Can we sit down and converse about powervalves?

R

rpduc

New member
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.
 
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.
 
rpduc said:
... 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..
Click to expand...
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.
 
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. :D
 
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.
 
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/showpost.php?p=62746&postcount=21

http://forums.everything2stroke.com/showthread.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...
 
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.
 
gasgasman said:
When the valve is closed, it increases back pressure in the cylinder-increasing torque.

http://www.youtube.com/watch?v=GD8QnsY925Y&list=LPpLpNCzNDdl8&index=1&feature=plcp
Click to expand...
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.
 
To elaborate on above; the backpressure is created by the stinger restriction & as you can see is considerably more influential than a change in port time area, which is sod all change to Backpressure.

Possible confusion may be that you are effectively changing the dynamic compression ratio. But this is in no way the same.
 
That was a particularly bad description.
Click to expand...

No kiddin'... And thanks for reminding me about my bashed up Doma. Replacing that pipe might make more diff than I realize...

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.
Click to expand...

According to that quad guy in the link i posted above the factory supposedly "adjusts" to a specified length, i.e. preload. No idea what qualifies him to make that statement. It is the internet.... Looking at parts lists one might surmise that the difference in shims required for different model years was predicated by a change in spec on the drive gear. Hard to know if any adjusting beyond that occurs unless everyone agrees to yank apart their unmolested PV governers and counts shims to see if there is any variance. So I guess we'll never know... though it seems like there ought to be someone, somewhere who could answer that definitively. There are enough posts on here by people who claim improvements to make me wonder...

I understand your method and have attempted to adjust out the slop in my PV that way, but nothing I've tried results in the as described very slight preload against the stop. Seems the shaft the pin arm fits on to must have a flat spot on it cuz the set screw finds home either with excessive freeplay or what I feel is too much pressure against the stop. No middle ground. I havent had that end apart but I spose I should see if I can take the arm on the right end of the shaft off and see if I can rotate it 180deg and expose a fresh seating surface for the set screw. I assume that would be possible?
 
Ross,

Yes, pull the bell crank assay and clean up the shaft where the setscrew seats.
 
I have not seen measured rpms mentioned anywhere. Only seat of the pants claims. Some claim more shims helped, some say pull 'em out...

I just had mine open and there are NO shims in there. The spring rides right against the drive gear. According to the parts fiche 01 thru 06 had 1.2mm of washers under the spring. I've never disassembled the governer so could be...
- Previous owner pulled them
- A factory "adjustment"
- Some undocumented variance in the MC models.

I did spin the shaft 180 degrees to get a clean surface for the set screw to seat on and retweaked the preload "Glenn style." And I'm even more curious about the shim deal now that I see there are none in mine. Using Glenn's suspension analogy... Insufficient preload = more static sag = PV opens too soon = soft bottom??
 
I suspect there is vaiability in the production governers, primarily in the springs. Ideally, you would want the PV opening curve to track the ignition curve. I suspect that this is the case in bikes that run really well. My '07 250 is a bike like this, just runs hard top to bottom, map selector works and makes enough power that I need to "map it down" at times. I don't want more power. The '12 is just as strong into the mid, but takes more time to get on the pipe even with a 10 oz weight vs 14 oz on the '07 (both 2K-2). Out of curiosity I installed the '07 CDI in the '12. Maybe a bit more but its tough to say. Map selector has very little effect. Its an effect you would expect if the PV was opening slower and/or at a higher RPM. The '07 has a head mod but it ran very close stock as well. When I have time I plan to swap the governers as an experiment, but perhaps before that observe the PV operation with the right side cover removed.

The problem with bench testing the governer is that you need something with decent speed control and high speed to measure or approximate the percentage of movement. Blitz was actually going to play with this along with his home brew CDI, not sure how far he ever got. I know he had a setup to spin the 2K-2 flywheel pretty fast for testing his CDI with the stator/pickup.
 
May be interesting to note that your 2007 supposedly has one .2mm shim and the 2012 has two .2 mm shims according to the parts lists. Would also be interesting to find out if your unmolested governers have the shims installed as specified.
 
rpduc said:
I have not seen measured rpms mentioned anywhere. Only seat of the pants claims. Some claim more shims helped, some say pull 'em out...

I just had mine open and there are NO shims in there. The spring rides right against the drive gear. According to the parts fiche 01 thru 06 had 1.2mm of washers under the spring. I've never disassembled the governer so could be...
- Previous owner pulled them
- A factory "adjustment"
- Some undocumented variance in the MC models.

I did spin the shaft 180 degrees to get a clean surface for the set screw to seat on and retweaked the preload "Glenn style." And I'm even more curious about the shim deal now that I see there are none in mine. Using Glenn's suspension analogy... QUOTE]

My 05 300 did not have any shims either. I was wondering the sane thing as you "Insufficient preload = more static sag = PV opens too soon = soft bottom??" I had a 250 kx that was exactly like this. By adding some preload it got rid of a the big hit in the midrange by filling in the middle. Now my 300 just goes a little flat and doesnt have the bog sound. So it really seems like the power valve should just open sooner. Whish it had some shims I could remove to test. I did get most of the flat spot out with jetting.
Click to expand...
 
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