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400exrapes
02-04-2007, 08:48 PM
On my friends 2001 300ex hes got a stock header with a d.g. slip-on R.A.C.E. pipe. Hes having some small engine problems needing a rejetting. What size jet does he need? How much do they cost? What brand should he buy? any help is great. thanks

300exOH
02-04-2007, 10:15 PM
Originally posted by 400exrapes
On my friends 2001 300ex hes got a stock header with a d.g. slip-on R.A.C.E. pipe. Hes having some small engine problems needing a rejetting. What size jet does he need? How much do they cost? What brand should he buy? any help is great. thanks

You may need to buy a few main jets to try to get it spot on but stock is 122 keihen(sp) so I would pick up a keihen 125,128, and ~130 from your local atv dealer or even C&D. They should only be around $3-$5 each.

400exrapes
02-05-2007, 05:35 AM
after the carbuerator is taken off and we take the old main jet out do we just install the new size jet or is there like more to it

300exOH
02-05-2007, 08:48 AM
Originally posted by 400exrapes
after the carbuerator is taken off and we take the old main jet out do we just install the new size jet or is there like more to it

Basically yes. You may be able to loosen the clamps on both sides of the carb and rotate it a bit to change jets instead of removing the carb completely. Also you will want to run the quad and check the plug afterward to make sure it is not too rich or lean. The plug should be a light tan color when you've got it right.

400exrapes
02-05-2007, 02:15 PM
okay so i take off the carb replace the jets and ive rejetted the quad? also how do i know if the jet size i buy is good cause you told me a couple sizes? and the header pipe was changed there was a stock one but then another used stock one was bought the quad backfires like crazy so its needed.

JIMFROMTEXCO
02-05-2007, 03:28 PM
when i put a sparks pipe and a uni filter on my stock 300 a 127 main jet was the magic number

bwamos
02-05-2007, 05:47 PM
Assuming the following.

Slip on Silencer.
A stock header.
Stock Filter.

I'd go with a 128 Keihin Main Jet (may be a hair rich, but a good starting point)
needle in stock position
stock pilot jet
air screw 2.5 turns out from seated position.

A quick rundown on what you need to do.

Remove plastics & tank as necessary.
remove carb from motor so you can access the bottom of the carb.
There will be a plug on the bottom of the bowl w/ like a 17mm hex head on it so you can use a wrench to remove it (counterclockwise). Once that's out you can access the main jet. Use a deepwell 6mm socket to remove it (counterclockwise). Install the new Main Jet. It will say 128K on the side of it. Probably the long Hex style.
Put the plug back in. That part is done. directly in front of it toward the motor side you will see a brass slotted set screw on the bottom of the carb. It will be pointing straight down. Get a flat-head screwdriver and turn it in until it very lightly seats (stops turning clockwise). Then I usually mark a dot on it with a marker at the slot for a reference point. Turn it out 2-1/2 turns.
Reinstall everything in the order you took it apart. (I'd wait to put the plastic on until you finish tuning it.) Start her up, let her warm up and test her out. Note: Idle will probably need adjusting slightly. I usually turn it up a few hundred RPM's while tuning.

Forgive my typing, I'm not on my own computer at the moment. ;)

400exrapes
02-05-2007, 05:55 PM
okay this is the first rejet im doing i have the owners manual what im pretty sure about at this point is that once the carb is off i pull the stock jet out and replace it with the new size jet that is right for the job and then it sounds like im done. and all these screws i have to twist is that necessary?

bwamos
02-06-2007, 07:16 AM
Yes. You have to adjust your other circuits.
The main jet on ly controls 3/4-Full throttle.

If your getting backfiring when letting off the gas, your pilot circuit (idle-1/4 throttle) is too lean. Fixable by adjsuting the air screw with this small amount of modification. No pilot jet change necessary.

If you get sputtering or boggin in the 1/4-3/4 range, you need to adjust your pilot circuit.

Here's a good guide that may help clear things up a bit.
http://www.duncanracing.com/techfaq/Tech_keihin-carburetion-jetting.phtml

Note: this if for a slightly different carb on a 2-stroke, but the concept is the same.