Rejetting after going Big Bore

[Hero_Wheels]

Hello all. This summer I bought a bike that had a blown top end for cheap and decided it would be best to replace it with a bigger bore (it was a yz250f but now it is 262cc).

When I went to start it, or tried to start it, it would not unless I pushed it down a big hill and it will barely run even with choke on. I adjusted my idle screw and the tapered needle jet to make it as rich as I could but it is still running dangerously lean.

What would I have to do to rejet it for the big bore (i.e. how many jet sizes larger and which jets specifically). However, what bothers me is that I already have idle speed as rich as i can get it.

Any useful insight would be greatfully appreciated.

[kawasakimx8]

i THINK big bores call for leaner jetting. not sure tho, but almost.

[kawasakimx8]

also, fro mwat ur saying, it seems as if its running too lean already. hmmm. im not a jetting expert.

[Wayward Son]

sounds like the carb is gummed up from sitting.

[qwert321]

i THINK big bores call for leaner jetting. not sure tho, but almost.

I had to go leaner with my big bore everywhere(athena yz144) but most other riders I found on line have gone a little richer from 410 stock to 420-480main. I went to a 380main,two leaner pilot 40 to 35, and the leaner(strainght section and longer leaner taper)05 yz125 needle. So I say leaner but most say richer?But I look at it this way a 2002 cr250 with 38tmx card had 390 main stock and I think cr 125 has 440 main same carb.bigger bores must pull fuel better through jets.

[Hero_Wheels]

the carb is not gummed. It was but then I disassembled it FULLY and cleaned it FULLY. The bike is showing all the symptoms of running lean...blueing pipe, popping on decell, white spark plug. Having a bigger bore means your engine can process larger volumes of ar at once, therefore more air in the cylinder means that more fuel also needs to be added to get the proper stoichiometric ratio. What I don't know is how much fuel to add and my main worry is that it won't even start by unless you roll it and open the throttle (thus activating other jets higher up in the throttle position) and have the choke closed. So my idle is way too lean and it is already adjusted to full richness and I was under the inpression that all fuel adjustment screws are the same (so i can't gat a different one). Meanwhile the jetting in the higher throttle positions is rich enough for the bike to at least run. (it stalls as soon as you let off the throttle and pull in the clutch, therefore idle is the main issue)