Hi All
Just wondering if someone can give me some advice, I have 2 A12 engines both with the same problem. They both run but are a little lumpy. Have done a compresion test on bothe engines with similar results. 2 cylinders have good compresion, 1 cylinder is a little bit down but is within 10% of the other 2, but cylinder 4 (nearest the flywheel) is down to about 30psi. Are these engines prone to rings going in this cylinder or am I missing something. Any sugestions or advice would be very helpful.

head is prob corroded (or loose or soft) and gasket leaking into cooling system
#4 has 2 big water passages behind it
check for bubbles in radiator when running
do a leakdown test or just make a fitting (from an old spark plug) and fill the cyl with air
it will come out the intake,exhaust,radiator or blow by into the sump highlighting where your problem is

These engines are not prone to ring problems. They are one of the most reliable engines every built. But after 30 years, they may need new rings. My A12 is 35 years old still running on the original rings.

Thanks for the pointers. The air test seems like a good idea, so I'll give that a try before going any further.
Thanks Again.