YZ125 Low Compression

Did you measure the cylinder or did it just "look good"?

90 psi is very low. The engine should not kick start with it that low, but it may bump start.
Did you do a "wet" compression test?
Have you checked your compression guage?
Have you checked it with another guage?

Did you check the ring end gap when you rebuilt the engine? What was that measurement?

There are a lot of things that can cause low compression.
Wear, leaks and etc.
Using the incorrect head piston and cylinder combo of parts.

Paw Paw

Just a note for reference: in an old dirt rider magazine I have, Yamaha engineer Ken Clark says to ALWAYS use the ‘C’ piston in ANY yz cylinder regardless of the letter the cylinder has on it because it’s safe and makes better power due to better tolerances. 🤷‍♂️ Makes you wonder why they bother with the lettering system then🤔

Wet compression test is putting a small amount ( 1/2 teaspoon) of oil into the cylinder and the do the compression test. You reading should be higher than the dry test.
You do need to have the cylinder measured.
So if the bike cranks up easy and runs good, then what is the issue?
Low power out put at your elevation?
What does your spark plug look like?
Do you just need to modify the engine for higher compression as needed for high elevations?

Paw Paw

125 should be between 130-160+. With a new piston should be closer to 160 IMO. When was the last time you replaced the clutch plates? Could it be your clutch is slipping and not getting the power to the ground?

Paw Paw and others have hit the relative points. 90 psi is way too low.

Since it sounds like you are doing this all yourself and mentioning piston size etc, I have a few suggestions to consider if you are going to be doing this long term on your own to take the guess work out it.

If it's in your budget, consider purchasing some proper measuring tools. A caliper and feeler gauge only takes you so far. I only do a few rebuilds a year, but made this investment. To figure out proper piston sizing as well determining the condition of your cylinder you need to be able to consistently measure them.

A T Bore gauge or Dial Bore gauge are the only ways to really measure you cylinder right. I tried for years to get consistent measurement with a T Bore snap gauge, but there is a lot of skill to get it right. Machinist seem to do it in their sleep. I never mastered it and bought a Dial Bore gauge.

You can't get a proper consistent measurement of your piston with an ordinary caliper. You really need a micrometer (which btw you will need for T Bore or Dial Bore anyways.) A decent quality used non digital one is very affordable.

With these you can take out the guess work out of piston sizing and inspecting your cylinder. You can insure you have the proper clearance too. A feeler gauge and caliper simply can't do it.

If you absolutely can't get these tools, you can use some feeler gauge stock strips to measure the piston to cylinder clearance of what you have, but not size for what you need to buy. Feeler gauge stock strips are 12" or longer and can be purchased for a few bucks a piece. You really can't just use a regular feeler gauge (too short) to get a proper clearance measurement up the length of your cylinder. The only thing a regular feeler gauge is good for in this process is ring gap.

I am not a pro mechanic, but these have helped me build motors for myself successfully. Lots of folks build their own motors without them, and 9 out of 10 times they will be successful. But if you really want to take out the guess work, a few more tools are very helpful in my experience. Used, about the price of top end parts and a good investment in my opinion.

Good luck!

New oem Yamaha clutch