Start with a compression check. Next I have seen those old 225 slant
six's wear cam lobes of sometimes (it actually happend to me once
about 30 years ago)
1. Bent/burned valve
2. Blown head gasket
A compression check, as SnoMan suggested, will tell you whether or not you
have a cylinder sealing issue. If you do, forcing compressed air into the
sparkplug hole will tell you where you're losing compression:
1. air coming out of tailpipe = burned/bent exhaust valve
2. air coming out of carburetor = bent intake valve
3. air coming out between cylinder block & head = blown head gasket.
4. air coming out of crankcase = bad rings
If items 1, 2, or 3, you'll be pulling the head. If item 4, you'll be
pulling the whole engine. To do the test, you'll need a source of
compressed air and a tool such as this:
You're welcome. I forgot to mention -- you need to test with both valves
closed. If you first remove the valve cover, you can tell when the lifters
are on the camshaft base-circle. You'll also be able to see if there's any
valvetrain damage (bent pushrods, broken/worn rockers, etc).
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