Well, at least a surprise in the sense that I ended up with jetting numbers a bit different than what I would have expected. My '06 KLX250S was bought used and came with a full Muzzy header/muffler setup, a fully opened airbox (screen also), Uni foam filter, and an FMF jet kit. The bike was notably rich, so I opened the carb and found the main jet was a DJ132 with a 6-position needle in the 4th slot from the top, and a 35 pilot jet. The slide was already properly drilled. The previous owner still had all stock parts and gave them to me at purchase. I changed the main jet to a DJ128 and dropped the needle to the third position from the top. The bike then ran crisply and cleanly with excellent power, no stumbling or detonation, and good mileage. The bike did have the occasional cold hard start, especially when temps were low. I did a compression check, and the 250 was at the max end of the recommended test pressure. I'll note that this bike ran great and would pull redline with 14/47 gearing in top gear. Plug readings at cruise throttle and WOT were ideal.

OK, move ahead to my installation of a new, stock 300 cylinder and piston. Everything else was left as is except for the pilot jet and the addition of a Kouba fuel screw. Without being able to adjust the fuel screw while idling, it's darned near impossible to tell the condition of the pilot/idle jetting. I installed a 40 pilot jet, as I thought this would be a no-brainer with the new cylinder and piston. I though richer jetting would be required overall but decided to leave everything as it was to see how lean it would be. Initial startup and a quick Mototune-style break-in have gone smoothly. Surprisingly the bike runs great at all throttle openings, it revs out hard, it even lugs cleanly, it pulls redline in top gear, has no detonation, and has notably better torque and overall power. The spark plug color was that nice magazine quality light brown/tan you always wish for. The only rough spot was at idle. Tuning the fuel screw clearly showed a too rich pilot jet, so I went down to a 38. Now the fuel screw would get the highest idle at 1/2 turn out from fully close. That indicated that the pilot was at the rich edge of optimum. I went to the stock 35 pilot and got a nice 1.25-1.5 fuel screw adjustment with a strong idle and excellent off-idle response. Valves have been adjusted dead in the middle of the recommended range both while this bike was a 250 and now in 300 setup.

So...my surprise is that I ended up with basically the same jetting as the 250cc setup was running. The bike runs very strong and pulls cleanly throughout the range. There is notable improvement in power throughout the range. It's not like it has a rocket booster attached to it, of course, but it is without question a stronger engine response. The most notable improvement is how much better the bike pulls out of slow corners without having to downshift as much...no real surprise there. My compression test produced almost the same exact number as the 250, so no surprise there. None of this is exactly earth shattering, but I'm still surprised the jetting remained bascially the same. From some of the other jetting results on this and other forums, I expected a need for slightly richer settings. I also realize that all bikes aren't exactly the same in their mods or components, but I still anticipated richer settings. I'm not complaining, just a little surprised.

Oh, and on the cold start issue, I drilled the OEM starter jet out from .017" to .022". With the same jetting as before, the starter jet drilling should be easier to detect its impact on cold starts. Now you can hardly get your thumb off the button before the engine has fired up with the choke on, no matter what else is going on. I think I would only go up to about .020" if I were to do it over, as the .022" drilling seems a bit rich. I can't leave the full choke on but 3 or 4 seconds before it starts to "blubber" richly. This has no influence on the overall jetting of the bike once you close the choke.

I still have the original receipt that shows the purchase of an FMF jetting kit. My research shows that FMF appears to use Dynojet parts from the info on their site, and the parts the previous owner gave me appear to be a DJ kit with the little plastic compartment box like I've received from DJ for other bikes of mine...including DJ main jets, drill bit, etc. Here's a pic of my needle. Does it look like the DJ needle supplied to you DJ users out there?

 

The needle indeed does look like a DJ needle from the KLX300 kit.

 

fmf jet kit is a rebadged dynojet kit with the most important part added, StIcKeRs.

 

Damn! I didn't get any stickers!

 

Damn, I wish I had a 300 or 331 or 351! Oh well, my bike is still fun.

 

good report TNC It's not unusual for a big bore to use the same or smaller jets, the motor can pull more fuel by the increase in vacuum with the same airbox restriction. If you were to further uncork the airbox side you might be bigger on the main.

For comparisons sake my 250 pulled almost a perfect fuel line on the dyno using a 128, 40 pj, 300 needle and 300 exhaust uncorked with billet tip, kdx snorkel. BUT it was never right riding. Rejetted it by ear and run quality and fell back to a 38, then 35 pilot just like you.

Lived with a slight lean surge at 6-7k on a cold morning that went to a totally unacceptable flat spot when I installed K&N (coming from a stock foam. Tried lifting the needle 1st but got nowhere. Threw in a 132 and bingo, perfecto. Could go even bigger with the box lid off, but I'm in too much water.

Point is, these things are very sensitive to intake and exhaust mods and you just have to go out and jet them. I wasted time online trying to find what everyone else thought was a good setup. Didn't work on my bike. Even the previous owners dyno time was wasted IMO.

 

Good to know...thanks for that insight. I was thinking that something like better efficiency was occurring...or something along those lines to produce these jetting results.

 

I really appreciate the way this thread was started - with well written detail and real paragraphs.

About jetting, I recently set up a KLX400SR (DRZ400) and corresponded with people in that forum. I ran into some dogma about jetting, with one-line absolutist statements on jet size, size of airbox cutout, etc. I considered to the advice but jetted my individual bike for what it wanted. Instead of a 155 main jet, I ended up at 147.5 from the stock 142.5, the 155 would have been over rich for my particular bike.

Surprising results happen because of the large number of variables involved regarding the flow of the atmosphere through the engine and with jetting subtleties.