Low compression cont'd

So here is a continuation of my thread in the carburetor forum. To catch everyone up to speed, I discovered that I had horribly low compression in ALL four cylinders, checked valve clearances and they were all within factory specifications.

WELL.... I was urged to try the compression test again after pouring a teaspoon or so of oil into the cylinder before testing.
Waddya know???
The compression jumped from 50 psi to about 180 psi instantly. I continued on to test the other 3 cylinders and the same thing...

Wired George (bless his soul) said that if this were the case then that points to my piston rings not sealing. My question now is: if all four cylinders needed new piston rings should I be concerned that the bore has been damaged on all the cylinders? Like some dumb ass ran the bike with no oil for a period of time, or something horrible like that?

Bicycle Lee wrote:

...horribly low compression in ALL four cylinders, checked valve clearances and they were all within factory specifications...compression test again after pouring a teaspoon or so of oil into the cylinder...jumped from 50 psi to about 180 psi instantly...other 3 cylinders and the same thing....

Just to be sure --- was the throttle held wide open during the firt test, with all plugs removed and spinning engine over rapidly with electric starter?

If any doubt, would repeat compression test without any oil added, and with throttle held wide open, to confirm the psi readings.

Does the motor now run with the low compression? How is the performance? Should also perform compression testing before engine has completely cooled down (as a basis for comparison).

Did the engine sit unused for an extended period of time so that the rings might have possibly become frozen inside their grooves?

Good Luck!

And what is mileage and maintenance history?

If it is actually the rings and bore causing the loss of compression, and not stuck rings, then more than likely you will have to bore to the next oversize.

I measured six 750 four cylinder blocks, and only one was still good enough to still be within service limits after running a glaze breaking hone through it. all the others were egg shaped. Good thing I have a set of .020" GPz750 pistons and rings, and a 810 kit.

IMHO, that large of a shift in compression means the engine needs work NOW. At minimum, hone and install new standard rings on the old pots. next up the list, hone and install new standard rings and new standard pistons. Best would be bore up 0.25mm and go with new oversize pistons and rings.

yes, the engine DOES need work NOW. And it is going to GET work NOW. I'll post pics soon, but I have removed the engine and set up a nice work station in my spare bedroom (my "garage" is not a safe place for this type of work)
I am getting new gaskets and piston rings for sure. Is there a way to determine whether boring it out is absolutely necessary? Like I mentioned on the other forum I'm not a rich man. More of a "paycheck to paycheck" man so I'm trying to do this piece meal. I don't want to OVERDO it when I don't need to.

Bicycle Lee wrote:

...getting new gaskets and piston rings for sure. Is there a way to determine whether boring it out is absolutely necessary? ....

Per FSM for 1978 KZ1000, determining whether cylinder bores are within specs requires six separate measurements inside each bore.

Being side-to-side and front-to-back measurements at three locations inside each cylinder bore:

10mm below top;
60mm below top;
20mm above bottom.

Service limit is 70.10mm or more than 0.05mm difference between any two measurements. (Compared to standard 70.000-70.019mm with less than 0.01mm difference between any two measurements.)

Standard piston/cylinder clearance is 0.050-0.069mm.

And here's a very good earlier thread about this.

Click Here

Good Luck!

Take it to a machine shop, along with the relevant specs (listed above) and ask them to check it for you. It only takes a few minutes, so they probably won't charge for it.

seeing as how the engine itself weighs a couple hundred pounds, what parts do I have to take? Can I just disassemble the head and take it down?

Jugs alone would be okay. They are removed anyway to replace rings. Would take head also and get checked for any truing needed.

Bicycle Lee wrote:

yes, the engine DOES need work NOW. And it is going to GET work NOW. I'll post pics soon, but I have removed the engine and set up a nice work station in my spare bedroom (my "garage" is not a safe place for this type of work)
I am getting new gaskets and piston rings for sure. Is there a way to determine whether boring it out is absolutely necessary? Like I mentioned on the other forum I'm not a rich man. More of a "paycheck to paycheck" man so I'm trying to do this piece meal. I don't want to OVERDO it when I don't need to.

It takes a precision inside diameter micrometer.

If the cylinders are not servicable without boring, you might want to look for a used cylinder from a wrecker. Boring menas about $75/hole machine shop cost plust all new pistons and rings (got to be at least $200). I found a place willing to sell me a good used head complete with valves, cams, cover, etc for $100. If you find a used cylinder that doesn't have too much wear it would let you put some new rings on your standard pistons and run it for a while longer.

Bicycle Lee wrote:

seeing as how the engine itself weighs a couple hundred pounds, what parts do I have to take? Can I just disassemble the head and take it down?

You need to remove the head and then the cylinder from the engine. Take both to the shop. They can check the ID of the cylinder and see if the cyl and haed are true (not warped). They will also check for cracks.

If you don't want to remove the cylinder, you can check the inside diameters (as listed above in another post) if you can borrow a micrometer. After you have the head off, rotate the crank to but the piston at the bottom and measure the cylinder wall diameters. Repeat for all four.