OK, I've spent the last few days reading up on KLX250S horsepower ratings, jetting mods, and dyno runs, so I know just enough to be dangerous. At the end of the day, what's the best formula (jet, needle, air/fuel mixture, etc) for the best gains, and can we qualify/quantify the amount of gain, which I understand varies by RPM (typically more gains above 7K?)??

 

KLXSTER has pretty well documented a best case scenario with the stock displacement.

There's too many variables to say what the default go-to set up should be though. It's going to depend on your altitude, fuel, mods, temperature... the list goes on.

Honestly, the best formula for gains is without a question: install a BB 351 kit, jet it, uncork the exhaust and intake and call it a day.

 

How much do you want to spend? That's always the question, isn't it?

BB351 Kit. Adds power everywhere $530
Jetting up to $150 adds some extra power.
Mikuni 36-68 Adds power and throttle response. $325
Pipe Adds noise and some power $50 (used KLX300 muff) to $800 Full FMF system
Marcello cam mod. Free and adds good power ~Be careful!
Other free mods. YMMV

Most, but not all would agree that the 351 kit is pretty cheap and rewarding but it requires some skills to install. I just did a friends bike and jetted it without a muffler. It's a lot better.

You can easily spend a couple of grand but for $1000 I'd go with a slip on, a 36-68 carb and the BB351 kit.

In the end it's about how much you want to spend.

 

If your near sea level, I can get you around 22 HP cheap and easy - and the mods will also serve to enhance a future move to BB351. So no money wasted.. This is a 5-7 HP increase depending on what you choose to believe is stock HP.

 

This is the only complete Dyno chart I've seen on stock power.
http://kustom-kraft.com/files/KLX250...rt_6_22_11.pdf


BTW: The power levels of their 330 kit are also on that chart. I have no idea how they tuned that 330 but I get the same power levels with my Dyno tuned 250..

 

KLXSTER - Thanks for the input. Your chart/graph information was primarily what I've been pouring over.

I'm at sea level. Fill me in on 22HP, emphasis on CHEAP and EASY.

I would almost bet that by the time I'm ready for the 351 mod, i'll be moving do a different bike; lighter, faster, ... that's not a blaspheme here, is it?

Quick background: I got the bike because it was street legal, off-road capable, and had an electric start. It was more responsive and lighter than the Honda 250L and 230L, and less expensive than the Suzuki. My primary use is to enter enduro events and trail ride with my teenage son.

I've also heard there are great gains to be made in the arena of suspension, but that's another story for another day.

Cheap? Easy? Talk to me, Goose.

 

So you feel the need.. the need for speed..lol

I assume your bike is stock and runs perfectly well? I don't pretend to troubleshoot or be able to fix carb/performance issues..

 

Runs well. A little tough to start in the cold weather. When it's hot, I hit the start button and roll the throttle on 10-20% and let off, and she starts like a charm without fail. I bought it with an FMF Q4 exhaust on it. The snorkel is off the air box, but the lid is on. I confess I haven't taken the carb apart yet to see what the jetting is. The *only* issue I have is that when I let off the throttle when it's under load, it pop-pop-pops.

I also have a buddy who owns three of these (one for himself, one for each of his teenage sons) and they're looking for the same gains. His are absolutely bone stock and work perfectly well. A little more difficult to start in cold weather than mine. I figure we'll experiment on his bikes and if it works then try it on mine. (what are the odds that he reads this, right? )

 

There are a few methods for improving cold start problems. One cure is to drill out the enricher/choke jet to 0.018". Something to consider while you're in the carb. Be sure to modify the correct jet. This is making it just a smidgen larger than stock.

 

Ok, Dyno run done. DJ150 went too rich..

So, Q4 eh? Stock header I assume..? De-Smogged I hope..?

Your decel pops are about to be cured and engine starting should improve..

Get a DJ 2152 kit, a DJ140 main jet , a #38 or #40 pilot jet. Optional is the Kouba Extended Fuel screw.

Install the pilot jet while installing the kit (as per instructions for stage 2) but use the DJ140 main and ignore the instructions for setting the fuel screw. Set the fuel screw to 1 turn out - it will be fine tuned later when the bike is idling at operating temp. Pull the lid off the airbox. When fully warmed up and idling, turn the fuel screw out (Richening) until idle drops, then in till idle drops, then back out to the midway point between the two drops.. If you don't have a leaky exhaust seal and/or header to midpipe seal, your "pops" are history - and starting issues should be gone..

That's 140/3N with the 2152 kit and larger pilot.. Good for around 22HP and around 15.5 lb ft TRQ , with a good slip-on + stock header with lid off , around sea level.
For you tech oriented guys - It'll provide an AFR @ WOT between 12:1 and 13:1 with lid off, stock header + slip-on, near sea level..