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hi all new guy here with kind of a issue maybe lol i dont know yet. i have a 95 yz 250 that had a bad piston and rings so i decided to ship it out have it replated and install new piston and rings and most of the upper seals. The problem im having or even if its a problem at all but was 95 just a slower less power full bike? the bike runs good starts first kick most times and so far for the 8-10 hours of riding ive done it has been a very reliable bike but my god is it gutless!! just feels like the power isnt there! my buddies 89 rm 250 feels like it has 2 or 3 times the power of my bike nad most 125s even seems to be faster and more powerful so im feeling like something is wrong. the bike feels like low end is good but mid range and high end it just falls off like the power just isnt there.is that just how a 95s run? so now im questioning what can i do to adjust the mid range and high end power? Or is there even any adjustment i can do? i kinda need to just be pointed in the right direction on what to check first or if i even need to check anything at all? Any info would be greatly appreciated! thanks a lot and ride safe!

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From my knowledge, the early - mid 90’s mx bikes were some of the snappiest... I had a 99’ CR 250 at one point and that was like riding a swarm of angry bees...

 

What was it running like before the rebuild?

 

I have heard stories over time that the power valve assembly / linkage bolts can vibrate loose and flog out the dowels / dowel holes that hold the powervalve ‘cam’ together.

 

Can’t really quantify, I have honed a few earlier YZ cylinders but thats about it to be honest. Never really played with them.

 

Next thing would be going back to basics. Check how your plug is burning, check fuel / mixtures etc.

 

 

 

 

 

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No shortage of power in the mid 90s Jap MX bikes including the YZ.

That was a period where a lot of engine changes were being made so its important the parts you get are for your year.

Power valve needs regular maintenance as mentioned. The governor bearing and thrust washers wears as well. 

Do a compression test to make sure everything's sealing OK and its got the right piston (height) in it. Shouldn't be lower than 190 psi and should be pushing 220+ psi if it's been tuned.

Check / clean carby, has it got stock jetting or close to it?

Check the reeds, they lose tension over time and can fray or chip. Usually they are changed at least at every couple of pistons.

Poor ignition can cause issues - but most will stop it running or make it run with a lot of breaking up if there's a problem.

 

 

 

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thanks for the responses and sorry it took me so long to respond back just been very busy with work. the bike is stock far as i know and i dont know how it ran before because when i bought it, it was not running. Millennium Technologies is who did all the cylinder work and thats who i got all the parts from. i have yet to do a compression check yet because i was unsure on where to start.Also im using castor 927 mixing it at 40:1 using unleaded gas is that a wrong mix ratio? i pulled the plug today and its oily black so im assuming my mix ratio must be wrong. when i search online i see a lot of people saying they run 50:1 which is almost a ounce difference in mixture or like .7  could one ounce of oil make that big of a difference on how the bike runs? for some reason i feel like its a power valve issue only because thats the only thing ive never installed on a bike before so i was kinda unsure on how it went. i installed it in the same position that i took it out but i dont even know if it was correct when i took it out. is there anyway it can be adjusted?  

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Sounds like the power valve isnt opening. Pull lid of the power valve linkage cover ,  and check it all looks good then start and rev the bike and watch it open . If it does not that is your problem .

 

 

 

 

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If you had the clutch cover off the power valve linkage may not be sitting in the centrifugal clutch correctly . But it's pretty hard to put the cover back on if Its not right too.

Down load a service manual free from yamaha it goes through how to adjust it etc 

 

It very important your power valve parts are in good condition ( in particular the stop ) if this is worn the power valve can hit the piston and it will be all over sooner rather than later .  I have one such motor in my shed . I'll take a pick of score in the barrel. I bought it like this so not my doing. 

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i just gotta say you guys are AWESOME!!!!!!! i pulled the linkage cover off started and reved the bike and sure enough the linkage was not moving at all. so i traced it down to i think was part of the governor it kinda looked almost like a gear looking thing with a spring behind it but i noticed there was like a key missing to keep that gear looking thing from spinning so i cut down a hex key n made a piece to fit and BLAM it now works GREAT! sorry for my terms as im not to forsure what the names of all them parts are lol but i really do appreciate all the responses thanks a ton. and the bike is no longer a slouch it rips!!!

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i just gotta say you guys are AWESOME!!!!!!! i pulled the linkage cover off started and reved the bike and sure enough the linkage was not moving at all. so i traced it down to i think was part of the governor it kinda looked almost like a gear looking thing with a spring behind it but i noticed there was like a key missing to keep that gear looking thing from spinning so i cut down a hex key n made a piece to fit and BLAM it now works GREAT! sorry for my terms as im not to forsure what the names of all them parts are lol but i really do appreciate all the responses thanks a ton. and the bike is no longer a slouch it rips!!!

Everyone likes a happy ending mate!

Top marks for bush mechanics.

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i just gotta say you guys are AWESOME!!!!!!! i pulled the linkage cover off started and reved the bike and sure enough the linkage was not moving at all. so i traced it down to i think was part of the governor it kinda looked almost like a gear looking thing with a spring behind it but i noticed there was like a key missing to keep that gear looking thing from spinning so i cut down a hex key n made a piece to fit and BLAM it now works GREAT! sorry for my terms as im not to forsure what the names of all them parts are lol but i really do appreciate all the responses thanks a ton. and the bike is no longer a slouch it rips!!!

If that pin was missing in the governer it could be missing the balls. It's critical the governer is assembled correctly for it to work correctly - so you know the spring in the governer and the number of balls it contains is how you adjust the power valve on a yz ( less balls comes on later as its lighter - softer spring ( a wr spring was made in the nineties ) also gives a different response - um makes it come on earlier.(open earlier)

I remember now - one of 2stroke wr's I bought the governer had been fiddled with and assembled incorrectly and this caused all sorts of damage . I still have all the bits - I'll take pics when I get a chance .

Glad I could help with your issue - cheers .

 

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