Recurring Cylinder Problem

[alaskan_trout]

I am in the middle of my annual of a 1948 C170 with a C145-2.
It is 335 hours since major overhaul. The number 1 and 2 cylinders have had problems before at about 225 hours. They seem to be having exhaust valve problems in that they do not keep compression. I lean when I fly, try to be smooth in the application of power, change the oil every 25 hours, oil anaylsis indicates okay, and yet the cylinders are again having problems with the exhaust valve.

Does anyone have any ideas? Any operational tips or guesses would be appreciated.

Ken

[mrpibb]

Ken, we need a little more info. Were the cylinders new or overhauled? where is your loss of compression, valves? rings? Is it consistant or intermittant such as several checks reveal a good then bad then good reading. There's a lot of knowlage in these forums on the continentals so be prepared.


First flight after engine overhaul

[Bruce Fenstermacher]

Ken, Vic is right we need a little more info. Having said that I'll go out on a limb and say your cylinders are good whether they are new or rebuilt.

You did say you had the same problem about 100 hours ago which indicates that someone fixed a problem. More than likely you would have indicated the cylinders needed to be removed for this repair if they had but you didn't so I'm guessing you had a carbon buildup on the exhaust valve or stem causing the exhaust valve to remain open and the cylinder leak. This is fairly easily fixed without cylinder removal.

Now if I guessed right and it is carbon and it's on the valve stem causing the valve to stick open and it's happened again I'd suspect the valve stem to valve guide clearance to be to tight on those two cylinders.

Of course I'm just babbling on here stabbing in the dark. You really need to give us more info.

[alaskan_trout]

Thanks for the responses.

Here's the history.
June 2002 - Major Overhaul with New Continental Cylinders.
July 2003 - Oil leaks at #1 intake valve & #6 exhaust valve repaired by engine rebuilder. 116 hours
September 2004 Removed cylinders #1 & #2 due to oil leaks at exhaust ports. Repaired. Reimbursed by TCM. Only 44 additional hours due to time down for rebuild of wings, tail, and new paint.
October 2006 - Today. Total hours of 335 on SMOH. The exhaust valve on #1 had low compression that was cleared by a smart rap with a rubber mallot. Apparently some carbon stuck. #2 was not so easily cleared. The cylinder has been removed and will be taken to the engine overhaul people for further analysis and reseating the valves.

Compression
2004 #1 75 #2 71
2005 #1 -77 #2 - 78
2006 #1 - 68 #2 - 40 & in for repairs.
The rest of the cylinders are in the 70's and have been consistently.

Along with mechanical advice, should I be operating anything differently?

[Bruce Fenstermacher]

Yep I'd have them check the valve stem to guide clearance. If they are to tight it will take less carbon to make them stick. Clearing the problem with a rap of a hammer indicates a carbon problem. BTW if the carbon buildup is on the stem and it is sticking, rapping with a hammer might close the valve for the compression test but it does not fix the problem of the carbon buildup. Only removing the valves and cleaning the stem and the valve guide will do that. While removing the cylinder to accomplish this is the most thorough way to do this the valves and guide can and often are cleaned without removing the cylinder.

I suspect the oil leaks you are talking about are from the push rod tubes not the valves themselves which is from a very old design that is less than desirable.

As far as operating differently well I'd be running 100% mogas (without ethenal) and MMO but that is another really BIG can of worms.

[spiro]

how's your baffling?

[GAHorn]

Ken, it's really difficult to offer good advice with so little info, but perhaps it's because your mechanic isn't giving you many details?
You wrote about valves with "oil leaks"...which are not likely what you are speaking of this time? Valves rarely develop "oil leaks". They usually leak compression or intake/exhaust gases. The only time they leak oil, is if their valve guides are cracked or worn beyond limits. If such is the case, the guides, and possibly the valves and valve seats, should be replaced.
Valves can leak compression and exhaust/intake gases for a variety of reasons, but one of those reasons can also be a cracked or worn guide. The valve guide ... get this.... guides the valve longitudinally as it opens and closes. If that guide is worn excessively or cracked, it will not support the valve stem well, and that can allow the valve to close on it's seat in an inconsistent manner and prevent it's sealing closed against it's valve-seat. This will allow gases to leak past the valve/valve-seat. If very worn, it' can allow oil from the rocker-box/cover area to leak past the valve stem and into the cylinder head, and run down into the intake manifolds and into the exhaust system.
The comment from Bruce regarding the pushrod tube leaks are actually not about the valves themselves leaking, and the repair for such a leak is a fairly simple one, involving using a special tool to re-seal the upper end of the pushrod tube to the head. While he may feel that design is "less than desireable", I don't think it's such a problem that much money should be thrown at it, when the repair is so simple and inexpensive. (Usually those upper pushrod housing leak because the cylinder was mis-handled during engine-assembly, by a mechanic who used them to transport/carry-around the cylinder. Once installed, and once the tubes are resealed, they rarely cause further problems during the overhaul-life of the engine.)

Now, back to your leaking exhaust valves: If you are speaking of a loss of compression or leaking exhast gases, the most likely fix will be a removal of the cylinder, and a repair involving reseating and/or replacement of that valve, guide, seat. If your mechanic has the proper valve-repair machines in his shop it's a job done in a few hours. But few mechanics actually do such engine work anymore, most of them send the cylinder back to the cylinder shop where the work will be performed. Your mechanic will likely remove/reinstall the cylinder, or he'll possibly recommend an "easy way out" of total replacement of that cylinder. Lots of owners/mechanics feel that is the shortest way to gettng rid of a problem. Unfortunately, it sometimes is simply a way of gettng someone elses problem cylinder. For that reason I usually recommend new cylinders at overhaul.
But if your engine was already overhauled using new cylinders, then a simple valve repair such as I described. may be the least expensive way out.

[alaskan_trout]

Thanks for all the input.
These should help with my communications to the engine shop.

The walls of the cylinder were also very smooth on the removed cylinder.

The initial reaction from the engine shop with a cursory look was that the bracket air filter was the culprit in that while it captured the small pieces of dirt, it let the large pieces of dirt go through. They favored a paper filter instead of a bracket filter.

My mechanic disagrees with that analysis.

The baffles look like they seal correctly.