250-500cc Wr250 300 kit 2009

Hello

This week i will mount a 300cc cilinder on mine 2009 wr250, what will it do with the jetting of the mikuni
Riding sea level.
Iddle jet 17.5
Main jet 460
Jd needle red
Maybe some people made this conversion allready.
What would be a nice start with the jetting for 300
I have also the HTM racing head and piston.
Fmf gnarly exhaust

Best regards

Oke thanks
The compression is almost stock but with a mutch better curved cilinderhead and piston with squish of 1.2.
I use only octane 102
The cold start is also verry poor becouse the 17.5 iddle jet but hot it runs perfect it could iddle for ours :-)
Is there no basic rule when going from 250 to 300
Let us say main 20 bigger and iddle 17.5 to 22.5 ore something to start with

Often you go smaller on the main when going up in displacement. I think 460 will be too big for the 300

It's a 300 now, so just jet it as such regardless of what's in the carb now. Adjust the float level so fuel comes out of overflow only when bike is leaned 45 degrees or more. This should improve starting. PITA cold start ritual with POS Mik is lean bike over until some fuel comes out of overflow, push kicker a couple of times until you find compression stroke, kick it like you mean it.

Oke thanks alot
I will first try it as it is Steve.
Cant wait to get the bike back :-)
A older hrc 2 stroke mechanic will build it together for me.
The jetting will i change when i get the bike back from him when i need to.

Unless the 300 has the race head installed it will have less compression than the 250. The cranking compression numbers on a 250 are typically 200 psi and the 300 hangs around 180 psi. unless the squish is modified or a race head is installed.

After this weekend the bike will run and i will let now how its running

I found I got really loud detonation when I went to the OEM 300 cylinder and OEM head on mine. I had to increase the main and went to the richer needle from the JD kit. But it was easy to tune out.

Oke thanks all
Bike is running great
I have 17.5 iddle jet
480 main
Red needle
The 300 is a little stronger than the 250 from low revs but i expect more from it.
My 250 with ch hihg conpression hed was really fast.

Little update
490 main iddle 17.5
Still i need more fuel i think i have to go to 520 or 530main jet.
With the 250 460 was ok but the 300 needs lots more.

i would say the pilot is a mile too lean
nobody can ride a 300 wfo all the time
suspect you are on the pilot more than anything else

I think i found the problem
After going from 470 to 480 and 490 and 500
Wot didnt change
The jd needle and needle holder cant suply more fuel.
I try the original needle the 6dey15-74 with 40 iddle and 480 main i think wot will be a lot better

Just tried it didnt help :-(
490 main 15-74 needle
Pilot 32.5
Bike is running perfect
The difference to jd needle and needle pipe is verry little
Only now when i am do some trail and go wot its spooks 2 or 3 gears becouse its full with fuel.
Then the bike is mutch to lean again.
Someone now what could be this issue
Bije runs rips like hell but why do i need a main jet over 500
I tried to order today they all told me 500 is maximum size something must be wrong with your bike.
I hope someone has a idee to solve my problem

The main only controls the fuel from about 3/4 to wide open throttle. The problem will be somewhere else than the main. Read this, http://www.iwt.com.au/mikunicarb.htm it is basically the same principal for all carburetors except the Lectron.

Yes i understand the principe and working thanks for the link. I will mount the jd kit again and wil try an 510 or 520 main the buke really rides good verry smooth and crisp with the jd kit but only the large main is a little strange.
I ride sealevel below. Ive seen some using 480main at 3000feet maybe is 500+ at sealevel not strange then.

suggest you make sure the tank filter is clean

someone here will know how much the petcock flows per minute- 200-300cc fuel per min at least- one thing less to puzzle you

That size of main is insane, there must be something else going on here like maybe the float needle is hanging and causing a low level under wide open throttle. I had this happen on my 09 wr250 and I had to adjust the clip on the float needle because it was hanging up and restrict the incoming fuel causing it to starve in high gear wide open.

If this is helpful, I looked up my best setting on my wr250 Gnarly pipe and it was 420 main, 32.5 pilot, GAY needle, 1 turn on the air screw. This was at 1500' air density altitude. If you are at sea level your air density altitude will not be zero.

Oke thanks
The too little pilot effecys also wot
When i have 17.5 or 37.5 pilot jet it is the same when having 490 or 470 main
Maybe i have to set the valve a little higher and maybe it will run better then with the bigger pilot jet.
Is there a stock setting for the valve of the carb for iddle speed?
Thanks to all for the help!!