Power valve operation RPM

[Motorman_55792]

Does anyone have any information on what RPM the power valve operates on our bikes? Mine is a 2018 xc250 specifically, but I would guess the RPM is similar across many models.

Mine is opening way early (about 2500 rpm) even with the adjuster screw all the way tight. I believe the normal operation should be at 5k or more, but this is just educated guess based on other brands and my snowmobile experience which is higher at 6750 rpm. I am going to dig into the side case and see what it looks like and I will update what I find at that time, but knowing what rpm to shoot for would be helpful.

 

[Gasser Nate]

How are you measuring engine rpm? Guessing is not really going to be accurate obviously. What sort of engine are you comparing the gasser to? They probably do not have the same characteristics so not really comparable. What is the bike actually doing wrong?

 

[Motorman_55792]

I have no low end. It runs like a 125 and way weaker than my 07 GasGas 250 I sold until in the upper part of the power band. I have a great deal of experience on many engines and this is obviously not working right. I used a sonic tachometer to check the operation with the linkage cover behind the pipe removed. I can ride around the big hole in the power, but it isn't good. If I set the adjuster at 2 turns out as recommended you would swear it is a 125, and with the adjuster screwed in tight it is bearable, but not right. Try setting your adjuster to about 6 turns out and it may run like mine.

 

[Pygmygod]

How old is the exhaust packing?
Do your exhaust o-rings leak/damaged?

Stock pipe or aftermarket?

 

[Motorman_55792]

How old is the exhaust packing?
Do your exhaust o-rings leak/damaged?

Stock pipe or aftermarket?

This is a 2018 with low hours. Packing is good. Stock xc fmf pipe and also same with fmf fatty. O rings seal well. I have it jetted perfectly and have also added an RK Tek head since the stock head is very poorly set up. That added enough power to make it ok, but it still is not right.

This is absolutely a power valve function issue. I can't believe I haven't found any manual that states what rpm the valve should start to open or fully open!

 

[gasgasman]

Maybe the power valve flap is not "timed" correctly.

Is the flap fully closed with the engine off?

 

[Motorman_55792]

Maybe the power valve flap is not "timed" correctly.

Is the flap fully closed with the engine off?

I did verify that. It closes fully at idle and opens fully when reved up. Good idea, though.

 

[(F5)]

With a centrifical governor it should progressively open to some extent. The addition of a spring adjustment will just add preload. Haven't pulled a later unit apart.

I'm guessing snowmobiles are pneumatic, from my tiny experience there they tend to open rapidly depending on pressure bleed, at least on my Cheetah engine that was the case until I converted it to servo motor.

First check for free play, if there's heaps airflow can open the valve. Then check all the balls and correct spring arrangement.

 

[Gasser Nate]

This is a 2018 with low hours. Packing is good. Stock xc fmf pipe and also same with fmf fatty. O rings seal well. I have it jetted perfectly and have also added an RK Tek head since the stock head is very poorly set up. That added enough power to make it ok, but it still is not right.

This is absolutely a power valve function issue. I can't believe I haven't found any manual that states what rpm the valve should start to open or fully open!

Probably because there is not any adjustments to correct if it is out of spec.
Have you checked ignition timing? If the key way is not aligned properly or sheared etc it will affect power.
Sorry I can not be more help but the gassy power valves are so simple they rarely go wrong. The only thing I have seen the early ones do was break off the small stud on the power valve plate.

 

[Motorman_55792]

Been a long time coming, but I finally took the side cover off and found the problem. The pin that the power valve adjuster screw pushes against was only 1/2 way in the shaft. The case must have held it from coming out completely. Now it runs like it should.

 

[Down2fix]

Been a long time coming, but I finally took the side cover off and found the problem. The pin that the power valve adjuster screw pushes against was only 1/2 way in the shaft. The case must have held it from coming out completely. Now it runs like it should.

I guess it's time to pull my cover and check this as well, I swear every ride it loses more and bottom end. Like something is slowing backing off or losing its adjustment.
Thanks for the update

 

[Motorman_55792]

I guess it's time to pull my cover and check this as well, I swear every ride it loses more and bottom end. Like something is slowing backing off or losing its adjustment.
Thanks for the update

I just had a long single track test session today. Works so much better and uses about 25% less fuel with no drool out the pipe. I may be a little lean actually it looks like based on the complete lack of drool after 3 hours of riding.