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my 450 will only run with the choke on, if you turn the choke off and keep it running with the throttle it pops and cuts in and out, it also sounds like it gasps for air at idle like a hick up almost.....the motor is stock i have an 08 head pipe (fatter diameter) and white bros xcr muffler, my airbox lid is half open ( i cut out half of it) and if i remember correctly i re jetted it with an 088 main and didnt touch anything else, im guessing its not getting enough fuel but before i go digging in i could use some good as advice, thank you in advance!
 

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I presume you have a TRX450R and not the TRX450FE or TRX450FM?

First off - it doesn't have a choke. It has an enrichening jet circuit that adds both fuel and air for a fast idle without need to change the throttle setting.

Now generally if it won't run without using the "choke", the carb needs servicing. Sounds like the slow jet might be clogged starving it for fuel. Could also be that the changes to the airbox and exhaust require getting more fuel into the air stream. You might be able to adjust the fuel/air screw and accomplish this without having the carb worked on. NOTE: The fuel/air screw is usually capped at the factory making it tamper proof. If the carb has been worked on this screw has usually been "uncapped". The screw is on the bottom of the carb between the engine and the float bowl.

Changing the main jet has no effect on starting, idling or running at a steady speed. The main jet only controls fuel flow at full throttle. Your main jet was a 118 from the factory. You probably should not have bigger than a 128. I am curious which jet you actually changed? There is 4 jets in the TRX450R carb. The main jet is a hexagon shape in the middle of the carb inside the float bowl and can be accessed by removing the plug from the bottom of the float bowl.
 

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Check for a loose carb clamp/cracks in the rubber manifold connecting the carb to the engine.These can let 'extra' air into the fuel mixture.G'luck:)
 
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