Hi all. I've popped this question on the main site, without any luck so far! My 54 Panhead is losing compression, and will not start. When I move the kick start pedal I get a whoosh from the oil breather hole just below the front cylinder exhaust valve spring. Have changed the base gasket, head gasket, checked the piston, bore and rings, checked and lapped the valves, it ran for a while then next day it went the same way! Compression test reads front 35 dry, 60 wet, rear 60 dry, 80 wet. Did a smoke test with no major leak.

Suspect the rings, but whistling in the dark! Getting really cheesed off as missing most of the summer. Any ideas? I think I'm right in saying the crankcase is under compression as well as the pots which would explain the permanent whoosh. HELP!!!

 

Have you checked the pushrods. If you have hydraulic lifters, one of them might have failed or collapsed.

 

Start with checking pushrods. Have you done a real leak down test? A smoke test doesn’t have enough pressure and is really made for troubleshooting vacuum line issues. A real leak down test will help you determine where it is leaking from. Hissing in exhaust=exhaust valve, hissing thru intake=intake valve, hissing thru breather=rings, hissing around the head=head gasket. Intake vs head gasket can be tricky to pin point, a stethoscope helps here tremendously.

 

Thanks guys. Solid pushrods, not hydraulic, and I get the hissing even when the pushrods are removed!. I have now taken the head off (again), and the hissing still comes up the oil return. Weird!
Not 100% sure what a 'real leak down test' is? Would be glad of some info on that. For now, as I've got the head off again I'm going to replace the rings, more new gaskets, and see what happens.
Fortunately, these are relatively simple bikes to work on, or I'm sure I would have torched it before now!
Any other suggestions would be gratefully received

 

https://www.mobil.com/en/lubricants/...-leakdown-test

 

you need a experienced set of eyes to look and check the condition of the cylinders and the valve seats

as its open again now and you can hand carry it and find out what they say