does anyone got troubles tuning the kelhin 39 pwk carb on modded scrambler 400?
what is the all around performance differences between the flatside mikuni?
back when first modded my scrambler in 99,i got great result with the tmx 38mm,got the 42mm then after that and not seem to get more power but better top end...
the kelhin fit very well on the 380 2 stroke powerplant but seem crisper at tuning to.

 

scramblered,

The major difference is the flow of the 39mm Kehin as opposed to the Mikunis. When using the Mikunis, they tended to hit a flat spot at about 3/4 throttle. From that point on, it just made more noise. They just wouldn't flow any more fuel and air into the cylinder. The 39mm PWK has no "flat spot". It continues to make power all the way to the WFO throttle setting.

Generally speaking, the was carb jetted quite nicely from HPD. They set it up a little on the fat (rich) side to be safe. I went down one main jet size and have been runnning it that way since.

The 42mm Mikuni is nothing more than a taper bored 38mm Mikuni TMX. You will get slightly more flow with the 42mm than the 38mm, but neither can match the 39mm Kehin PWK. If the 42mm were a true 42mm, you would see an huge increase in top end but a corresponding decrease in low end power. You wouldn't get the intake velocity that you would in a smaller carb at lower RPMNs. This is why the Yamaha Raptor has two smaller carbs instead of one big one.

Yes, the settings on a 39mm Kehin are slightly more sensitive than the Mikumis.

Hope this helped!

 

In your northern area, buy the 42. This has been hashed out over and over on another forum. Unless you're only racing....ONLY RACING up there, you'll have to mod the needle to get rid of the 3/4 throttle lean. The 42 equipped with a leaner slide, and a richer needle (available from Rob at Hot Seat) can't be beat for the northern tier. The hair has grown back, but a lot was lost. Any winter riding with the 39 and you'll be changing out pistons. Do you want a summer only, or a year round carb. What Mike says about the cripsness is true, but for all around range of tuning, get the 42 with the different slide and needle.


Ken

 

hey mike thanks a lot for the cool explanations,i've tried them all (the tmx 38,42,)and now i run the pwk 39,the kehin have a lot more powerband i've found on the 380 2 stroke,it is crisper,more torque,and top end power really scream.
i've brake down 3 engines with a 42 in the past on race mods from hot seat...

 

I was just wondering what kind of difference this carb would make on a stock engine,piped and jetted and all that other little stuff. pretty much everything but not touching the engine. ive heard it would increase the hp a little but was not sure.

 

hpd claim a 5 hp gains with the pwk 39mm carb + the velocity accelerator on the 2 stroke scram(3+2).