?Sticking Valve or Carb Ice?

[56alpha]

My 170 had a similar symptoms a few years ago. The engine had about 150 hrs SMOH with new cylinders installed at OH. After the normal diagnostics (inspect plugs, compression test, etc.) I found that Cyl. #2's piston rings were stuck (all of them stuck... very stuck). Since the oil ring was stuck I believe oil was fouling the plugs and causing about 200-300 loss of RPM. After a few minutes the plugs would clear and full power was back for a while, but the cycle continued. I attribute the fundamental cause of the problem to terribly leaky baffles.

[GAHorn]

Stuck rings should be evident with a compression-test. (lots of leaking air sounds at the oil-filler cap)
Stuck rings would not likely cause intermittent low-RPM.... but would show up consistently, as low compression on one cylinder with the stuck rings.

[fangzz]

To our invaluable moderators, and all you guys watching this, and, like me, learning from it, I'm waiting for some tools I've ordered. Next Friday, after the tools have arrived, I'm set up with a local IA (we're way far away from home and our own tool crib) to incrementally attack this thing. Pulling the plugs and looking at each one very closely. Compression test. Pull the valve covers; watch everything go in and out. Measure stuff (like valve stems). And, then probably do the rope trick to all exhaust valves (drop valve, ream, etc.).

The wait is killing me. You guys should see this flying weather here in N.E. Washington. Really tempted to load some MMO, and try and force another sticky episode. I'm REALLY curious if, when, after a good cruise, pulling a little power off, which is when it's always happened (except once), by diving a bit, to up the airspeed and rpm, it would resolve the episode. But, maybe this old age is good for something. Judgement seeping through.

I'll keep you all posted, but don't stop those cards and letters; might be something one of you recognizes for sure. Thanks a lot for your interest and time. Drew

[GAHorn]

Don't ream valves that are not visibly stuck.
You don't want to enlarge valve guides unless you can confirm they are undersize, or seriously
coked.
Theres little to be gained and much to risk arbitrarily doing the rope trick.

[fangzz]

Since this is 'intermittent,' and only after some serious cruise, and since ground operation has to this date always been flawless, I don't expect to find a visibly stuck valve. Would you say I might expect one or more to 'hang up' while propping it over slowly?

[Bruce Fenstermacher]

Drew,

I've had several stuck valves. They all stuck open after the previous shut down and of course were discovered on the next start. I have also seen a valve that was stuck open and freed up using the rope trick to push the valve closed but only stick again when hand propped. I can not say that I have absolutely diagnosed sticky valves in an engine that runs and intermittently has a stuck valve but I suppose there is no reason it can not happen.

Your symtoms seem to indicate a cylinder not producing power or being dead. There are few things that can do this intermittently. Ignition is one but you would have to have a rare case of a both systems in a redundant system failing at the same time. This can happen but it is slim. Another is a valve not closing for any number of reasons, a coked stem and guide being one.

Besides the normal risk of breaking something any time you disassemble something there is one gotcha, and it is a big one, when messing with the valve train. You see there are hydraulic units out there that were not designed with a clip to hold the cup in the top. The push rod rides in this cup. Oil holds the cup to the pushrod. If you remove the pushrods during the valve train disassembly process, and the cup sticks to the rod, and there is no clip, the cup will likely rotate. And when it does and you turn over your engine by hand the cam will force the cup into the hydraulic unit and break it. You may not even know you did it. The only way to fix this is slit the case. And if you do this you might as well rebuild.

Unless you are absolutely sure you have hydraulic units with clips, do not remove the pushrod when you remove the rocker arm. BE VERY CAREFUL removing the rocker arm because the same oil and suction that holds the cup to the pushrod also hold the pushrod to the rocker arm.

Now the other thing you want to understand about cleaning the guide. And assuming the guide was sized right all you are doing is cleaning the guide. You really are not reaming it in the sense that you want to remove metal. I've posted before most of the time I've done this I do not use a reamer to clean the guide. I use a piece length of 1/4" soft copper plumbing pipe. I use a #11 Exacto knife blade to scrap the inside of one end of the tube so the edges are sharp. I then use the sharp tube edge to scrap the carbon out of the guide. The soft copper can not hurt the guide.

Of course once you have cleaned the guide and the valve stem you want to check the fit. You want to answer the question as the whether your guides were reamed to the correct size to begin with. I've heard of guides that where left to tight from the start. If they are deemed to be to tight only then would I actually use a reamer and size them correctly.

BTW there are at least 2 other excellent threads where I discussed my technique and I think there is a picture of my 1/4" cleaning pipe as well as others have discussed the correct reamer size.

[ginbug92b]

You only said it was overhauled in 2009. Did it get factory new cylinders or were the old ones "overhauled" If the cylinders were overhauled did they replace ALL valve springs with brand new or did they check the springs for tension and put them back in because they were "within limits?" It sounds just like what I had and the cure was all new valve springs. Mine had only 200 hours since overhaul with new springs but sat for 10 years before being run again. FWIW

[canav8]

Please save yourself some grief and look at the airbox and the control closely before you take anything apart at the engine. Been here and done that already!

[fangzz]

Will do, and thanks.

[blueldr]

Bruce,
it would be a little less confusing if you kept in mind that "1/4 inch pipe" measures 1/4 inch inside diameter and "1/4 inch tube" measures 1/4 inch outside diameter. In your above note about your reaming device, you seem to use two different sizes of copper to make your reamer.
Frankly, I have found that a brass rifle cleaning brush, on my Makita drill motor, does a fine job on cleaning out valve guides.

[marathonrunner]

So does a reamer with a little grease on it so you dont have fod in the cylinder. Yes I know it would blow out but, better to keep; things as sanitary as you can

[Bruce Fenstermacher]

You only said it was overhauled in 2009. Did it get factory new cylinders or were the old ones "overhauled" If the cylinders were overhauled did they replace ALL valve springs with brand new or did they check the springs for tension and put them back in because they were "within limits?" It sounds just like what I had and the cure was all new valve springs. Mine had only 200 hours since overhaul with new springs but sat for 10 years before being run again. FWIW

Drew, this is the power of the forum. Ginbug92b thinks he had the same symptoms. I you can't varify the springs used in overhaul to be new at the time, for the price I'd be ordering new. Aircraft Specilties prices for the three springs you will need for each valve are as follows:

SuperiorAECTCM
PN240315.134.839.85
PN240294.404.556.59
PN6259582.703.086.29
Per valve12.2312.4622.73

As the exhaust valve gets dirty, and they naturally do and some carbon is normal, a weak valve spring will not seat it. Symptoms would be like a sticking valve. And the symptoms could come and go. This could be the key that was missing in my mind. Cleaning the valves and guides would be a temporary fix.

[Bruce Fenstermacher]

Bruce,
it would be a little less confusing if you kept in mind that "1/4 inch pipe" measures 1/4 inch inside diameter and "1/4 inch tube" measures 1/4 inch outside diameter. In your above note about your reaming device, you seem to use two different sizes of copper to make your reamer.
Frankly, I have found that a brass rifle cleaning brush, on my Makita drill motor, does a fine job on cleaning out valve guides.

I don't think I specified inside or outside just 1/4" and it wouldn't matter so long as the pipe fits inside the guide. The pipe is just a scraper stiff enough to rid the guide of carbon but soft enough not to damage the guide.

A brass rifle cleaning brush would work nicely as well at your home shop but you likely don't want to carry it in your travel tool bag as I do my trusty 1/4 coper pipe.

[fangzz]

Bruce, no question about it; a wealth of experience and knowledge here. Trying to soak it up like a sponge. I have read every thread and post that came up after searching stuck+valve and also rope+trick going back years. Tons of great discussion. Entertaining. I picked up early on the fact that you do guide cleaning (using your tool) every 250 hours or so. Seems like a very wise thing to do, given the history of this engine to have this issue, and the really dangerous situation it can cause if the spirits are at the wrong place at the wrong time. A year ago, at the RAF fly-in at Ryan Field (mountain strip), a perfectly good 172 crashed on takeoff because of a valve that stuck at the wrong time. I haven't forgotten that and I never will. No fatalities, fortunately, BTW, but injuries the good folks are still recovering from. This procedure seems relatively easy and straight forward (I'll know for sure Friday), and I will probably adopt your practice. For sure, if it ever hangs up in flight again, I'll repeat the guide cleaning. This assumes we find coking to have caused this in the first place. Something did. And, it could have been deadly. Gotta find the culprit. I'm hoping it turns out to be something obvious, like a hanging valve due to dirt.

I will probably go for the new springs - good suggestion. Don't know for sure in 09 if they were replaced. I'm thinking they were not. I really appreciate the strong heads-up on the clips or no clips. My homebase IA had the same strong caution. He thinks we're on the right track here and has firmly stated that cleaning out the guides should provide lasting effect. And, unless you say different, Bruce, I'm going to assume your tool is soft-copper TUBING.

Bluldr, great idea with the rifle-cleaning brush. Is that .30 cal?

Thank you all

[blueldr]

I can't remember whether the rifle brush was .27 or .30 caliber. I know that I had both sizes in my cleaning kit.