A compression test on my 1992 FXR, Evo 1340cc of course, returned 150psi for the rear cylinder and 125psi for the front cylinder.

I understood a 10% variance was fine, but more than that is something to give pause. Is this true? Is the 17% variance something to worry about? Also, any ideas on why there might be such a variance? Push rods and valves are good.

Background is that the bike runs fine, except for a recent hiccup with the oil pump. Seems to have given out. So I am about to have it replaced with a high flow pump, etc. The bike has been through a Stage II, so a piston boring, new Mikuni carb, Ev-27 cam, single fire ignition, higher amp starting system etc etc. The bike is a pocket rocket. The rear base gasket does leak, so I was thinking that, while she is getting opened up (heads come off with my push rod type, etc) to replace the oil pump etc, I might put in James gaskets etc. I was wondering if doing the top end gaskets, aside from fixing the little leak, might also be relevant to the compression variance?

 

you don't have to pull the heads to do the oil pump. and if i was tearing it down and decided to pull the heads, i would go back with Cometic MLS gaskets. and replace the cyl base gaskets also.

 

thats pushing it on the difference IMO. you did have the throttle wide open so she could breath while doing the test right ?
no matter what base gasket you use , I highly recommend copper coat . and if you are going so far as putting a cam in it , why not use adjustable pushrods ?

 

To figure out where the problem on compression is, repeat the test after dripping some oil down into the cylinder and working the piston a couple times. If the compression goes back up, then your rings/piston is not making a good seal. If it still reads low, then your valves/seats are the problem.

 

Yep, throttled wide. But in your opinion, WHY is a 17% variance too much? That's what I'd like to get a handle on, as it will help me know how to interpret the bike (can't think of how else to say it). Push rods are adjustable, and I'll look into copper coated gaskets, so thanks for that tip.

Thanks for that tip. I knew there could be an issue with the rings/pistons and the valves/seats, but was not aware further compression testing could help isolate things.

 

copper coat is great stuff.

I say 17 % is to much because ( not that I'm a book thumper but.... ) tolerance for our engines is 10 % don't get me wrong , 125 is acceptable , but if your other cylinder is pumping 150 there probably is a reason and you might as well fix it now that you know somethings up or it gets worse

 

Opened her up. It seems the piston rings were not quite seated properly (to spec, etc).

Thanks for the advice, zukeeper.