Power Valve Setting Debate

[Bill Hibbs]

I was having a debate with my friend on the best way to set up the powervalve on my KTM. Currently I have it set up to open late (up in the revs). When it does open and since the revs are high, you really feel a power surge. Generally it's not a problem but on the MX track I have to remember to be low in the power or I'll shoot off the jump Nose high. My friend was saying it may be better to set it up to open early so I don't have that sudden surge but I don't want too much power when I'm dancing in the slippery woods.

How do you guys have your's set up?

 

[BigBore]

Mine is at the stock setting. The last time I went riding I tried turning it all the way out, and it made the bike run like a slug, so I put it back to stock. Feels fine to me.

 

[Gardener]

I have mine set one turn out from stock. It doesn't change too much but seems a little better than stock for the woods. Try it, you can always put it back. The best thing I did was change away from the NOZX needle to a straight taper. The NOZX series needles, if that's what your using, produce an evil hit for the bush.

 

[ktmracer]

I switched to a cek single taper needle (along with a much larger main) and it really comes on the pipe now. I have been wanting to mess with my powervalve setting ever since my last top end job. when i put the jug back on the motor, the pv actuator arm was in the fully closed position. (which doesn't alow it to line up with the arm coming up from the case) so, when i rode the bike up the street for a quick test run, i noticed that low end power was WAY up. i know it didnt' have any top or mid but i have been wondering if i could change the starting position of the pv flap to increase low end, without effecting (too much) the high rpm pull. any thoughts??

will

 

[dirt bike dave]

It's a personal preference, but the PV settings and jetting make a huge difference in the power delivery on the 300.

At least six of my friends have 300 Exc's (mostly '99 and '00) and ride single track. With stock jetting and PV settings, the bikes have a hit in the midrange and got poor fuel range. After rejetting and adjusting the PV to open later, the bikes have excellent, smooth low and midrange and get about 70 miles per tank.

FWIW, the fastest guys I know like the smoother power delivery of the late opening PV. On single track, these guys ride a gear high, keeping the revs low and the speed fast, clutching as needed and not shifting much.

When the trail widens or we hit a dirt road, they open up those powervalves and the bikes are rockets. From a stop to 55 mph or so they will pull my CR250 by a couple of bike lengths if traction is excellent.

With the late opening PV's, the 300 is still as fast as anybike out there in a WFO drag race, but you get a nice manageable motor in the tight stuff.

 

[Barbarian]

Dave, I used to get hardware store springs and hook one end to my power-valve linkage and ancker the other end between the cylinder and the power valve cover. This made my cr's as smooth as a 4-stroke.

 

[Bill Hibbs]

That's pretty much how I have mine set up DBD. It's definitely a blast when you get into a nice straight to open her up! I'll have to play with it a little more. Thanks for all the input!

 

[socal scott]

ktmracer,
my 200 is obviously different, but when I had the flap at its lowest height, it ran like a terd. I thought fully closed shouldn't affect the full range of movement, but something went weird after bottom end rush.

Big,
Try smaller adjustments. Your valves opened way to early.

Dave and Bill,
Same adjustment different results? now I'm confused again. I was convinced that delaying would make power more linear, but I do remember Moose Racing sold a CR kit years ago. The grass track spring was the softest of the spring set. I guess if you open it soon enough, you stay far away from that optimum torque/hp crossover point.
Openning closer to that optimum point will create a bigger hit than stock. I think my 200's lack of torque makes it a different animal though.

 

[Humai]

Good thread guys!

My personal preference is based on my dislike of being cheated out of horsepower at any given point in the power curve.

There has got to be a point in every bike's power curve where there is a cross over between the hypothetical drop off of power with the valve permanently closed and the rise where the power valve is always open. My approach is to try to set the powervalve opening to take place where this theoretical cross over occurs.

If I feel a definite hit when the valve opens (you'll hear that point as an increase in expansion chamber wall noise emission), I'll adjust the opening point downwards until the transition is less noticeable. If the power falls into a hole at the point where the valve opens, I'll adjust it to open at a higher rev.

I'm experimenting right now with my '03 KTM 200 so this is very timely.

My apologies for making a simple thing sound complex, but that's what works for me!