Bored 250 Jetting

[pickledkiwi]

Hi guys. I've just bored my 99 KX250 out to 265cc. It currently has a 170 main and 42 pilot. Needle is on the middle clip. Because this motor is on a kart, the majority of its running tends to be WOT. The last piston in it (at 250cc) had been a bit too hot as there were signs of excessive heat on the crown and black burn marks under the crown.
When I get it back on the kart, it will also have a PC pipe which it didn't have before.
I'm guessing that the larger bore and PC pipe will mean I need more fuel, but how much more. I don't want to buy a whole shop load of jets, so any pointers would be helpful.
Thanks
PK

[Friar-Tuck]

P.K.
 Pro Circuit has specs for their pipe on a stocker,
 Stock KX250     158     45     N3YK     3RD     2     55                       
 W/PC Pipe      STD      STD      N3WK     3RD    1 1/4    58

16159-1056 (POWER JET)             16187-1079  (NEEDLE)

Hope this helps,
 A few more jets would certainly be alot cheaper than a new top-end.
Is there anyone at the track who would let you borrow a few jets to get close?  I realize the Racing community may be a little hesitant to "Share" with a competitor, but it may be worth a shot.
  Did the shop who did your engine work offer any suggestions?
Tuck\o/

[DoldGuy]

Good info from Tuck. Also keep in mind that a bigger bore will "Generally" run a bit richer than a stock bore as it has a stronger vacuum signal on the carb which will pull more full thru the same jet. Of course this all depends on who & how the cylinder/head was setup. As Tuck stated, get 5 mains (1 down & 4 up) as it's not much $ compared to what you just invested. I would also purchase some of the powerjet jets (if memory serves me I think I paid $10 each) as this can really help out the mid without causing to lean of a condition up top. Also keep an eye on the spark plug ground strap to determine you are running the correct heat range (the color band should be right at the bend, if it's closer to the plug it's too hot of a plug). Shifter Kart??? I wanna try!!!
Hope some of this helps

[pickledkiwi]

Thanks Guys. You're quite right about the jets being cheaper than a top end job and I never checked the powerjet to see what size it was. Funny how you'll spend $100 of dollars on a barrel and then try to save $20 on a couple of jets! The motor pulls really well on the kart but I've gotta stop overrevving it.
The guys that did the bore and replate are not engine tuners so I didn't bother asking them. I'm sure that if I start off conservatively and slowly run it leaner then I should be right.

Thanks Again
PK

[pickledkiwi]

Hi DG

You were exactly right about the stronger signal. I started way over-jetted and ended-up back at the 168 main that I had before the bore job. It runs crisp and lively and the plug-chop shows a nice mid tan colour.

Wheelstands in a kart is a buzz!

Thanks
PK