Carb on the KX?

[kawdude]

Just for piece of mind...

Isn't a 39mm standard for the big KX.  Someone telling me should be a 38 and not a 39mm

[demographic]

Just for piece of mind...

Isn't a 39mm standard for the big KX.  Someone telling me should be a 38 and not a 39mm

Kehin PWK 39 is standard on mine and I think on all the later ones.
Although I have heard that PWK 39s are really 39.5 mil anyway, could be wrong about that though Not gonna lose sleep over it either

[Paul]

39.5 PWK is correct

[doordie]

I wonder if it?s the best carburator size (39,5mm)for a 500cc when you use it at MX-tracks?
Look at mossbargers site www.mossbargerracing.com,they says 36-38mm would be best for the higher velocity(airspeed) it produce with smaller carburator.Maybe someone will try a smaller carb.. size?

[kawdude]

I'm no expert but I would think that a 36 would be too small although it could be made to work.  I was under the impression that this size of engine was not so concerned with velocity but volume.  Granted I understand the benefit for higher velocity but at mid range and above the smaller carbs would allow enough air to pass.  Interesting topic!

[mikesmith]

I read Destry Abbott runs a KX250 carb bored out to 38.5,and he's racing desert.

[kawdude]

Doesn't the 250 carb have the pump on it?

[doordie]

The reedcage and the area of the reeds decide how much air going through to the intake.If a reedcage have less area/flow than the carburatorsize, it should be only airspeed who can press more air into engine.So, too big carburator produce less airspeed,less hp.
The problem is to determind what size is too big!

Just my thoughts  

[mikesmith]

You mean the power jet?You have to have electrical power to run it (I think at one time on the 125 Kawasaki had the power jet carb on the bike,but the power jet wasnt even wired up) and Destry's bike didnt have a lighting coil,so even if it was a power jet carb it wasnt wired up.You think as long as the 500 has been around there'd be more info on pipes and carbs or differnt combos ect.

[kawdude]

What is the stock slide and needle that is used on the 500?

[mikesmith]

Slide is a 7.0,needle's N82R.I'm sure Rick cut some of his slide to lean it out a little and it worked well for him.Any one else try this or a differnt slide than stock?

[Twist-a-grip]

There is a trade off on sizes I think.  If I remember right my '86 had a 40mm mikuni.  '91 has a 39 kehien.  Factory must have been looking for better low end or the latter is more efficient.  The smaller the carb the better velocity/low end throttle response.  The bigger the better top end breathing capability.  You gain in one area and lose in another unless modifications to the engine create a different demand.

[John]

Where does the moose spacer effect fit in to all of this? Increasing the distance between the carb and cylinder give you a similar effect.

//John

[YUNGGUNNAZ]

John,spacer does help the bottom to midrange.tie this in with a polish rod beams to ward off some deflection of incoming charge with smooth crankcase and you should feel the size 14 to the rearend effect.now this was two cents worth.also play with spacer width.