Low compression

[Monty]

1969 late model A-4, Cylinder # 3 has 30 lbs pressure all other have 85 pounds. Tried added oil to cylinder #3. Saw no increase in compression. Engine backfires under load. ? time to remove head and check values. what should I look for?

[Don Moyer]

Monty,

The oil check to determine whether poor compression is due to valves or rings is not really fool-proof. For example, if you poured the oil directly into the spark plug hole, most of it simply left via the exhaust manifold and didn't work its way in and around the pistons.

Removing the head will give you the opportunity to examine the valve seats in the block, and if you find the seats very irregular and/or corroded, it's probably worth the effort to remove at least the valves from cylinder #3 to reface the seats and replace the valves.

Compression of 85 psi is not unusually low for an early model engine, so if you can get the third cylinder values back up to the same as its neighbors, you should be able to continue using your engine for at least several more years.

Don

[Monty]

I removed the head and found that the exhust vale was not seating properly,.
On cyl #3, I have since removed the rest of the values and will resurface the face and seat's

Several years ago I added eletronice ignation and fresh water cooling because the inside block was so clean. This time I see the block has a black color on it.
The engine was salt water cooled and is now fresh water cooled with antifreeze. I Also used a 90 Degree fitting on the cooling plate with out any water being diverted. The temperature is control at the heat exchange . No bypass hose is used to the thermostat.