What to expect?

[Billing]

Hi all, as some of you may know, I have a '91 TZR125 which I've been slowly rebuilding and generally tarting up. I've had a few goes on it down a private road, and it's very much dead up until 7000rpm, after which point it goes ballistic up until about 10500rpm.

I've just bought a powervalve servo and control box etc, providing all goes smoothly it should hook up fine and I'll have a powervalve that turns

My question to those of you in the know is what changes in the characteristics of the engine should I experience?
Will it make more power? I.E 20bhp instead of 15bhp (for example)
Will the power be available sooner in the rev range I.E 'boost' at 5000rpm not 7000rpm?
Will the power curve be smoother, so there is a less noticable 'boost' if at all?
Or a mixture of all 3?

I'm unsure of the exact effect, but am imagining a mixture of all 3 as the valve will be closed (for low down power) to begin with, it should move as the rpm's increase, meaning that the valve is in the perfect place all the time as the revs increase, and should open fully meaning maximum power at the top end. Is this the correct train of thought? It seems logical to me, but then again I'm nowhere near being an expert!
____________________
'84 TS50X, '91 TZR125, '89 CBR400RR, '91 VFR400R NC30, '98 R1
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[Vincent]

As you guessed, the PV was there to give you more power lower down the rev range. It does this by changing the height of the exhaust port.You will find that you need less revs to pull off etc.

YPVS Yamaha engines, Yamaha Power Valve System: Yamaha engineers realised that by altering the height of the exhaust port they could effectively change the engine power delivery thereby having optimal power and torque across the entire rev range, so it was that the YPVS was born. The valve is of a cylindrical design running across the top of the exhaust port, it is turned by a servo motor controlled from a control box taking information from the cdi (and other locations). The valve is a sort of oval shape. This changes the height and size of the exhaust port at different engine speeds maximising the available power at all rev ranges. It was fitted to all of the later models of the RZ/RD two-stroke road bikes (125, 250 350 and 500 cc), the TZR range, as well as most of the YZ series of motocross bikes. The YPVS is only found on the liquid cooled bikes not air cooled versions. Yamaha have also used a guillotine version in some of their later models such as the TZR250R SPR model. Yamaha DT125R is the most powerful street-legal 125cc enduro motorbike.

Yamaha was actually the first company to produce consistent results with their YPVS in their race bikes. The 1977 OW35K was the first race bike to incorporate the power valve system and it won the Finnish GP in 1977. The Kadenacy effect was harnessed and controlled to a point that gave Yamaha great advantage over all the other manufacturers throughout the late 70's and into the mid 80's. The first street bikes with YPVS were the RZ/RD350 YPVS (LC2), and RZ500 GP Replica in 1983-84.

[ms51ves3]

If the powervalve was in the open position which it sounds like it was, it'll already be full power. The power surge at 7,000rpm will still exist but it won't be as harsh.