compression test

Just did a compression test yesterday and had 180# on cyl. 3,4,2, but 1 only had 90#. Re-tested today and with a little motor oil I was getting 120#. As it stands now I need to do a top end rebuild.

I was planning on doing both(top and bottom) anyway but a friend had questioned on why not do a compression test first .At least see what we have to work with.

BTW this was also a good reason to check out electrical system.Starter works great but no fire to plugs?

This is the 1980 KZ750E or SCUBA bike as we call it.This bike went under water!

Thanx

rayellis wrote:

Just did a compression test yesterday and had 180# on cyl. 3,4,2, but 1 only had 90#. Re-tested today and with a little motor oil I was getting 120#. As it stands now I need to do a top end rebuild....

Or valve clearance adjustments on #1 cylinder.

Good Luck!

Ray, compression #'s can vary greatly w/deaders. for example, a stuck float valve allowing raw fuel to pass or a stuck ring/s. will drop comp. in a single cyl. often simply getting running will restore #'s. check valve clearances, clean carbs and run a tank of gas thru, and re-check comp. before launching into a re-build. you might get lucky.G

hmm if it went under water , there is a good possibility that there is some rust/ debris under the valve.
I would check the vlave clearance like was said before, and chase down the elct. problems.
I would get it running first, then make a decision on what needs to be done to fix it.

If compression went up with oil it would be rings. If the valves open not seated compression would leak out through them.. That was an open throttle comp. check ?/

carbs not on bike at all.

Sounds to me a Rusty ring.Sea foam or a good penitrating oil to let soak in the cylinder. I haven't had my coffee yet.

Thanks for the advice I will try that!

I installed a MarkII engine on my bike and i had one cylinder down on pressure after checking valve clearance.

I have bad valve stem seals and i could see oil build up on the exhaust valve that was low.

I put some ringfree in the gas tank and have about two hours of running the engine and riding.

The cylinder pressure has come up 90 lbs.

I still have to install valve stem seals.Roy

you may want to do a leakdown check then you would be able to tell if it's coming from a valve or through the crankcase. Since it would show if you have a valve that is being held open due to not enough clearance. Just a thought.

roy-b-boy-b wrote:

I installed a MarkII engine on my bike and i had one cylinder down on pressure after checking valve clearance.

I have bad valve stem seals and i could see oil build up on the exhaust valve that was low.

Same thing got my 750. You may notoce the valve clearance actually get wider as the gunk builds up and the valve doesn't fully close.