All 2st Dumb 2st questions...

Basically Dean....when the piston is forced down by firing...a high pressure shock wave leaves the exhaust port....this expands in the expansion chamber at high speed....until it reaches the stinger portion(small Dia) of the pipe....the pressure pulse(or shockwave) is then forced backward (via the restriction)towards the piston and bounces off it to create a scavenging effect(pulling in fresh mixture).....the power valve sits in the exhaust port and varies the aperture or height so as to achieve the desired effect(scavenging) in most of the RPM range.......

I said basically right..???rofessor:



found this

http://en.wikipedia.org/wiki/Expansion_chamber

narrower (height) exhaust port means more low end ......bigger (Higher height)more open means more top end high HP but this makes it variable and gives best of both worlds. Makes old farts still able to ride a 125 and get to work on Monday without broken bones!
Joe

im so confused, wheres my cam at then?

Dean, I'm dumb with you. Having never owned or ridden a 2 stroke bike before, I often wondered what the "power valve" doohickey was myself. Thx 2s wizards! Now, about those reed thingamajigs......? I remember Honda advertising something back in the 80's that was supposedly revolutionary for power delivery in the 2s's. Was that the beginning of the power valve?

Yes..that would be an early version called the ATAC system(80's Hondas)...it never caught on...basically(here I go again:smirk they had a resonance chamber behind a throttle plate set up right at the exhaust port....they opened it up in in stages to acoustically alter the exhaust ....I remember some guys ran them closed off.....so it wasnt that effective.....but it was a start

Call it "semantics" but if the torque is increased, so would be the spread of power and hence a widening of the power band. The PV inceases the torque which increases the width of the powerband. One begat the other........BTW which came first.........the chicke or the egg?

https://www.halls-cycles.com/Catalog/PDF/Husqvarna .PDFs/2008/CR_125_2008_spare_parts.pdf

see 2008 pdf parts catalog CR/WR125 Fig3 page20
items 26 and 27 (exhaust valves) plus all the actuating linkage. This Valve set up is virtually identical to my son's 97 RM125, at least the dual valve with common activating shaft is,,,the centrifugal engine case portion is a bit different.
For some, the valves open and close following engine RPM in this set up using a centrifugal governor.

FYI, A run down on my recent rebuild (RM125)If you look the item 26 (right valve) the shaft portion is what broke off from the main body of the valve piston and the valve piston portion slid down freely and chafed the exhaust side of the piston, seized the ring and made for some strange rattling sounds in the motor. This an example!! It was on an RM125 suzuki,, that is quite old as in 1997 model. the fatigue life of the valve was reached and it finally failed after alot of years.

Hey..you forgot the loose nut holding on to the bars.....

Hee Hee :smirk:, I was beginning to worry about you Coffee. I remember learning about reed valves while tearing apart my old Cox .049 model airplane engines as a kid in the early '70's!

I had the "Sand Blaster" dune buggy with the Cox .049 in it. It had the front wheels that you could set to run straight or in a circle. Plus the engine had a little recoil pull start cable on it,...not a propeller trying to chop off your index finger!

I had a lot of fun with that thing. All my airplanes were toast after about 3 flights ending with nose dive crashes into the back yard!