Low compression

[dirkmc3]

Ok...I just got good news/bad news from my mechanic... I have a 73 20 hp Chrysler in pretty good shape physically

Good news..my engine has new points,condensers already in it and the carb has been re-built somewhat recently. Coils are in great condition.

Bad news..50psi on the top cyl...
my question....I dont have alot of $$ into this motor so I'm considering the option of repair.

What are the main causes for one cyl to drop this psi ? If there is an easy solution it would be worth investigating..Are these angines prone to anything in particular?

Head gasket?

Stuck ring?

Bad piston?

Cracked Head?

Thanks!!!!!

 

[jaybirddog11]

Re: Low compression

It could be any of those listed, best way to find out is to pull the head. Maybe you get a little lucky and only need a head gasket. If all else looks ok, a decarb may help unstick your rings.

 

[dirkmc3]

Re: Low compression

It could be any of those listed, best way to find out is to pull the head. Maybe you get a little lucky and only need a head gasket. If all else looks ok, a decarb may help unstick your rings.

Would there be any telltale signs of a bad head gasket vs stuck rings? The head gasket line looks clean and dry.


Also..

thanks!

 

[jaybirddog11]

Re: Low compression

Not that I know of. If your motor runs but you are hesitant to pull the head, you could do a decarb first to see if that brings up your compression.

 

[dirkmc3]

Re: Low compression

Ok...so I finally got around to pulling the head and noticed a couple things right away..the top cylinder (50psi) piston is super clean and shiny like it was scrubbed clean. the bottom cyl has the normal carbon type deposits (not bad though). I pull the gasket off and there is a 1.5 inch long area on the edge adjacent to the cylinder wall that looks slightly burnt and the internal metal of the gasket is exposed. So...my conclusion is a bad head gasket.

The cylinder walls are not scored and are quite smooth and polished.

do head gaskets just blow after time or does something usually cause this? I'm going to look very carefully at the powerhead where the blown gasket portion is to see if there is any crack. Would it be obvious??

 

[jaybirddog11]

Re: Low compression

I would think a crack would be fairly obvious, but it never hurts to go over it carefully just to make sure. If the gasket is old and the motor has not been run in a while it could blow a gasket. Also, if the bolts were not tightened according to specs, that could allow water to seep into the pistons. Hopefully one of the guru's can add their 2 cents.

 

[dirkmc3]

Re: Low compression

thanks for the input. After close inspection prior to dissasembly the paint was chipped on the head bolts in a way that would indicate prior dissasembly. So....could be that the head was not torqued properly. Inspection of the powerhead doesn't sow anything resembling a crack so I'll chalk it up to a head gasket and drop the $40 to replace it.


I just ordered a head gasket and would like to know if there are any things I need to mindful of when replacing it ?