atc 250sx with 350x carb swap - whose done it successfully??

[userj8670]

I've read here that this mod has been done before but has anyone done it successfully? If so please post up how you dialed in the 350x carb.


I have been messing with the pilot jets and the needle to get her jetted right and it's a bit of a pain . Another forum member has been helping me a tremendous amount but I wanted to grab other people's thoughts as well.

I have a dead spot in the throttle when I punch it from idle...boggs down. My pilot is a 40 and the needle is going to be clipped at the top to lean it out. I have tried 38 and 42 jet sizes as well. The 40 seems to work the best with the least amount of popping on decel

[Dirtcrasher]

Unless you mod the SX, the 350X carb with SX jets fixes the cold start crap. Simple swap.

Or within a couple sizes.

If you mod the ex. pipe or the airbox, you can jump a couple sizes on the main jet.

I haven't even started my SX in 8 years.......lol

[userj8670]

Unless you mod the SX, the 350X carb with SX jets fixes the cold start crap. Simple swap.

Or within a couple sizes.

If you mod the ex. pipe or the airbox, you can jump a couple sizes on the main jet.

I haven't even started my SX in 8 years.......lol

DC I was hoping you might chime in!

I fixed the cold start issue by drilling out the starter jet so thats not a factor. I put the X carb on strictly in hopes of performance gain. All i have for mods are a full supertrapp on my sx, a uni filter, and a Rocky piston courtesty of Jeff who doesnt seem to be around anymore? My jetting on the X carb is a 138main and a 40 pilot at the moment. My problem is there is a dead spot where if i mash the throttle from an idle it bogs down and will die. If i build a little RPM and mash it, it works no problem. I tried a 38 pilot but it seemed to lean as i would need choke just to keep the bike running and make it idle - additionally lots of popping on decel. THen i tried a 42 or was it a 43. Either way that was no good as it was popping alot on decel as well and the dead spot was still very evident. The 40 seemed to be good with minimal popping on decel. I then moved my attention to tthe needle. When i started it was in the middle clip. I moved it 1 down on the clip (richer) and didnt notice any change. It would still bog and i also used some choke to see if more fuel would help the dead spot...no avail.

I then moved the clip to the very top (leaner) and it still bogs when i mash the throttle BUT i do need significantly more RPM's built before mashing the throttle to prevent it from bogging down. Additionally the popping on decel has become more noticable here. I also used the choke here with reving it up to see if more fuel made a difference - it did not. I have not touched the air/fuel screw yet as i figure that is for fine tuning once i am somewhat close. I am really stumped. Not sure whether to go back to trying pilots or what.

In talking with another memeber it was an idea that due to the smaller engine, it might not have the vaccum to power this bigger carb so we might need to lean it out hence my moving the clip to the top but again it seems that i need more RPMs this way to mash the throttle??? Would i want to try my jets from the sx carb in the 350x carb? The jets in the carb now are what was in my 350x.

[Dirtcrasher]

No, that crab will run perfect.

The 1st thing I would have done was removed and checked the air screw.

Make sure the order goes: screw, spring, washer, o-ring. Set it at 1.5 turns out.

If it does nothing when turned, the pilots too big. If turning it out 3 turns seems to run well, your pilot is to big.

If you want that SX to move, cut a big hole in the air box lid or remove it.

That Rockey 11:1 and the Super Trapp exhaust will wake that SX up!!

You WILL be lean at that point!

And remember, change 1 jet at a time and write down the results....