?Sticking Valve or Carb Ice?

[fangzz]

good to know. I'll have both ready.

[GAHorn]

I am still not convinced this it's a stuck valve problem....unless theres is an associated heavy
vibration to the point you think the inst panel is coming loose.
Have you changed spark plugs recently?

[marathonrunner]

I agree George, a stuck valve will shake the thing like crazy. I have had it happen and know others who have too. It also usually does it on start up and then clears up in the early stages and becomes progressively worse as time goes on.

[Bruce Fenstermacher]

George and Joel, I agree. As this thread has progressed I'm leaning 66% to 75% towards a valve issue. Could be carbon, could be carbon and a weak spring, could be a loose guide.

I agree some things don't add up with Drew's description if you are comparing it to a completely stuck valve. And engine running with a completely dead cylinder with shake pretty good specially if throttle is increased in order to increase engine RPM.

Drew the pipe I use is a soft copper pipe. The kind you can bend and form with your hands.

I don't really fear a stuck valve. I feel that if the engine is making power on the gorund when throttle is advanced for takeoff a valve will not stick. A balked landing is another story because of the cooling and contraction of the cylinder on decent.

[GAHorn]

.... Thinking back, ...it was after passing through a light shower. ... Onset is immediate, 1-2 seconds. No gradual degradation of rpm. After 5-10 minutes of degraded engine operation, with mild to moderate vibration, it resolves. Resolution is not quite as immediate as onset, perhaps 3-5 seconds back to full rpm. ....

Is this also another example of the other post which reminded me of water in the scat/sceet hose?

See: http://www.cessna170.org/forums/viewtop ... cial+alert

[fangzz]

I remember reading your post, George, to the fella who had washed his newly bought 170 and then had these symptoms. They occurred right after getting back to homebase and giving his wife a dollar ride. And then the bath, which complicated diagnosis for all you guys; it was the 'only thing that had changed.' Next day his engine ran like crap (his words). After all said and done - TWO stuck exhaust valves. Again, this was after shutdown and sitting. I realize this is the classic development, but, it is not the one I'm seeing.

You, George, asked about vibration. As I wrote, mild to moderate. Ok, what does that mean. It means, to me, shuddering I would expect with one dead cylinder and power set for cruise flight. Actually, while troubleshooting in flight, the shuddering was evident even if I retarded the power; it really did not change much. Just the frequency. But, it was not such that the 'inst panel was going to depart.' I would perhaps expect move violence if TWO cylinders quit at the same time. Yes, I can see that. Mag check didn't change anything during all this.

And, George, you asked about spark plugs. No changes. Annualed in April. Plugs looked fine. Cleaned, gapped, returned to service. Engine operation excellent -- until now, and now only very intermittently 'off.'

The consistent thing, so far, is cold engine start-up and ground ops are perfect. Definitely lulls me into thinking it might be ok to 'take 'er up.' Til I remember that poor guy and his wife at Ryan Field. Engine was functioning fine, til his valve decided to stick open. Cheated death, but barely. Wasted a perfectly good airplane.

I've been hearing (all of this anecdotal -til now) that intermittently sticking valves are not uncommon. This from experienced hands. So, even tho this is not the classic, I'm pretty well satisfied it's the culprit. Better be; some spectacular flying weather is getting away. It is the generally flawless engine performance that points me toward one coked valve, which likes to hang up when everything is up to temp ---and then I close the power, just a bit. Must be something to do with a slight reduction in CHP at that point that sets up what happens next. That has been consistent (except once). I'm actually not bothered by the fact that the valve isn't stuck upon RTB, because I would probably go for 'de-coking' the one, instead of them all, which is the current plan, at least in my mind. Armed with Bruce's excellent cautions, coupled with some excellent do's and don't's from homebase, I feel confident in the 'open-heart' about to take place. But, that doesn't mean I'm not reading (actually making copies, as well, for the IA that's taking me under his wing, so to speak) and considering very carefully all the possibilities our guys put forth.

Bluldr, did you/do you use any kind of solvent with the rifle bore brush adaptation?? That really sounds like a beauty.

Guys? Any possibilities we haven't considered? BTW, we're going to really start slow and check airbox apparati and all the other stuff we've thought of here, very carefully. I'll be a happy pilot, canav, if that is the culprit (might take longer to fix, tho).

[Bruce Fenstermacher]

I certianly wouldn't flood the guide with solvent. That would be particulary hard to keep out to the engine. If done dry most if not all the carbon crap can be blown/sucked out of the cylinder.

[ginbug92b]

Weak springs caused the valve to float, not stick. I would lose 200-300 RPM with a mild but VERY noticeable vibration or shudder. In my case it was probably 1 cylinder that had the weak springs but I replaced all the springs in the motor. I am NOT trying to say for sure that this is what his problem is, just giving something else to consider. Mark

[Bruce Fenstermacher]

Mark, you could be right, I'm listening and agree. A weak spring could cause valve float with or without a dirty stem and guide and it would fit with the scenario Drew is experiencing.

[fangzz]

Hmmmm. Think that cylinder or exhaust would now be obvious on inspection?

[FredMa]

It would not be obvious, it it were a weak valve spring. Your options are to test the springs as I described earlier or replace them all.

[FredMa]

That is if you don't find any of the obvious things wrong with it that were already mentioned.

[blueldr]

I am of the opinion that an engine that turns up a maximum of only 2700 rpm is unlikely to encounter valve float unless the springs were so weak that they probably could be "thumb pushed".

[GAHorn]

I am of the opinion that an engine that turns up a maximum of only 2700 rpm is unlikely to encounter valve float unless the springs were so weak that they probably could be "thumb pushed".

The hydraulic lifters depend upon the proper spring pressures to regulate lift. If springs are below specification the valves may not seat (called float) in a timely fashion. Many engines are field repaired ("overhauled") and have existing valve springs re-installed time after time without any checking by the mechanic. Engines may also sit for decades and the springs slowly fall out out spec.

I have witnessed a set of new springs sit in a freshly rebuilt and pickled engine for 14 years fail test and required replacement before the engine could be returned to service.

Page 41 of the Overhaul Manual specifies the spring pressures:
Inner spring: compress to 1.075" and the min acceptable pressure for used spring is 24 lbs.
Intermediate spring: compress to 1.137" min pressure is 37 lbs.
Outer spring: compress to 1.168 min pressure is 74 lbs.

One method to accomplish this task is with a calibrated scale placed under a shop press (arbor or hydraulic press).
With the spring on the scale, compress the spring to the specified length (use a caliper to measure) and read the scale.

However, while we don't know the health of his springs.... for some reason the symptoms don't lead me to suspect springs.

[fangzz]

No, if a weak spring could cause such a power loss, and the associated danger, it would be negligent to fail to replace it. But, first to prove it and find it.