve guide diameter. That's all they're good for.
See the Lyc table of limits. Curiously, helicopter valve guides are MUCH bigger. Ever
wonder why?

Oh yeah. Dental floss. Because I am so monumentally stupid compared to any
Canadian, I figured out that you want some dental floss that has good tensile strength
in the presence of petroleum distillates.