Extremely low compression - help plz

[audionick12]

Hey all, Im pretty mechanically inclined but im getting stumped on this problem and I cant figure it out. I did a top end rebuild (piston rings, gaskets, o-rings, ball honed, etc)... now Im at a loss for compression. I checked my valves and rechecked my ring staggering but still getting extremely low compression. My manual calls for 150-215psi but im getting 80 max from all cylinders, any ideas??? Thanks in advance.

[Fingers]

Did you check if the bores were out of round when you redid the top end? Did you check your valve clearances when COLD? Did you make sure that the cams are timed and set correctly?

[md86]

Wow , 80 ? I thought my 110-120 was bad . Damn . What about a leakdown test to tell you exactly WHERE the problem is ?

[The CyberPoet]

Did you take the time to at least check the bottom end (the crank & journal bearings)?

What caused you to rebuild the top end in the first place? Do you know what the pressures were before you started? Did you use over-sized rings when you did the rings, or did you get a set of the same size? Normally you need to go up a size to compensate for the extra material you removed in the bore during the rehone...

Cheers,
=-= The CyberPoet

[audionick12]

Did you take the time to at least check the bottom end (the crank & journal bearings)?

Normally you need to go up a size to compensate for the extra material you removed in the bore during the rehone...

Cheers,
=-= The CyberPoet

Did that exactly, I got one size larger rings then had the cylinders honed.
Put on new head gasket and o-rings... only thing I can think of is my valves. Its just strange that I had good compression before this rebuild and I havent changed the valves at all.

[Anonymous]

ok this is gonna sound dumb but have to even fired this engine up since the rebuild you do realize that rings have to seat in before you get max compression right. just a suggestion

[rwcreigh]

good point sin +1+1+1+1+1

[audionick12]

ok this is gonna sound dumb but have to even fired this engine up since the rebuild you do realize that rings have to seat in before you get max compression right. just a suggestion

Doesnt sound dumb at all, however, I should be getting more than 80psi. Thats pretty damn low

[Anonymous]

ok this is gonna sound dumb but have to even fired this engine up since the rebuild you do realize that rings have to seat in before you get max compression right. just a suggestion

Doesnt sound dumb at all, however, I should be getting more than 80psi. Thats pretty damn low

ok this still doesnt answer my question have you fired the motor up and ran it at all it takes 20 to 60 miles to get new rings to seat properly and no on a fresh rebuild with no miles 80 sounds about right add a tsp of oil to each cylinder and try checking again does it up go if so fire the motor and take it for a test drive fluxuating the rpms in the first 4 gears not redlining it but revving it up threw the rpm range a good bit after 20 or so miles check the compression see if its starting to build up more thats why they say it take 50 to 200 miles to break in a new engine

[Fingers]

Adding oil will bump the compression but it will go back to crap as soon as it burns off.

Check your cam timing, as I said. If you have overlap, you will get poor compression because the exhaust cams will open too early.

[The CyberPoet]

Check your cam timing, as I said. If you have overlap, you will get poor compression because the exhaust cams will open too early.

The katana engine (and most motorcycle engines) have quite a bit of valve timing overlap -- intentionally. The exhaust opens, and then before it closes, the intake opens, using the suction of the exhaust leaving to pull in a larger charge (in theory; at low RPM's it doesn't benefit, but at higher RPM's it does). If he's checking his compression with his valve covers off, he'll be eyeballing the cam positions and know that both the intake and exhaust are closed... if not, suggest you do so.

Cheers,
=-= The CyberPoet