96 sx 250 power problems

[kiwi_rider]

Hi everyone, i have a 96 sx 250 with a power issue, the bike has just been rebuilt by a ktm dealer 4 hours riding ago, i bought it from my stepbrother.

The bike starts well, and goes ok thru the midrange, but flattens off in powerband, and it does not go like a bike that is meant to be 56hp/41kw, i have checked the reeds, they are not cracked(although the are open about 1mm each just bolted to the vee block)

i have checked the exhaust powevalve and without the control rod connected to it it moves freely, i took the pipe off to have a look and can see its nice and clean, pipe is fmf gold series.

a friend has a 91 kx 250 that just eats my bike at the moment. does anyone know if the powervalve control mechanism on this bike is centrifugal? the rod is tight to try move by hand, and i havent been able to move the rod (i havent forced it tho)

i would like some suggestions as what to check next..
the bike jumps real nice and is well balanced, i just need to figure out this minor problem thats robbing me of about 8 hp

Cheers

Matt

 

[elf]

Your problem sounds like a power valve that is not opening, unless your main jet is way off. I dont know about your 96 but on my ktm the acuator arm will move up and down. It just feels like it has some spring tension on it. Also you should probably replace your reeds they should lay flat. But that would be more of a low end issue.Does your bike have a cover plate that can be removed so you can see if the pv is moving while you rev it up?

 

[kiwi_rider]

Hi Elf

Hi Elf,

thanks so much for the reply, there is in fact a cover over the power valve actuator that can be removed, a bit of oil will come out, but as long as i dont run it for long it should be ok,( i will warm the bike up first, bring it back and take the cover off) the reason i hadn't tried this so far is that i thought the powervalve probably would not move unless the engine was under load?. the bike actually has a real solid bottom end, but then flattens off. where would i get a manual for the bike to check where the powervalve actuator is meant to be adjusted to, as the end of it is threaded.

Also my bike wasnt smoking a lot, even tho i was using a 40:1 mix, after a bit of investigation i found the air bypass screw was out about 3 turns, i wound it in and it seems to be running a fair bit richer, im yet to ride it to see if that made any diff.

thanks for the help, i will check the powervalve...an owners manual would be helpful if i had one.

 

[elf]

The power valve opens at a certain rpm, it does not matter if your engine is warm or under load. there is a spring in it and when the engine reaches a certain rpm the pressure overcomes the spring and moves the acuator arm. Your set up is different than the one on my bike, but looks the same as the pv on the new 250s. Also, your air screw will have very little effect at wide open throttle.It has the biggest effect at 0 to 1/4 throttle.

 

[kiwi_rider]

I found the problem

I manually adjusted the powervalve and the bike went heaps better, i then started to strip the bike, i pulled off the pipe,powervalve cover, powervalve spring, i was about to start ripping the rest of the bike apart when i took a closer look at the spring, in one end it had a dimple, the other end a slot, the dimple end was sitting on the actuator, the slotted end in the cap, after shoving my finger in there and descovering there was a stub that would fit in the slot, it seems the TARDS at the local KTM dealer had put the spring in backwards (when engine was rebuilt) giving it another 8mm or so of loading on the spring, making the actuator rod almost impossible to move by hand, now it moves with not much effort. Im yet to ride the bike, but i think (and im pretty sure) i have it fixed now, im yet to ride it. hope tomorrow is a nice day so i can go take it for a ride and feel what a 56hp bike that has been modded my shayne kings race mechanic, goes like

 

[elf]

Sounds like you fixed the problem. Have fun riding!!

 

[blanc]

So whats she like then??????

 

[kiwi_rider]

omg insane machine

this bike is an insane machine now, it flys. its currently 9:16 am sunday morning, i am going riding today, i will have more info, im going to drag my friend on his modded 2002 yz250. i had a good rip on the bike on friday and had to learn to ride it all over again, it was lifting the wheel in 4th gear under power, it is soo much fun, glad i just put new sbs brake pads in :)

Thanks for the help Elf, you put me on the right track. I am soo happy with the bike now. i just cant wit for the black clouds to bugger off and the sun to come out.

One other thing, my zokes have started leaking (they are the 55mm telescopic marzocchi magnums) aparently the seals are not that great, anyone got any tips/mods on these forks (apart from fitting some usd whitepower shocks)

anyway, hopefully i wont hurt myself today and can beat the yz250 and the yz400 they owned me on the beach the other week

Cheers :yeehaw:

 

[elf]

That bike sounds like a blast!! Post a question in the suspension forum about your forks,you will probably get some replies..I don't know a bloody thing about marzocchis.

 

[kiwi_rider]

thanks

a blast it is!! cheers elf