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Hey guy’s just picked up a 2000 trx400EX
It sat for 5+ years, ran before that, did a major carb clean and new battery, as well as pulled tank and flushed and cleaned. The bike has a stock airbox, but is running yoshimura RS3 exhaust appears to be a full exhaust not just slip on, i cant seem to get the bike tuned correctly. I currently live in utah, but was told to jet it for sea level because of the extra air, currently has a 38, and 145 for jets, but will partially run but had fuel cuts in half and quarter throttle. Any jet recommendations and air screw turns, to get her where she needs to be?
 

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Welcome to the forum and the world of ATV's

Jetting is as individual as the modifications - if it ran good before setting, no jetting changes are required (unless additional recent changes have been made) - a professional carb cleaning would be all that was needed. A "major carb clean" means nothing. Spraying the outside of the carb with Simple Green and hosing it off is a major cleaning. The carb needs to be removed, fully disassembled, cleaned in a heated sonic cleaner with a solvent recommended by the sonic cleaner manufacturer.

What you referred to as an air screw is a fuel screw. The setting of the air screw has to be done when the engine is hot (after 20 minutes of riding) - it is for is for idle only - the initial setting recommended in the service manual is just to get it close enough to be able to run it prior to the final setting. The pilot jet is for idle only - seldom needs to be changed unless the carb is being modified by boring, polishing or some other physical change is made to the casting or machining. The slide needle and needle jet are responsible for mixture control from 1/8 to 3/4 throttle. A bog or cutting out in that range is usually related to an incorrect needle e-clip setting, wear on the needle and jet or an ignition problem. Many times just a new spark plug can cure that problem. The main jet is for fuel mixture calibration at top speed operation. Putting on a low restriction exhaust may require a 2 to 4 incremental size increase.of the main jet, but the average elevation of Utah is about 3000 feet above sea level so standard jetting is a bit rich to begin with so adding a slip-on would make it closer to optimal without changing the main jet from the factory - the stock 38 pilot and 148 main should be fine, but if the air filter is changed from the stock foam to a K&N (which is not recommended) then the main might need bumped up to a 150 or 155 and if riding above 4000 feet, jetting down to a 145 might be optimal.

Other than that, the proper way to set the idle fuel screw is to ride the vehicle for about 15 to 20 minutes - then with the engine idling at the desired idle speed and with a window fan blowing air across the engine, turn the fuel screw in (in 1/8 inch increments waiting 5 to 10 seconds between each change) - when the engine drops 50 rpm turn the fuel screw out one full turn from the setting where it lost 50 rpm and the adjustment is finalized.
 
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