Rough running engine at 100?

[terrykohler]

The lead has likely been happily building up on the piston heads for some time. If you switched to hotter plugs, it would make sense that the bottom plugs are the ones loading up (gravity at work). Is this the case? If not, something else is causing the cylinders to run hotter and melt the lead.
Are you monitoring/tracking EGTs and CHTs? Are you seeing a spike?
Terry, CFI
FR9A N323TP

[Jerrygemma]

Thanks for the heads up on the ?cracked cylinder? post. I?ll look for that.

[Jerrygemma]

I finally got time to spend on the engine.

I ran the engine and as before. Once it got hot ran rough. Did a hot compression check and all cylinders were 87 or better. So no stuck valve. But thought of a cylinder running lean. So I pulled the number one cylinder intake tube and it had oil in it. Pulled the plugs and they were black. So my final analysis is the intake valve guide is leaking oil and causing the plug to miss fire. I am going to pull the cylinder and have it IRAN. Hope that is the end.

[Mark Burns]

Originally Posted by Jerrygemma

I finally got time to spend on the engine.

I ran the engine and as before. Once it got hot ran rough. Did a hot compression check and all cylinders were 87 or better. So no stuck valve. But thought of a cylinder running lean. So I pulled the number one cylinder intake tube and it had oil in it. Pulled the plugs and they were black. So my final analysis is the intake valve guide is leaking oil and causing the plug to miss fire. I am going to pull the cylinder and have it IRAN. Hope that is the end.

Jerry,
Some oil in the intake tube is normal for all the aircraft engines I've worked on.
There is no seal on the valve stem like car engines.

You say "once it got hot ran rough". Could be a magneto coil breaking down with the heat. I still think it's your mags.

Mark

[lr172]

Originally Posted by Jerrygemma

I finally got time to spend on the engine.

I ran the engine and as before. Once it got hot ran rough. Did a hot compression check and all cylinders were 87 or better. So no stuck valve. But thought of a cylinder running lean. So I pulled the number one cylinder intake tube and it had oil in it. Pulled the plugs and they were black. So my final analysis is the intake valve guide is leaking oil and causing the plug to miss fire. I am going to pull the cylinder and have it IRAN. Hope that is the end.

Unfortunately a leak down test is not definative for a stuck valve. It often starts by sticking intermittantly and will close up before you can do the leak down test. The only definative test is checking the valve guide clearance (i.e. the wobble test). Once it sticks permanently, the leak down will confirm it if it is stuck partially open (not if stuck fully closed though) but by then you usually have bent push rod. If this were the case, and the issues was a permanently stuck (opwn)valve, your roughness would not come and go. Due to this fact and your intermittent condition, valve clearance measurement is required to rule out sticking valves. Also, excess oil in the combustion chamber will speed up the condition that causes exhaust valves to stick.

Not saying you definately have a stuck valve, only that your test doesn't eliminate it. Usually the valve will stick when cold and free up when hot, so it is only a possibility for your symptoms. As mentioned above, ignition is a possibility, as coils often fail only when hot. However, the cylinder fires pretty well on only one mag. You should be able to eliminate ignition by running on each mag individually when it is running rough if you have a good ear for your engine. Also, a bad coil will wipe out the whole ignition and would not present on only one cylinder.

Larry

[lr172]

Originally Posted by Jerrygemma

I finally got time to spend on the engine.

I ran the engine and as before. Once it got hot ran rough. Did a hot compression check and all cylinders were 87 or better. So no stuck valve. But thought of a cylinder running lean. So I pulled the number one cylinder intake tube and it had oil in it. Pulled the plugs and they were black. So my final analysis is the intake valve guide is leaking oil and causing the plug to miss fire. I am going to pull the cylinder and have it IRAN. Hope that is the end.

Plugs with dry black coating are typically from running excessively rich. Excess oil in combustion chamber will also result in black plugs, but they will be wet and shinny, assuming the oil issue is ongoing. You can also usually see oil pooling in the low point of the cylinder near the piston. Also, at higher RPMs, the combustion temp will typically burn the oil off fast enough to prevent misfires.

Also, if you can make it run rough on the ground, you can put a timing light on both the ignition leads to confirm if the plug if firing or not.

Larry