generic jetting ?...Please dont shoot me!

Im pretty mechanically inclined, but jetting baffles me. so now that thats out there, I bought a 450x that had been jetted, the air box was modded on the top,  but was sporting the stock exhaust. I installed a two bros exhaust and once i fired it up, the exhaust got way hot. I know its lean now, but ballpark, how much bigger should i go with the main jet should i go as a starting point? Also I come from the 2t world, so im not sure how hot  the exhaust is normally

yea, i looked through those, im just curious, on average how many sizes do you go up when putting an exhaust on?

yea, i looked through those, im just curious, on average how many sizes do you go up when putting an exhaust on?


Stick to the jetting thread.

The main jet is not your issue, it's the pilot and needle.

Can't guess, you have to get in there and change the needle, pilot, main, fuel screw, leak jet, and apump upgrade (pre '07).

You have to change everything at once: intake, exhaust, jetting. 

Any other way leaves you with something not right.





JD Red or NCVS needle on 3rd or 4th

50 leak

R&D fuel screw

Edited by Krannie

yes, i understand i cant guess, so since i cant guess im asking where to start do i go up by 2 or 20 for a start.


kind of hard to change everything at once when i bought the bike with the pilot, needle and main already done for the stock exhaust.


Apparently its not clear that I already looked at the jetting sticky, but am curious roughly where to start after instaalling a new exhaust, I can look at the jetting thread all day, but it doesnt say after installing new exhaust do looking for a little advice, not a database

Rather than have a formula for a pipe and or air box mod. The pipe installed (or any mod for that matter) might not flow as well as stock. So an increase in jetting size might not be the fix needed. Also judging a bikes lean/ rich condition by exhaust heat is not valid. A 4 stroke will glow red on a stand at night. No air flow. So what is the bike doing? Will it start? With or without the choke? Does it idle? Does it pop on decel? How is the top end? Does it seem to pull evenly throughout the rpm's (in one gear)?

yea, i looked through those, im just curious, on average how many sizes do you go up when putting an exhaust on?


 Understand that there is no specific rule of thumb.  Besides the main jet, there is also the needle used (which can be fat or thin and have different tapers from one needle to the next), which to a certain extent, has a bearing on the main jet size.


 However at a guess some where around a 170.   Carb tuning is done from the top down: you start at wide open throttle, get the main jet correct, then work down to 3/4 and mid-range and get the needle and clip position set, then down to the low end, when your running on the slow jet and fuel screw.   Here's a graphic of what affects what and when:





Don't worry about where the jetting is currenty.  I don't know where you are in CA but if you are below 6000 ft., go with 165 main, 45 pilot and JD red needle even with the new exhaust.  When the bike is jetted correctly it will still get hot while idleing.

It's hard to tell you how many sizes to go up or down on your jetting until you know what's in there. Go with what Krannie suggested as a starting point and your bike should work great. Good luck enjoy

Yeah, it's best to not follow 8 years of experience available here and find a NEW more complicated way to re-jet your bike.

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