justinbravo

Member
Oct 27, 2007
3
0
i just recently bought a 93 yz 125 and it ran perfect. then the other day i was riding some trails and i lost power in first two gears. if i really stay on it i can reach the power band in second but really i dont have any power until i hit the power band at the end of third then it takes off like a bat out of hell. ive already changed the spark plug. cleaned the carb. checked the packing. did a compression test. im at a loss. ive heard it could be the power valvle and govenor setup but i need some advice as to what to look for on these parts. the fuel mixture is 40:1 this is what the guy i bought it from said he has ran it at since he bought the bike new.
 

rmc_olderthandirt

~SPONSOR~
Apr 18, 2006
1,533
8
Sounds like the power valve to me.

On the right hand side of the engine (Kick starter side), just above the where the exhaust pipe attaches to the cylinder there is a plastic cover over the mechanism for the power valve that will be held on with a couple of screws. Remove that cover and you will be able to see the power valve in operation.

Go ahead and start the bike up with this cover off, it will be fine. As you rev the engine up you should be able to see the mechansim from the governor (buried down below) operate the power valve (you can see the end of the power valve shaft). If the power valve rotates freely as you rev the engine up then it is probably okay. If it doesn't move, however, then it it probably stuck.

You should probably get a manual to get the proper directions for how to remove the power valve and especially the linkage without damaging anything. On my bike (an older YZ125) there is a spot that you can insert a pin (like a nail) to lock the linkage so that you can remove the nut on the end of the power valve without transfering the torque down to the govoenor.

Once the linkage is off the right hand side you remove the cover on the left hand side. Reach in with a "allen wrench" to access the screw buried deep in the center of the power valve. Remove the screw and then the power valve will split down the center allowing each half to come out its respective side.

Rod
 

slodad

Member
Sep 4, 2005
99
0
Sounds to me like the wheel is spinning in 1st & 2nd gear and it's finally hooking up in 3rd so that you can feel the acceleration. Try riding it somewhere with good traction to see if it's an engine issue or a traction one. There's no reason that the engine behavior would vary from gear to gear. If there's no power in 1st or 2nd, there should be no power in the other gears either.
 

a454elk

Mexicutioner
LIFETIME SPONSOR
Jun 5, 2001
7,538
18
Doesn't make sense that it works in 3rd gear but not 1st and 2nd. It has to do with the engine not the gearing, unless your clutch is slipping in 1st and 2nd. I would check that route, it sounds like yor power valve is working if it kicks in at the top end of 3rd. Can you hear it rev up high in 1st and 2nd gear like it does in 3rd, but you have no power? Or does it bog in 1st and 2nd then opens up in 3rd? Put it in 2nd and hold the brake, rev it, let the clutch out slowly and see of it slips. Dont' pop the clutch or you may loop it! Good luck
 

justinbravo

Member
Oct 27, 2007
3
0
my dad was invited to a private track and had a 2 stroke guy look at the bike, it was the little plastic piece that holds the needle in position in the main jet, so it was loading up until higher rpms where the gas was needed rich. 25 dollars later im back and running something so simple thanks for all the help everyone
 

a454elk

Mexicutioner
LIFETIME SPONSOR
Jun 5, 2001
7,538
18
Nice, thanks for the update. ;)
 
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