Can anyone explain how the power valve works? Cheers taz.
Modern power valves work like a shutter to adjust the height of a stroker's exhaust port. At low revs they prevent power erosion by restricting the premature exhaust of unburned fuel/air. When the revs increase they open to allow better breathing giving optimum power. The effect is to give a wider useful power band across the rev range but they don't actually increase the engine's BHP. Before power valves you needed to rev the guts out of a stroker to make it fly. Anyone who's owned a Yammie RD400 will know the lethal kick when the rest of the horses come charging in at 6000 rpm. Consequently they've all been "up the road" at one time or another with an undented example difficult to find. Not that this ruined the machine as taming the beast was part of it's appeal. The YPVS system that followed worked using a servo motor to turn a spool-shaped port valve thus constantly retuning the exhaust port. It offered much improved flexibilty but the design appeared to ignore the heat and clag generated under hard thrashing making them sticky and unreliable bastards to maintain. riccip
I'd have thought it was slow speed work that causes gumming up. Thrashing them would generate enough heat to keep them clean. I've never had YPVS but I've had a number of two strokes.
Broadly, you can shape the ports of a two-stroke to give decent mid-range and torque and not much top end, *or* sod-all mid-range and torque and a screaming top end. MZ is a classic example of the former, and an old Kawasaki H1 of the latter. If you wanted a fast two-stroke, you had to make do with the engine deficiencies elsewhere in the rev range. What a power-valve does is adjust the port height by means of a revolving barrel at the edge of the port. So you can have a screaming top end *and* decent mid-range. It doesn't actually increase power by itself, as you say. It just makes the bike more driveable. Control is via a servo motor, yes, and a sensor system which monitors engine revs etc. However, power-valves used to seize up if the bike was driven too *slowly*, because in a two-stroke that's when you get unburned crap gumming things up. Also, on the 350, the power-valves were right at the front of the block (well, they had to be) and the bike's inadequate mudguarding allowed all sorts of road crap to splatter onto them, which didn't do them much good. On the TZR250 and RD500LC they were better shielded, and gave much less trouble.
Cheers guys. Do the manufacturers use a similar type of system or do they vary from make to make? Whilst looking for bikes that are for sale I have seen once or twice a reference to the power valve being disconnected. Are some of them a manual thing? Cheers taz.
KTM use a simple centrifugal device with ball bearings running up a track against springs in their 2 stroke competition off road machines. I spent many a happy hour cleaning the clag off the valves on my last one. Ian
That's because Yamaha still hold all the patents on their own system which was, and remains, the best.
They vary. As I've said elsewhere in this thread, Yamaha invented it and the Yamaha system remains the best. Suzuki's RGV250 used a sort of guillotine affair, IOIRC, and Kawasaki and Honda had their own versions. None is a manual system AFAIK, though you can convert Yamaha ones to manual via a handlebar lever instead of a servo motor. Hardly an ideal solution, though.
The Older Gentleman was seen penning the following ode to ... whatever: That's the one that occasionally guillotined its engine, right?
The Older Gentleman was seen penning the following ode to ... whatever: Even I'm not that mental. Occasionally tempted by the Aprillia RS250, but I've managed to resist so far.
You're right. It's arriving at slow traffic in town while they're still hot that gets the PV seals hissing like a Madagascan cockroach which is why they're a nasty commuter. Maybe I was unlucky but mine was a bitch. Terrible build quality, ropey electrics, and it would routinely seize then unseize on the motorway with no apparent reason (or damage). Strictly for solo scratching as an anorexic on the pillion would turn the headlamp into an anti-aircraft searchlight. The earlier LC's (4L1) were a much better bike provided you looked after the cooling system. But the old aircoolers are still by far the best IMHO. A pair of Allspeeds and an induction kit can beef up the mid-range power almost as good as powervalves, and god they can take some thrape. riccip
Same engine, so same chance of the infamous piston hitting the valve problem that some later RGVs had.
mike. buckley was seen penning the following ode to ... whatever: I know that. Which is why, curiously enough, I managed to resist the temptation so far.
