kz750 twin pistons/rings

Okay so After I started having a spark plug problem in one of my other topics, I started to think it wasn't the plug. I did some research and I am thinking my piston rings are wearing out to the point that oil is getting onto my spark plugs. I started the bike, ran it around the block a couple times till it was hot and gently removed plugs for a compression test. These are the results sitting on the bike. Left chamber is a hair line above 90 and right chamber is a hair line below 90 . Can anyone tell me if these readings are an indication that I need new pistons and rings? Whats the compression suppose to be at? -Tyler

It should be above 100 cold.
Have you checked valve clearance?
Valve guide seals could have dried out.
Letting oil pass thru to the valve guide to the combustion chamber.
When you checked compression did you pull both plugs,throttle wide open?
Check your compression cold and add a tea spoon of oil to each cylinder.
If the reading is higher it's rings that are bad.

.

I suggest you also perform a leak down test. This may help identify if compression is leaking past valves or rings. Ed

Ill try the leak test . im hoping its the valves

Schmeck wrote: Okay so After I started having a spark plug problem in one of my other topics, I started to think it wasn't the plug. I did some research and I am thinking my piston rings are wearing out to the point that oil is getting onto my spark plugs. I started the bike, ran it around the block a couple times till it was hot and gently removed plugs for a compression test. These are the results sitting on the bike. Left chamber is a hair line above 90 and right chamber is a hair line below 90 . Can anyone tell me if these readings are an indication that I need new pistons and rings? Whats the compression suppose to be at? -Tyler

Service limit for 750 twin is 115 psi HOT. Low compression can be rings but it can also be valves.

Oil on the plugs is almost always worn out valve guide seals.

Okay so I sprayed alittle bit of oil in the chamber, turned it a couple times to get it around and then checked compression, Hasn't changed at all. Actually when the bike is warm or cold the compression doesn't change much. Cold Im at 90 and hot im just a hair line above. The compression would have went up if it was the rings. Sooo I take it my valves need to be serviced? Maybe that valve guide you mentioned is worn out?

Valve guides or seals probably don't affect compression. The valves need to be lapped (at least) so they seat properly.

If the valves clearances arent within spec that could lower compression right?

Yes. If there is not enough clearance the valves may not be closing all the way or may not be closing until late and opening early. Either of those conditions can reduce compression. Ed

650ed wrote: Yes. If there is not enough clearance the valves may not be closing all the way or may not be closing until late and opening early. Either of those conditions can reduce compression. Ed

+1 , a little loose is better than a little tight


.

what feeler guage size do i need ti measure my clearance ? smalled i have is .008 then .010 and neither of them fit . the lobes are up and the shim is touching.

The acceptable clearance range on my K650 is .00315 -.0071 inches or .08 - .18 mm. Yours may be somewhat different, but it sounds like you need a different set of feeler gauges if the smallest you have is .008 inch. Ed