All 2st Dumb 2st questions...

[Coffee]

This thread was split off from another thread cause I took it off topic Sorry!

Any anecdotal info?

Ha! Thought you had me on that one didn't ya.

Here is one for you, I don't even know what a power valve is. I especially don't know if them cracking or breaking will cause any other major damage.

IR dubm.


I'll keep watching this thread.

 

[Troy F Collins]

Here is one for you, I don't even know what a power valve is. I especially don't know if them cracking or breaking will cause any other major damage.

IR dubm.


I'll keep watching this thread.

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

 

[HuskyDude]

Or simply put.

A power valve is just a piece of metal or shaft that moves down and covers part of exhaust port making it smaller. Big ports mean big horsepower, but they also mean narrow power band. By making the port smaller, the power valve helps make the power band wider. It does this by keeping more of the fuel mixture in the cylinder, and out of the exhaust pipe, at lower RPM.

my words are smaller

 

[Motosportz]

tunes the exhaust port for broader power.

- Dean can even understand that one.

 

[Joe Chod]

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

 

[Coffee]

Or simply put.

A power valve is just a piece of metal that moves down and covers part of exhaust port making it smaller. Big ports mean big horsepower, but they also mean narrow power band. By making the port smaller, the power valve helps make the power band wider. It does this by keeping more of the fuel mixture in the cylinder, and out of the exhaust pipe, at lower RPM.

my words are smaller

Ok - a plate covering part of the exhaust but not all of it i.e. it is not closed at low rpms but smaller. Is this plate 'static' at a certain rpm? Or is it rapidly moving with each stroke of the engine?

 

[razornpc]

im so confused, wheres my cam at then?

 

[ioneater]

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?

 

[rasputin]

actually, the power vale does not make the power band wider.

on a two-stroke engine you will find a torque minimum at around 2/3 rpm of the torque maximum. due to this minimum the torque at the beginning of the power band increases rapidly, giving the two-stroke that "hardly rideable" image.

the power valve eliminates this minimum. so the power valve does not make a wider power band, but it increases torque below the power band, pus it delivers a smoother transition into the power band. the result is a more predictable and controllable bike. on the 125 you will feel the effect mostly at 7000-8500 rpm, on the 300 at 5000-6500 rpm.

r

 

[HuskyDude]

im so confused, wheres my cam at then?

Four strokes have cams above the piston.

Two strokes have nothing on top the piston except a spark plug.

The power valve is locates between the piston and the exhaust port.

Hope that helps.

 

[Troy F Collins]

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

 

[Joe Chod]

actually, the power vale does not make the power band wider.

the power valve eliminates this minimum. so the power valve does not make a wider power band, but it increases torque below the power band,

r

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?

 

[robertaccio]

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.

 

[Coffee]

Thanks for the pf link Robert, it helped

I think I'm clear enough on what a power valve is at the moment for my purposes.

What is a reed valve?

BTW - I know how generic 2st engines work and have re-built many in my youth, just not the motorcycle specific terms like power & reed valves.

 

[Motosportz]

Google is your friend...

pv - http://en.wikipedia.org/wiki/2-stroke_power_valve_system

http://science.howstuffworks.com/two-stroke5.htm

 

[Troy F Collins]

Four strokes have cams above the piston.

Two strokes have nothing on top the piston except a spark plug.

The power valve is locates between the piston and the exhaust port.

Hope that helps.

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

 

[Coffee]

Google is your friend...

http://science.howstuffworks.com/two-stroke5.htm

Never mind about the reed valve question. All the other 2st that I owned had reed valves.



A mind is a terrible thing to waste.

 

[krieg]

Never mind about the reed valve question. All the other 2st that I owned had reed valves.



A mind is a terrible thing to waste.

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!

 

[HuskyDude]

I remember learning about reed valves while tearing apart my old Cox .049 model airplane engines as a kid in the early '70's!

Wow does that bring back memories.

 

[Rusty 2]

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!