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Discussion Starter #1
Hi guys,
I know powervalve set up is probably one of those topics that comes up time and time again but I have been searching and searching but can't find what I need! I own an 08 exc 250 and at the moment its not got much bottom end pull but lots up top. This is with the red spring and the brass screw in maybe 2 or 3 threads I can't quite remember. Basically I want lots of tractable low end torque so that I can lug my way up stuff without spinning up the back wheel so much!
 

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get your head modified, make sure your reeds are in good nick, fit a gnarley, jet it slightly rich.
 

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Shows you what I know ...
No no. I was just clarifying what that setting does but it does make the bike easier to ride in technical terrain. Apparently Jarvis runs the green spring.

Green spring kills the peak torque at around 8-9k revs. Flush adjuster lowers the torque around 5500-6000. No difference at all below that.
 

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Al_Orange: not quite. The brass adjuster determines when the power valve opens and the spring determines how long it takes to open fully. At factory setting (about 1.5 turns in from flush) it starts opening at roughly 5500rpm.

At factory setting:
Green = PV Opens @ 5,500 rpm ; PV is fully open @ 8,300 rpm.
Yellow = PV Opens @ 5,500 rpm ; PV is fully open @ 7,800 rpm.
Red = PV Opens @ 5,500 rpm ; PV is fully open @ 7,300 rpm.

When you turn the brass preload adjuster anti clockwise it starts opening much earlier! Green spring and flush gives you the most amount of useable low end grunt because what it does is open the power valve as early as possible and lengthens the time it takes to open so you have the most amount of full power spread over the widest margin as early as possible. It's the most powerful setting at the earliest in the Rev range but with the smoothest delivery. It makes the most amount of useable low end torque of all settings which is probably why Jarvis uses it this way.

Anything else gives the grunt higher up the Rev range.

Keep in mind, the earlier the power valve opens the less you will feel a 'kick'.
 

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Agreed with the above, but what's low end grunt?
The power valve is fully closed until around the 5000 - 5500 RPM mark (depending on preload of dolley screw) so no amount of fiddling with settings will have any effect below 5000.
I would define low end as from idle to 4500, mid range 4500 to 7000, and top end 7000 to rev limiter.

The green spring makes it 'feel' like more bottom end, as the hit above 5500 is strangled, so making the torque increase flatter and more linear. So easier to put the power down when slippy.
Same as the red spring makes it 'feel' like less bottom end as the torque increase is massive and steep. So easier to spin up. When I started to get used to riding a 2 stroke again, I found that when slippy, I was rolling off the throttle as the revs climbed to keep the torque constant, and wasn't spinning up anywhere near as bad as when I came to 2 stroke from a 4 banger.
 

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I reckon stumos on the money with richer jetting just off idle. Set the pv for the feel/delivery you want but give it enough fuel so it can generate the torque you want. If it's towards the leaner end of things it'll be edgier and more snappy but this then means your soon through this and onto the needle more.

Try a one step richer pilot first and adjust air screw accordingly. One step richer needle starting diameter or possible one clip richer- worth a try.
 

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Al_Orange: not quite. The brass adjuster determines when the power valve opens and the spring determines how long it takes to open fully. At factory setting (about 1.5 turns in from flush) it starts opening at roughly 5500rpm.

At factory setting:
Green = PV Opens @ 5,500 rpm ; PV is fully open @ 8,300 rpm.
Yellow = PV Opens @ 5,500 rpm ; PV is fully open @ 7,800 rpm.
Red = PV Opens @ 5,500 rpm ; PV is fully open @ 7,300 rpm.

When you turn the brass preload adjuster anti clockwise it starts opening much earlier! Green spring and flush gives you the most amount of useable low end grunt because what it does is open the power valve as early as possible and lengthens the time it takes to open so you have the most amount of full power spread over the widest margin as early as possible. It's the most powerful setting at the earliest in the Rev range but with the smoothest delivery. It makes the most amount of useable low end torque of all settings which is probably why Jarvis uses it this way.

Anything else gives the grunt higher up the Rev range.

Keep in mind, the earlier the power valve opens the less you will feel a 'kick'.
Sorry frogger, the theory you've outlined sounds reasonable but what I said is proven to be true.

Bear mind that a power valve is actually there to increase bottom range torque - when it starts opening too early (flush) you miss out on some torque that would be available from the lower revs. Likewise, when it takes too long to fully open (green spring) your miss out on torque that could be available at higher revs.

I'll loom for the link/graph that shows this later.

However, the FEEL of the green spring flush is a bike that's easy to control as it's making less torque everywhere.
 

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Discussion Starter #14
Agreed with the above, but what's low end grunt?
The power valve is fully closed until around the 5000 - 5500 RPM mark (depending on preload of dolley screw) so no amount of fiddling with settings will have any effect below 5000.
I would define low end as from idle to 4500, mid range 4500 to 7000, and top end 7000 to rev limiter.

The green spring makes it 'feel' like more bottom end, as the hit above 5500 is strangled, so making the torque increase flatter and more linear. So easier to put the power down when slippy.
Same as the red spring makes it 'feel' like less bottom end as the torque increase is massive and steep. So easier to spin up. When I started to get used to riding a 2 stroke again, I found that when slippy, I was rolling off the throttle as the revs climbed to keep the torque constant, and wasn't spinning up anywhere near as bad as when I came to 2 stroke from a 4 banger.
So what is the best setting for what I'm after? Gearing and mods aside
 

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Don't ignore gearing and jetting - they have a much bigger affect than the power valve spring.

Assuming you've got proper jetting and sensible gearing then try yellow spring 1.5 turns in from flush.

If the bike is really lacking then there's a number of other things to check, like compression etc.
 

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Sorry frogger, the theory you've outlined sounds reasonable but what I said is proven to be true.

Bear mind that a power valve is actually there to increase bottom range torque - when it starts opening too early (flush) you miss out on some torque that would be available from the lower revs. Likewise, when it takes too long to fully open (green spring) your miss out on torque that could be available at higher revs.

I'll loom for the link/graph that shows this later.

However, the FEEL of the green spring flush is a bike that's easy to control as it's making less torque everywhere.
I absolutely agree, the green spring is rumoured to be a torque monster, but it isnt and any Dyno can prove that.

When you look at the torque curves at 5600 RPM (which is the standard opening point of the PV) you will see that shortly after this "starting point" the engine produces a lot more torque with the red spring.

 

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I absolutely agree, the green spring is rumoured to be a torque monster, but it isnt and any Dyno can prove that.

When you look at the torque curves at 5600 RPM (which is the standard opening point of the PV) you will see that shortly after this "starting point" the engine produces a lot more torque with the red spring.


That graph says it all really, there is nothing in it between any of the springs from 3.5k to about 6k. (Maybe the green spring giving a 'fraction' more torque, but not enough to notice whilst riding the bike. The different springs then just flatten off the torque curve above 6k different mounts depending on the spring. The green spring more or less kills the top end. The yellow spring being a 'half way house' and the red spring allowing the engine to produce a 'natural' torque curve for a 2 stroke. I can see how the green spring will allow the rider to use the rev range above 6k without spinning the rear tyre but only because it is causing the PV to 'strangle' the engine.

I'm no expert by any means, I've only dicked about on green lanes at the moment but I am learning to ride the bike with the red spring, it just takes some throttle control which to me is at least half the point of having an endure bike with a big 2 stroke engine!
 

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That's the graph I was going to post - it shows the affect on the top end.

There's another one that shows the adjuster affect on the 5-6K range and the most torque is around 1.5 - 2 turns. Flush makes less. I think that's why in the manual it says not to fuck with it.

Of course, raw figures don't tell the whole story - it's how it translates to usable power for the type of riding and ability of the rider. Combined with gearing and jetting, these bikes are very tuneable.

That said, I'm just clubman level and after lots of dicking around, I pretty much always come back to near standard setup so I guess KTM do know a thing or two about it. lol.
 

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Torque graph is interesting, do you have the Power BHP graph out of interest?
The torque graph is far more meaningful than the power one, as it shows exactly how the seat of the pants feels, and where things might get lairy and spin up or wheelie.
The power graph muddies the waters by the fact that power is a function of torque and RPM. The power curve lags behind the torque curve, as even though torque can be dropping, the RPM is still increasing, so power increases. The power curve maxes out and starts to drop when the torque is decreasing faster than RPM is increasing.

A diesel engine may have max torque at 2000 RPM, and max power at 4500 RPM, so the two curves are totally different.
 
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