Can we sit down and converse about powervalves?

[rpduc]

OK I've searched and read ad nauseum trying to determine how exactly my powervalve should function and maybe I'm dense but I'm not getting the picture...

I understand how to alter function by adding removing shims but I do not, at a very basic level, understand what I'm trying to acheive... I would say it takes a pretty good crack on the throttle to get my powervalve to move and then it kinda snaps open. I've seen posts here that seem to suggest their powervalve opens more progressively. In theory if I remove shims the PV should open sooner and "maybe" more progressively but is that necessarily what I'm after?

It seems the MC 250 has various "enhancements" for MX like different head, ignition, CDI etc. that make it a bit umanageable at times in the woods... I'm wondering if perhaps the powervalve is involved as well...

Taking out the slop (did that) will prevent rattle when the rpm is too low to activate governer but the way I see it should have little affect on when the powervalve actually opens.

[gasgasman]

The power valve is opened by centrifugal force of the balls acting on the ramp in the governer.

IMO, I don't think moving shims would affect the opening and closing timing of the p/v. When you move the shims, you will have to adjust the free play on the p/v actuator bell crank, which puts it back in the same position.

I supposed you can change the weight of the balls and also the profile angle on the governer ramp to alter the power valve operation.

Or, you can get the Trusty adjustable power valve cover which preloads the p/v governer to open slowly.

[AZRickD]

http://www.trialendurodirect.com/pro...alve_Cover_139

Sorry, but 299 British pounds ($466 US dollars), is just stooopid expensive.

[(F5)]

Originally Posted by rpduc

... I would say it takes a pretty good crack on the throttle to get my powervalve to move and then it kinda snaps open. I've seen posts here that seem to suggest their powervalve opens more progressively..

It should be revs operated. The idea is that it changes the time area of the exhaust port more suitable to the revs. It also disrupts the returning wave when the pipe is out of tuned range & would be mistimed causing more damage than good. Part of this strategy opens a chamber that helps disrupt the wave at low revs. it isn't a Helmholtz chamber or anything.

When you are under load (riding) it will open slower than if you are blipping the throttle.

[rpduc]

Like is said I understand mechanically how it works. Centrifugal force causes the balls to move out spreads the plates on the governer, applies pressure to the pin that opens the powervalve.

But it seems to me if you remove shims you would lessen the preload on the plates thus it should require less centifugal force to move the balls the same amount. Or vice versa. In theory one could preload that spring so much that the PV would not open at all...If removing them would have no effect why are they in there in the first place?
Or put in a stiffer spring... Isn't that how the KTM adjustable PVs work?

But my questions are more basic theory. F5 is getting at it with his comments. If it opens too soon what is the effect? If it opens too late what is the effect? If it snaps open rather than opens "progressively" what is the effect? I've seen some say yank all the shims, some add shims, most don't mess with it at all... But inquiring minds want to know.

[gasgasman]

When the valve is closed, it increases back pressure in the cylinder-increasing torque.

http://www.youtube.com/watch?v=GD8Qn...1&feature=plcp

[Jakobi]

I haven't had a look at the governor so not 100% certain on how the spring works in relation to the bearings.

The powervalve itself is nothing more than a variable sized exhaust port. A large port will flow better at higher rpm, but will have less back pressure at low rpm. A small port will give good bottom end response, but will sign off early.

Using that theory, if you opened the power valve early and fast you would get a soft bottom end, with a good strong top end. Think old school 2T which had no powervalves. If you held off until really late in the rpm and then opened fast it would be like the bike would fall flat, and then surge/snap up top. Making it open slower would simply make the transition smoother until its too slow which would then result in some lost power but should still be smooth. Ideally the powervalve is meant to take the hit/transition out of the engine by opening in time with the revs allowing optimium conditions for all rpm. But... The powervalve is still only 1 variable in the engine.

[rpduc]

Thanks for that basic theory lesson Jakobi. As usual I am prolly way overthinking it but here's a fun fact. I compared parts list for MC, EC different model years and it's interesting... GoFasters only goes back to 2001 and doesn't list my 2005 MC 250 but anyway...

2001 thru 2006 there is one #15 washer at .2mm and two #16 washers at .5mm for a total of 1.2mm

In 2007 only they went to just one .2mm washer (#15)

In 2008 they went to two .2mm washers

All other parts appear to remain constant but it's hard to tell from the GoFaster online parts fiche cuz actual part #s aren't there. At any rate it appears that GG has felt it necessary to vary that pile of washers in there for some reason.

http://www.gasgasrider.org/forum/sho...6&postcount=21

http://forums.everything2stroke.com/...ad.php?t=45719

Sounds like the Quad guy in that last link has some pertinent info. He's saying that .1mm shim equates to a 100rpm adjustment in powervalve timing and he gives a stock preloaded lenght of 14.8mm. (seems short) The older bikes evidently had a plastic drive gear instead of metal so maybe the different washer stacks are just making up that difference.

Guess there's really only one way to find out how changing preload shims would affect my bike...

[GMP]

PV spring physics is like any spring physics including suspension: PRELOAD adjustment affects when the spring starts to compress or in this case when the PV starts to open. SPRING RATE affects the rate of compression relative to force or rate of PV opening relative to speed. So, if you have a light PV spring with a lot of preload, it should stay closed to a certain RPM and then open rapidly, very non-linear. A stiffer spring and less preload should have the opposite result.

Think about how your fork reacts with different spring/preload setups and its easy to understand.

That said, I wonder how tight the QC is on the governor assembly at the factory, if pre-assembly testing is done on a test fixture, and if so, is preload adjusted on a case by case basis to meet a spec for an opening profile based on RPM. The practical range of adjustment for acceptable performance may be quite narrow.

In theory, the PV opening profile should always be so that the engine is making max torque at the current RPM. Same with ignition timing(and jetting for that matter). Ideally, these should all be in sync, and a no doubt the reason behind Honda's electronic PV on the CR250.

[(F5)]

Originally Posted by gasgasman

When the valve is closed, it increases back pressure in the cylinder-increasing torque.

http://www.youtube.com/watch?v=GD8Qn...1&feature=plcp

That was a particularly bad description. backpressure is a wash too.


GMP wrote what was forming in my head. Spring vs centrifugal PCs are a compromise. Ideally you'd have a different opening rate for every gear. But it doesn't really matter that much as they are such an improvement over no PV.