Anyone familiar with yz125?

[km4nxa]

Had a project setting to the side for a while. Decided to start on it today. 2000 era race kart frame with an 89 yz125 motor on it, 2T. Never dealt much with dirt bikes. But for most part, a small engine is a small engine, two stroke or four way I try to look at it.

Guy claimed motor had a fresh build on it when it was brought by. I can visually see new components, gaskets, seals etc. Also top of piston looks new as new can be. So, I'm going to assume (for time being) it's a fresh build.

With that being said, couple questions for anyone that has experience with these. 1) what should compression be. I've googled till I'm blue in the face, nothing fount for a 88-89 us specifically. But it seems 130-160 is where a typical 2T dirt bike of that size should be. I'm getting 72psi at best wot. Before I focus too much on that, I want to make sure that the 88-89 models weren't an exception to the general 130-160 rule. Second, timing...I know on any four stroke scooter, I can pull the wire off plug, hook up timing light to plug wire, turn it over and light works properly. Do the same on the 125 motor, nothing happens. Light is set up for two stroke when I try it. There is spark, but it seems weak. Tried to different cdi boxes. Brand new stator. Do these engines have to be running in order for the timing light to work?

[moofus02]

The compression is low but I would guess the motor is dry. Shoot a little 2t oil in plug hole and try again. If it is fresh and dry the rings probably aren't seated yet and the oil should give you a seal that will give you a better idea

[km4nxa]

Will do. I took top off this evening after reading, turned piston about 3/4 down and put a few shots of it around wall. Will recheck in the morning if compression comes up any.

It leaked down to around 35 in three hours. Again, don't know if any leak down is acceptable on these or not.

[moofus02]

Jan 16, 2018 19:57:05 GMT -5 km4nxa said:

Will do. I took top off this evening after reading, turned piston about 3/4 down and put a few shots of it around wall. Will recheck in the morning if compression comes up any.

It leaked down to around 35 in three hours. Again, don't know if any leak down is acceptable on these or not.

You did a leak down test? Ideally they will hold pressure forever lol. But from 6psi to 3.5 in over 3 hours sounds like it is pretty air tight

[ThaiGyro]

Sounds like you are on the right track...many things can fool you. Some manufacturers give cold compressions...dumb in my book. Most often you should check at "normal"operating temps, hot and wet and sexy.

Run it...do a plug chop shutdown and hook up as warm as possible. Hot/warm leakdown will show better results. Moofus2 said it...looks OK anyway.

We just did a 125 not long ago. Did not need to check compression. Our project had carb shitty-ness. New word.

As for the ignition, they can be weak if tool old, but would be hard to start and/or not rev worth a shit. Doesn't sound that way?