Many readers have questions regarding the 340 mods for the KLX250s, 06 and newer..

A few months ago I moded my 06 KLX 250s, aka "Queeny", with a 340 kit from Thumper Racing. I transported my bike to Thumper and Mr Gary Hazel took it from there. I live about 3 hrs from his shop. He provided a 300 cylinder, performed all the labor and tuning and testing. I picked it up and clearly noticed significantly more power at first ride.
Prior to the 340 upgrade Queeny had a HMF pipe with jetting which probably added 1.5 horsepower to the 250 motor. So, I started with 19 horses.
After a 300 mile breakin, Queeny runs super, with lots of extra low end torque. The HMF pipe is retained along with stock carb. Stock turn signals went away after the 3rd jump, replaced with lighter LED types.
However, popping from "lean needle" is common after a few minutes of riding. Although, I think this is cool, the popping can be controlled with the throttle control. Apparantly dynojet has a part available to stop this.
I estimate the horse power increase is in line with the displacement ratio of 340/250 x 19, or 26 HP. I never got around to a dyno.
2nd gear wheelies are effortless. Without the headlight, 3rd gear wheelies are not too hard. Queeny is still a littly lazy, but at least she keeps me from having bad surprises.
Power comes on strong at 2000 rpm and stays hot until 7000 rpm. On road, Queeny cruises 65 up hills against 30 mph wind. Sprockets are left stock.
The downside is that mileage dropped from an easy 70 mpg to 45-50 mpg.
Oddly, due to the higher obesity rate around here over the past decade, Queeny is better looking than most women, especially when she was brand new and wet from the first ride in the rain. Even covered in Denton County Texas mud, she ain't too bad.
I really like the luggage rack, cooling fan/light, passenger pegs, and tach too. I would have went with a KTM if it had these features. I take my kid to elem school on it,and to work. For off road I remove the taillight amd mirrors. Overall, I managed to buy Queeny new for $900 under sticker, and put in about $1,100 in mods. I think I'm in bargain territory considering the features and new performance vs the extra cost.
I am extremely happy with Queeny. With the extra power, getting air is much easier. Next step is to stiffen the forks.
Anybody know where I can get stiffer fork springs, manufacturer, vendor, part numbers?

Frankklx250s

 

Moto-Pro.com

 

Good work, Frank. Sounds like you're on your way to having a great DP bike with plenty of punch.
My suspension is being re-worked by Jerry Leighton with Four Stroke Works right now. I should have it back this week. Jerry has years of experience with the KLX. Give him a call at 760 948-0325 and tell him you heard he was doing my suspension work.

Bill Dragoo
Norman, Oklahoma
KLX 331
Dyna Sport

 

Thanks for the report. I would say your HP estimate is in the ball park. Nobrakes' 331cc kit with FCR35 and full muzzy eshaust dynoed at over 27HP. With aftermarker cams, one could probably bump that figure up to about 30hp. So much depends a upon temp and humidity though, so it's hard to compare unless you use the same dyno on the same day, at the same time [8D]

Also, if you decide to bring it in for a power check, putting a street tire on the back makes a huge difference, because knobbies -especially the aggressive ones- don't engage the drum very well.