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Join Date: Jan 2011
Location: Smack in the middle of Illinois
Thanked 290 Times in 285 Posts
Jetting has to be done in stages with the engine fully warm (after or while being ridden for about 20 minutes).
Changes to the exhaust doesn't affect jetting except on the top end - usually bumping up the main jet 2 to 4 sizes will accommodate a change of the exhaust, but a poorly designed or modified exhaust can affect engine performance at all speeds except idle.
I acquired an 05 TRX400EX with a gutted HMF slip on. It idled, but backfired horribly, was obnoxiously loud (hurt my ears to ride) was low on power and would not run over 1/2 throttle. I first attacked the carb - the fuel screw was 1/4 turn from lightly seated, it had a 45 pilot jet (stock is a 38), a 170 main jet (148 is stock) and the slide needle was as high as it could go. While the carb was in the cleaner, I adjusted the valves, checked the compression, installed a new spark plug and verified the condition and operation of the air cut valve. When the carb was ready for assembly, I verified the operation of the accelerator pump and float level (there was a change to the carb at some point and the length of the float needle was changed - if an unknowing mechanic installed a pre 2001 needle, the float level may be wrong). The float needles look the same and the same needles were used in models other than ATV - the float needle in the 05 and 07 TRX400EX was also used in the VT600, VTX1300 and TRX500 through 2014.
I assembled the carb repairing the choke operator, resetting the slide needle to the stock position, installing stock jets and setting the fuel screw to the initial setting position. I removed the K&N air filter and installed a UNI, installed a new air box lid and stock snorkel. The engine started better and idled smoothly. I ran the quad around the drive for about 15 minutes then adjusted the fuel screw for the smoothest idle. I took off and tried to accelerate down the street. At about mid throttle there was a noticeable power loss and anything above 1/2 throttle resulted in horrible backfiring and the engine tried to stall. I removed the POS HMF can and ran off the head pipe. The mid range no longer had the power loss and it accelerated smoothly to about 3/4 throttle and then fell off again. At this point, I was satisfied with the idle and mid-range performance. I went up 2 sizes (155) main jet - it then accelerated cleanly till it hit the rev limiter. I got a DG slip on, installed it and never made any other changes - it performed quite well.
I suggest you examine the slip on and if you can't determine that it was designed for the application or if appears to have been modified from it's original design, chuck it and get one designed for the 400EX. Go back to stock jets and start from the beginning - get the idle set, set the mid range then jet for top end and the job is done, but the carb has to be clean, the spark plug near new and the fuel fresh 91 to 93 octane non-ethanol.
You can't use old yeast and eggs to bake a delicious cake and you can't make an engine run right with ethanol fuel that is over 3 weeks old or has a lower octane then the engine was designed for. If you use ethanol fuel, it has to be fresh, not contain more than 10% ethanol and should be 2 octane points higher than the engine manufacturer specifies, but not more than 4 octane points higher.
Good luck and a recommendation - only make one change at a time so it is easy to change back if the result is unsatisfactory.