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Discussion Starter · #1 ·
hey guys im new here but ive got a question, i have a 1987 trx250x that isnt running right, it has a popping/breaking up efect from half to full throttle.. everything has been thoroughly gone through, air filter clean and carb was cleaned twice. it seems to come down to the carb isnt jetted right. but the problem is the jet thats in there has no size on it so i have no baseline to start off and know what size jets to buy..

i was wondering if anyone can give me a rough range of what size jets to try in this carb to get it running good heres the mods: uni filter, coffmans race exhaust, hc piston, high lift cam, head ported, stock carb.. the pilot jet is a 35..id also like to rejet it with the airbox lid off for better performance. thansk in advance for anything you guys got!
 

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welcome to the forums..as for the jet size..i'm not into racing crap..lol..but..the jet size itself will be stamped on top or the side of the jet itself..sorry i couldn't be more help.
 

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Welcome to the forums.
 

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Welcome to the forums... I would start by changining the slowjet to a 40 then turn the airscrew in until lightly seated then back it out 3 turns. the main jet will be somewhere in the 145 to 150 range. The needle clip should remain in the third clip position. I would go to ATV Parts, ATV Tires, Dirt Bike Parts & Motocross Gear | Rocky Mountain ATV/MC and buy the hex jets for the Keihin carb start with a 138 and go up to a 160 skipping a size in between so you don't buy so many. Also buy a 38, 40 and 42 slow jet. That should help you out.
To set the air screw you can do like I suggested or simply start the engine turn the idle up just a tad enough to hear a difference in engine tone. now turn the screw all the way in until it seats.. the engine should stay running (barely) now back it out in 1/2 turn increments allowing about 15 sec in between for the engine to balance out. when you get to a point where the engine RPMs increase all that they are going to do turn it back in 1/4 turn and reset the Idle. You're done! good luck if there's anything else we can help you with fell free to ask.
 

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Welcome to the forums!
 

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Discussion Starter · #8 ·
sorry to bump this thread back up but thought id update you guys! after cleaning the carb 4 times and trying different jets and needle positions finally found out the accelorator pump was the problem! it was engaging to quick and flooding the motor, took it off and bent the lever a bit and put main jet from 125 to 130 ruinning amazing now :D
 

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that's good news. but keep in mind you need to do a plug check a lean engine will run like a scalded dog...for a little while.lol make sure your plug color after the test is a nice med tan like cardboard.
 

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Discussion Starter · #11 ·
yeepper plug is looking good! i ran it for months with a 125 jet in it, we figured why this happened was the emolsion tubes were clogged with dirt, leaning out some of the fuel and allowing it to run until some of the dirt broke loose dumping more fuel and causing the problems, after cleaning the carb completely it became extremely fat and worsened the problem, until finding the ac pump problem which fixed it! :D then took the air box lid off and boosted the jet size a bit more and its great! this was all done with my friend who builds motors of all kinds from llawnmowers to quads and dirtbikes to our 850 horsepower big block engines in our dirtcars, so im sure its done right, gunna be great to get back on this thing and ride it!
 
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