so why does any one want bottom end on a 2stroke u got a 2 stroke for a reason ye ***SNAP*** pv are used to make the 2 stroke like a four stroke so why not ride a 4 stroke then, i removed the power valves in my 95 rmx 250 and replaced them with lugs What a difference not a lot of bottom end but it still gets me round and ***SNAP*** pure power the way a 2 stroke should be, CR500 dont have pv either and they are off there heads one of the best bikes made and theyre bringin them back out now as the AF or service bike why ?    A: reliabilty .
pv are a pain sittin close to ure piston and gunken up just get a 4 banger and there u go u got urself a low rpm bike still though outlet valves wich will doo what? A: give u problems and slow the rpm of the bike. no vavles and it wont run, 4 strokes, so Why incorporate outlet valves on to the motors that run fine with out? to turn them into what theyre not. take the outlet or pv off and its an original 2 stroke with nothin dangling waiting to blow up ur bike and nothing to slow ur rpm.



ANY WAYS what r ur thoughts on Power valves? and if u think they increase performance, why does a cr 500 have none and flog everything?

Well i think its cool you removed the powervalve from your rmx, could you give a bit more detail to how much bottom end you lost? So you basically remove all the moving parts of the PV system? Hmmm Interesting....


With the whole CR500 thing, it produces similar horespower in the midrange (where peak hp occurs) to the kx500, many dynos say even a little bit more than the kx. But the kx doesnt fall as flat in the upper RPM's where the CR500has to be upshifted if you wanna keep haulin ass, where the kx will be able to hold a gear longer whilst still producing power. I believe this id due to the PV. I dont know which engine produces more off idle.

Edited by Natho, 04 June 2012 - 10:14 PM.

Zaqster, on 04 June 2012 - 08:34 PM, said:

For your information the function of the PV is to make more TOP END. It opens at higher rpm to reduce compression as 2 strokes make more top end with lower compression. Quicker your PV opens, the snappier your bike is. So you've accomplished the opposite of what you were looking for...

Because of part availability and the desire to go riding one weekend, I installed a RM250 PV system in my '96 RMX (I don't remember the RM year).  By itself it wasn't that bad but when compared to an unmodified RMX the power difference was huge.  Also, considering that Dirt Bike magazine wayyyyy back in 1990 actually installed a RMX power valve in a RM and liked it better, I personally believe that you should keep it.

Power valves broaden the powerband, properly set up they will never cost you power.  In the case of a CR500, keep in mind that it's a big bore, so they never rev'd very high anyways.  This is also why the KX500 had a wider powerband and more power in the later years as they did use a PV.

If you need to remove it because you cannot afford to replace it ~ that's one thing, but removing it because you are looking for that pure two stroke experience, well, ... that's another deal and I do not recommend it.  Sure, it's fun to play around with a hard hitting bike, but while your spinning and throwing an impressive roost your competitors will be hooking up and waving goodbye.

I fail to see how you will get more "punch" with the PV blocked off. It will be making the same compression as if the PV was closed, so same low and mid. But as RPM's increase you would still be making higher compression and the power will fall off. Only driven part here is the PV governor off the crank which isn't a whole lot of parasitic loss. Were else would you find your gain??
In my eyes the PV just makes the powerband wider.

tribalbc, on 05 June 2012 - 08:29 AM, said:

I though that is not the main operation of a PV. It hardy increases TOP END. I dont mean to get in an anrgument but dont they just alter the size of the exhuast port? i.e low rpm, PV closed small exhuast port high = good torque/crispness. high rpm PV open, ports wide open, good top end power......If the PV was removed wouldnt it retain all the topend, and loose the ability to 'lug' and be torquey and crisp at low rpm?

Any chanceyou guys could help me out with the topic I started recently? You guys have already taught me ALOT about how pv s work without even talking to me... Help a fellow Thumper talker lol

00Rm 125 new piston+ rings NO COMPRESSION

Natho, on 05 June 2012 - 01:21 PM, said:

I think we're on the same page but he said he plugged off his PV so I'm thinking permanently closed. If it's permanently open then we will have the opposite effect. No bottom, mid lots of top end.

tribalbc, on 05 June 2012 - 03:52 PM, said:

Oh ok, I want him reply to get more info on the whole deal. I wonder how a big bore say 300 Big Bore would go with the PV removed, it would obviously still suffer down low but not as much IMO. The suzuki A TEV exhaust/power valve is very reliable and not as prone to carbon buildup/siezure as some of the other systems.........I could see removing the PV on something like an old KX KIPS system like a mid 90s KX250 if your going for pure simplicity and reliablitiy but IMO with the rm's leave em alone

Natho, on 05 June 2012 - 01:21 PM, said:

Not arguing at all, so all good!      , although I would like to add on to what you just said.

Think of it this way, the purpose of the PV is an engineering attempt at having your cake and eating it too.

What a PV actually does is "raise" the height of the exhaust port.  The higher the port the more top end power you will get (to a point), but this also comes at the expense of low end and how it rolls into the mid range power.  So if you did a dyno pull with the PV perminately closed (in the down position) the power will start out sooner, have a decent amount of mid range and then go flat on the top end.  Take the same motor with the PV locked open (all the way up) and the curve will not be as strong down low and as it rolled into the mid range, however, at a certain RPM (and it's different for every engine combination) the power will then exceed the closed dyno curve and then continue to make more power up to the mechanical red line.

So the real trick here is to get the PV to open at the point where the dyno curves cross, as it's at this point where that engine design is always making the most power.

OK people Firstly, my power valves are removed and the lugs sit like small plates across the holes where they were so they are permanently open they cant open faster than that, secondly the pv dosnt make the port bigger as it takes up space it makes it smaller and adjusts it back to normal (fully open)/

SOOOOOOOO PV TO MAKE A WIDER BAND LIKE A 4 STROKE . WHY? JUST GET A 4 STROKE  IF U RIDE A PV BIKE IT AINT A REAL 2 STROKE IS IT ITS BEEN MODED TO HAVE TORQUE AT LOW RPM LIKE A 4 STROKE SO EFECTIVLY ANYONE RIDING A PV BIKE IS WASTING MONEY ON 2 STROKE OIL WHEN THEY CAN GET THE SAME THING ACTUALY PROBABLY BETTER LOW RPM TORQUE WITH A 4 STROKE.

why bother with pv?

Edited by Zaqster, 09 June 2012 - 01:54 AM.

hope i havent shut down the topic with that. can any one prove my theory wrong.

Zaqster, on 06 June 2012 - 07:19 AM, said:

We all see what you mean, but two strokes with PV are still going to be more snappy and less smooth. Plus, most people have two strokes for the ease of maintenance, cost, and weight. Explosive powerbands are not what everyone wants, even though they ride a two stroke.

Zaqster, on 04 June 2012 - 08:34 PM, said:

Snap involves a fast transition from low rpm to high rpm, which can only be optimized with the powervalve. If you remove it, you're hurting low rpm power and slowing down the transition to high rpm power. Also, don't confuse low end power with a 4t, just look at any 2t trials bike. Placement of power is the result of a rider's wants/needs and/or a benefit to certain terrain types. If you like hyperactive input, why aren't you on a 125? If the reason is power, then why aren't you on a 500? The answer is simply that you've found a bike that works for you, but for some reason are unable to undestand how anyone would want anything different from yourself. Believe it or not, there is actually a world out there aside from your own.

Edited by corndog, 09 June 2012 - 12:25 PM.

75mx125, on 09 June 2012 - 07:16 AM, said:

+1, there are more reasons for owning a 2 stroke than just to have a bike that is all or nothing.
I don't agree that a PV makes a 2T more like a 4T, it just improves an already awesome engine design.

Exhaust valves in two strokes in most cases alter the port height (i think the Kawasaki KIPS system also added a resonance chamber to alter pressure waves in the expansion chamber also)

2 strokes with large width exhaust posts chew rings up in no time as the ring expands into the large exhaust port then gets stuffed back into the piston ring groove. So to make them reliable you would make the port not so wide but then you would drop power...so then maybe you could make the port narrow but taller but then you get a very narrow power-band which isn't really fun. So one answer was to make a tall port but with a type of valve that altered port height.

So the valve would alter port height which in turn would alter exhaust duration etc. You could then run fancy porting or pipes without sacrificing mid range power. Honda most likely achieved power and torque figures they were looking for without the use of the exhaust valve in the CR500 motors.

Edited by Spocko, 09 June 2012 - 11:52 PM.

So it would seem a lot of people realy like there pv, but they are extra maintenance , less reliable and i dont believe more snapy, yes more low rpm yes smoother no they dont make the port larger they make it smaller and open to peak port hieght they arnt part of the barrel and the rings dont touch or fill too them.. so no pv is different to expanding the port size because the port is the size it already was, yes there is no pv to add extra compression and backpressure (fill exhaust port) so the low rpm is bad mid is awsome and high dosnt want to stop , the faster flow also causes it to run richer, and seems that u need good throtle control because it wants to rev out so quick , i can peak rpm in neutral at 1/3rd throtle sooo fast , similar to if u adjust pv springs for high rpm (barely any tension) If u ask me there a gimik designed to turn profit on what can be a reliable bike

Zaqster, on 13 June 2012 - 10:09 PM, said:

Wide exhaust ports cause the ring the bulge into the port. This is well documented and was an issue in early two stroke designs. As stated before running skinny exhaust ports saved rings but flow area of the port was reduced hence  the ports got taller. Running taller ports gave you a narrow power band which was undesirable. Exhaust valves vary the port height without the downside of narrowing the power band. Also just to add a bit of bonus info, removing the exhaust valve system completely leaving a large void in the exhaust port will most likely impede the function of the pressure wave entering the expansion chamber. Allowing the exiting exhaust from the cylinder to expand and cool within a larger exhaust valve-less space is less than ideal. You would want the exhaust and its friendly pressure wave to remain hot and fast as it makes it way to the expansion chamber ensuring a high amplitude of the pulse wave.

Edited by Spocko, 14 June 2012 - 02:11 AM.

Zaqster, on 04 June 2012 - 08:34 PM, said:

I keep reading in the news about people using "bath salts" and losing their minds. Perhaps you can tell me, are bath salts slang for some wacky new drug, or are people just snorting Epsom Salt to  get high.