KZ400 Compression Rebuild Questions:

Hey all,

75' KZ400, D3, 5k miles, kick-only.

The engine has oil from the 80's in it... and that's about it.

I took the carbs off, with open-intake, and took out both plugs. I then threw the compression tester in, and after 10-15 kicks, the needle stopped moving around 100psi on the left cylinder, and 105psi on the right. I then threw about a teaspoon of 30w in both cylinders and checked it again and got 122psi on the left, and 125psi on the right. Stock we should be around 140-160 on a warm engine, so this shows a bit of promise.

I plan on tearing down the engine to do a full rebuild, gaskets, valve seals, and piston rings.

The kit of rings I received has 5 rings per cylinder.... 2 guide and 1 wavy for the oil ring, one SILVER ring with mild chamfers top and bottom, and one BLACK ring with a Teflon(?) silver edge. Both have markings for "top", but I can't find anywhere if the black or silver ring is the compression ring or the wiper ring... anyone know?

Also...(back to compression testing)... is the 100 dry and 120 wet on a cold engine a good sign? Should I even bother with the rings?

Thanks!

That's excellent compression for an engine that hasn't run in so long. I'd torque the head, adjust the valves, and leave it alone for now. The compression will come up some after an italian tune up.

Putting a teaspoon of oil in each cylinder will raise the compression a bunch even in a brand new, perfect condition engine; keep in mind that a teaspoon is roughly 5cc of oil. Here's why............

Even a very small amount of oil added to a cylinder can make a big difference in the compression readings. I have seen more than one person 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. As an example of 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. You can see how quickly the compression ratio jumps as oil is added to the cylinder. Ed

Nessism wrote: That's excellent compression for an engine that hasn't run in so long. I'd torque the head, adjust the valves, and leave it alone for now. The compression will come up some after an italian tune up.

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Thanks for the replies! I think an "Italian Tune Up" will suffice. Haha.

Not like it's hard to remove the motor and do a complete rebuild after I get the project running, a weekend job really... lol.