DT125R ARCHIVE

fused420 · DTR Senior Member

P.Valve FAIL!.

fused420 · DTR Senior Member

Mate as much as i love you for all of the help and advise you have given me and everyone else on here, i fear you are still missing my point..

IF the PV was set to turn AWAY from the piston, like the pic below, surely there would be no need to grind the PV down?!?

-- Edited by fused420 on Friday 30th of March 2012 12:09:55 PM

fused420 · DTR Senior Member

So am i right in thinking that all you guys that have had to grind down your valve also have the same problem? Or what would be the point of grinding if it turns away?

NORFE · DTR Trusted Engine Builder

only have to re profile it if the cylinder bored larger mate

fused420 · DTR Senior Member

Even with the valve fully open, i can't see why you would need to grind down any more than the sharpe edge that faces the piston, at most..

NORFE · DTR Trusted Engine Builder

if ur pining it open u dont need to grind it al all only if its operational pal

it only mooves about 10mm from closed to open

look at the 2 pics on my old post http://dt125r.activeboard.com/t47212329/power-valve-modification-for-over-sized-bores/

thats all that u need to modify in the red for 1mm piston clearance when valve closed biggrin

NORFE · DTR Trusted Engine Builder

how that pv is in the pic is never how it is.

when its open its flush with the top of the exhaust port so its a smooth line all way out of the port

when its closed it drops down infront of the port

if thats how it it in ur bike how its sat there it will be restricting the flow of gas big time....

fused420 · DTR Senior Member

Right, so you are saying it is suppossed to turn towards the piston? like this but not so far..

NORFE · DTR Trusted Engine Builder

thats now in the correct position of it when its closed ! ignition off!

as you can see if u put the piston in the barel now and slide it up its sits really close to the valve drum

when the bore is bored the pv drum needs to be re profiled to give a clearance of 1mm

then when the rpm rises the pv opens - turns clockwise until fully open flush with the top edge of the ex port mate

have you triied turning the valve 180 degres and then putting the pulley back on

tripr · Guru

good read after having a mooch i think iv been running mine backwards for a long time.
cables 2 to 1 1 to 2 with the valve turning towards the front wheel an sitting anti clockwise on idle. with a closed off the exhaust edge and an raised piston edge turning clockwise with rising rpm's opening by raising the front edge an lowering the piston edge. the bike went lovly an sounds sweet like this but had a boost still like a pegged system.
i have no rebore or o/s piston on the athena block so the pv shouldnt need a dremmel
swapped the cables arround yesterday now my bikes lost all major grunt, but the pv seemed to pull the bike as suggested it should right through the rev range. after a long ride tho
theres an evil knocking noise that sounds like someone describes above but only on idle after some use. when you first start it up, its running an sounding sweet ( well no knocks )
doesnt sound as angry. or powerfull.

and its disconected now due to me snapping cable 1s adjuster lol damned magic touch

-- Edited by tripr on Monday 2nd of April 2012 08:26:05 AM

-- Edited by tripr on Monday 2nd of April 2012 08:28:14 AM

AndyYam · Guru

This is the thread I have been looking for but I'm still unsure.

To clear a few things up.

The powervalve is, as we can see matching the shape of the exhuast port. 2 strokes operate well at high revs and poorly at low revs.

This is because at low revs a proportion of the fresh mixture is lost out of the exhaust port as the port is not completely covered by the piston by the time the mixture is coming in. In one side and straight out of the other.

The powervalve works by turning in the exhaust port and effectivly lowering the height of the top of the exhaust port, this means that when the mixture comes in the port is covered by the piston and we keep the whole lot for a better bang. The piston is not coming up fast enough to block the port off in time. Therefore the valve turns to block this hole that the mixture is escaping from.

It turns anti clockwise ( if looking from servo side of bike) away from the piston as the revs increase...BUT....it starts turned in "towards the piston". It set's itself to thi position when it goes wherrr wherrr, when you turn the key in the ignition.

As the revs increase and the piston is moving up and down faster and able to block the mixture from escaping from a higher point therefore the exhaust port needs to be higher and so the valve turns anti clockwise or "open". It will continue to turn as the revs increase until it matches the 'real port' shape, where it rests. I would imagine full open would be around 5-6krpm as from then on your in the powerband and this is what it would be like with the PV pinned open. Based on this I don't understand why people would pin their powervalves open unless they do no have a servo. Surely all it will do is lose you bottom end power and gain nothing from it other than a big lump at 5-6krpm.

Having the powervalve turn the other way is pointless. The powervalve self cleans when the ignition is turned on and re sets itself to 1krpm( roughly) which is closed. It is in the closed position that the powervalve is turned "towards the piston" and so the rings will be hitting at low revs, not high revs. At high revs the valve will be fully open and matching the exhaust port, clear of the rings.

If you have oversize and are worried the best thing to do would be to discconnect the servo and pinn the valve fully open until you have resolved the issue.

I'm going to get some of my barrels from the shed which are of varying re bore sizes and see what i can figure out!

Andy

-- Edited by AndyYam on Wednesday 11th of July 2012 04:57:56 PM

NORFE · DTR Trusted Engine Builder

deffo get best of both worlds with the servo if its setup correctly

even if its a small ammount out of tune/allignment it will alter the powerband

1mm os bores need the pv drum re-profiling to give a clearance off 1mm from the drum to the piston rings smile

AndyYam · Guru

So base on that a 1.25mm oversize piston needs 1.25mm shaving?

NORFE · DTR Trusted Engine Builder

hard to say exact mate

as each bore/cylinder and pv drum will differ in specs as no 2 are identickle

best thing to do is have the barrel bored then fit each pv drum half at a time then reprofile to suit lookin from each side u can see how much u need to re profile

http://dt125r.activeboard.com/t47212329/power-valve-modification-for-over-sized-bores/

thats a quick idea of how to plan what to mod