'77 KZ650 Troubles and Questions

650ed wrote: ezrider714 wrote:
"Also a compression test is not a volumetric test, makes no difference what amount of the compression chamber the oil may take up."

What??? No offense intended -you may have done this on hundreds of motors, but it defies the laws of physics.

Please try this - dump 1 teaspoon of oil in one cylinder and dump 3 teaspoons of oil in another cylinder then run a compression test on the engine to see if the compression of the 2 cylinders varies. You will quickly prove that the amount of the compression chamber the oil may take up matters a great deal. Ed

Boy let's not take it to the extreme... Let's fill one cylinder all the way up with oil then we'll know that cylinder has bad rings, cause the pressure would be so low :dry: Who would ever do that????? :ohmy: And it still only shows the volumetric difference between cylinders with various volumes of oil Not cylinder pressure in PSI Bottom line is you are looking for numbers close together, any cylinder up or down by alot is almost always a problem :blink:

Forgive me for being dense, but I still don't get it.

It sounds like you are saying the following:

-Let's say I have an engine with perfectly good rings, valves, etc.
-In one cylinder I put a small amount of oil
-In another cylinder I put a considerably larger amount of oil
-Doing this changes the volume of the cylinders quite a bit when the pistons are at TDC, so there now is a volumetric difference between cylinders
-But even though the volume of one cylinder is now greater than that of the other are you saying the compression will be the same?

If that is your theory, how do you explain the fact that installing higher domed pistons in an engine will raise the compression even when no other changes are made? I believe the reduction of volume in the cylinder will have the same affect on the compression whether the volume is displaced by added oil or a higher domed piston. Ed

The theory is if you are experimenting anyone that had any clue to what they were doing would not put such a vastly different amount of oil in different cylinders :ohmy: I was just trying to point out that the small difference in oil per cylinder you were referring to is for all intents and purposes immaterial :dry: Also oil will leak between the piston and the cylinder wall until it hits the first ring, whereas a domed piston will not change shape or lose volume.You have a penchant for comparing apples to oranges :blink:
Alot of your answers seem to forget these are 30+ years old motorcycles not a new NASA lunar spacecraft, where things are measured in thousandth of a mm.... :whistle:

As an aside a domed piston does not change the compression of an engine it changes the compression RATIO, and to that point a compression test does not measure the compression ration, but the rise in air pressure in PSI thru the stroke of the piston.....

Unfortunately, I suspect many (most) folks who add oil during a compression test do not precisely measure the amount of oil they add. As I'm sure you are aware, even a relatively small variance (a couple cc's) in the amount of oil added can make a pretty big difference in the compression readings. I have seen more than one person (not you) advocate adding "about a teaspoon" of oil to each cylinder when doing a "wet" compression test. That may not sound like much oil to many folks, but a teaspoon is actually right around 5cc, and that is a lot. For anyone reading this thread who may not have an appreciation for how little oil it takes to change the compression ratio I offer the following diagram. It uses a hypothetical cylinder with a total volume (cylinder & head) of 100cc and a dry compression ratio of 10:1. A little oil added changes things quite a bit, so if different amounts of oil were added to different cylinders one can see that the compression ratios might vary a lot. This example is not precise and is not any specific engine and it doesn't take into account oil that may seep past the rings, etc. It only is meant to demonstrate in very general terms how adding oil can change compression ratios. Ed

Very awesome diagrams a great visual to what i was saying that the oil only changes the compression ratio....
And yes the third one is just a carton, :ohmy: there is no need to put a teaspoon of oil in any cylinder :huh:
If you follow through the math on the first 2 examples you will see the PSI readings will be 147 PSI and 161 PSI which fall in to the 10% rule..... But always remember for every rule there is an exception... :blink:
Since few of us are able to work in a climate controlled/,ultra clean environment tolerances and measurements must adapt to the world they are being used in....

Kraneeum, both 650ed, and ezrider714 are giving you good info !! To help you, I recommend getting a factory service manual for your bike, and follow there guidelines on checking compression, and the manual will also help you to understand most everything you want to know about carbs. Then if the manual still leaves you with questions, which it will, then post them here, and you will get the best advice to be found, and you will also have the manual to help you understand what the guys here are trying to teach you. Good luck !!

Just my .02

Hey all,

Sorry for the delay, but I am finally able to give an update. First off, thank you all for the help and great discussion, I've learned a lot.

After doing a leak-down test, NO LEAKS were detected!!! Great compression in a ll cylinders. Suffice it to say, I am overjoyed that i don't need any major engine work.

This leaves me to believe i simply need a rejet and carb balancing. A valve clearance adjustment couldn't hurt I'm sure.

Not related, but since I last posted I added some new drag bars! imgur.com/a/ZhYy0

Thanks again!!!

I'm glad to hear the compression is good across all cylinders. I suspected that might be the case. Ed