Yz 125 hey just bought a 98 model yz 125 and its fouling plugs pretty quick and it has low compression bout 100 - 105 psi should i do the top end and powervalvues cause it stillstartsfirst kick and runs like a champ

I would be checking compression it may not be visible score on cylinder but it does not take much to loose compression on small bore motor with one piston ring and 11 years old

120 psi means you need top end( should be around 160psi) if you can feel the score marks with your finger you will have to have the cylinder bored and install a over-sized piston kit,if you cant feel them than you can have it honed.....now the stuck power valve is why you didn't have any power,take them out and clean all the carbon deposits off them and reinstall, pay close attention to how they come out so you can put them back the same way...if you have a manual you can check the adjustment specs on them also.....now on to the crank,the first indication of bad crank seals or bearings is losing your trany fluid,if you don't feel any unusual vibration or hear any unusual whining or have a large amount of lost trany fluid your probably OK with the crank.......yz-maniac

Hey E..
176psi is fine... I have been a factory trained Yamaha for 43 years...I have seen engines run fine with 90psi..(not racing engines).. Compression is a good indicator of overall top condition of an engine. Compression Ratio is different than compression psi. Now that you have established your baseline.. you want to maintain it!!
....If you want the engine to last.. run at least a 28:1 OIL/FUEL ratio.. even a 24:1 or 20:1 is better *... settle on a ratio and RUN this ratio at ALL times... if you are fouling plugs.. get a PROFESSIONAL that KNOWS how to "jet" an engine.. you should be able to drop the main jet one size..and maybe raise the needle jet clip UP one groove..this will lean out the mixture to the correct FUEL/AIR ratio and you won't foul spark plugs and the engine will LAST a LOT longer.. >> ... *Remember..the two-cycle oil you mix with the gas ALSO lubricates the main crankshaft bearings and the rod bearings.. (replacing those are $$$$$!!)
.. If you have more questions, reply to me on here...

Fouled plugs are typically caused by jetting that is too rich. This could also be from a dirty air filter which can make the bike run rich. Clean up your filter and see if that is it. If the filter is not too bad, you might want to drop a size on your pilot or main.

Another possibility is poor compression - this can make a 2-stroke run richer than normal as the fuel can not be burned properly. Rings don't last long in these 125s. I used to re-ring each week when I was racing 125s.

You might want to run a B8ES plug, too - this is one step hotter and will help the plug from fouling as often.

A compression test will tell you your condition. (Throttle wide open while doing that) If that's good, your jetting is too rich, could be the issue.

NGK BR9EG is the stock plug for your bike. You are one heat range hotter already. I suggest you go to a BR8EG to keep you in the racing plug style but one range hotter than stock. I question what 2 stroke engine oil you are using. Stick with Yamalube 2 stroke oil and you should be okay. If you are using standard motor oil like you have in the gearbox then the fouling is definitely the oil.

You have a 50/50 chance that they will work okay. All the mounting holes need to line up and the cooling holes as well. Get a head gasket for the '98 and see if all lines up. Will the exhaust pipe fit the '95?

sounds like crankcase seals may be bad

2 stroke bike are very dependent on compression. and being a bike that is 12 years old my best educated guess is that the top end needs to be rebuilt. i usually have to have the top end done on my honda about every three years.