Bad Compression (head gasket?)

[elaninfuture]

I've been slowly getting my '66 FHC on the road. The car sat for many years with PO. I went through the fuel system and got it running on they key. Doesn't idle well, but didn't run so poorly that I was worried. It had an exhaust leak, so I pull the exhaust manifold to install new gaskets, at which point I noticed what looks like baked on oil that appears below the head at the two most rear cylinder. It starts where the head meets the block.

A buddy came over with a Sears compression tester. The results (they are cold without wide-open throttle) are worrisome --135, 135, 55, 110 (lowest findings corresponding with cylinders that match the oil on block).

[tdskip]

Oh bugger - not encouraging but at least you figured out some of the backstory.

How do the oil and coolant look?

Have you done a leak down, compression change when oil put in the spark plug hole?

[elaninfuture]

Oil and coolant look fine. I haven't done a leakdown. I didn't add oil to the cylinders to check the rings. But I did notice the plug in the bad cylinder was heavily covered with oil.

[alan.barker]

Just take the Head off which is a very quick job. How many miles has the Engine done. If it's done more than 60,000 miles since last rebuild it will need a new set of Piston Rings before some of them break and score the Cylinders.
Alan

[dougal9887]

This sounds typical of worn valve guides.
Excessive oil running down the stems cokes the seats and results in a poor seal and low compression reading. The coke in the port would seam to confirm such. A leak down test and a stethoscope held to the port would confirm.
Pistons and rings will generally wear evenly and should not usually be the cause of uneven cylinder pressures.
IMHO !!
Dougal.

[alan.barker]

If the problem is the guides you will have deposit on the back of the Inlet Valves. Where the oil is sucked done when you close the thottle. I have found when a Twink has done more than 60,000 miles ring landings wear and some Rings break. While you have the Head off i would suggest you remove and check Pistons before Cylinders get scored. I would not say the compression problem was te Rings before dismantling Engine. Many times compression problems are burnt Exhaust Valves usually number 4 or number 1 where the Vacuum connections are, causing weak mixture which burns Valves

Alan

[2cams70]

You must do the squirt oil in cylinders and retest compressions check to determine if it's a bottom or top end problem. Twin Cam engines when built properly are very reliable. Especially the bottom end. Rings generally aren't prone to breaking unless they are overly stressed when fitting or the ignition timing is wrong leading to detonation. Many cases I've come across where a shoddy machinist has not machined the valve seats properly - eg. not concentric with valve guide or seat angles wrong. Sometimes they will machine the seats without replacing the guides. The resulting excessive runout of the cutter means the cut isn't concentric or aligned properly with the guide. When first assembled the engine may appear to run ok but it won't last long and problems will soon start to appear.

[alan.barker]

Imho 60,000 miles for a Twink between refreshing is not too bad.
when i bought my Sprint it had 68,000 miles with oil leaks. So i dismantled Engine to fit new gaskets and seals. The Pistons were standard but the Piston Grooves were worn and 2 rings broken. I bought a nosset of standard Pistons and deglazed the Cylinders no problem since.
When i bought my Plus Two S it had 65,000 miles on the clock and i did the same finding 3 broken Rings and a burnt Exhaust Valve number 4. After no problems.
Both cars running on standard bores.
Alan

[2cams70]

Which rings were broken as a matter of interest? Compression or oil control?. If you break a compression ring you usually know about it - horrendous blowby!! The fumes out of the breather on a car I once had that was so afflicted were so bad that I had to run a length of heater hose from the breather to the outside to avoid suffocation until I had a chance to dismantle the engine. I've had some pretty ropey modes of transport in my past!