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I'm new to this stuff I put a fmf powercore 4 slipon on my 400ex I have a uni filter not sure if and what jets I need to use on the carb.
 

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Welcome to the site, Some of these guys can spit jet sizes out like nothing
 

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Welcome to the forums.
 

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Got this on another forum. Its a copy
There's been a lot of threads about rejetting lately so i will try and help out by describing how and why to rejet. I'll leave out how to do a pilot jet because there was already a very good post about it. When ever you free up air flow in an engine it is necessary to do. Whether it is putting on an after market pipe, K&N/UNI, porting and polishing, or different piston you are going to lean out the fuel mixture in the engine and will make it run hotter and not get the full power out of it you desire that's why you need to jet it richer.

Main jet:
It is not necessary to take your carb off to replace the main jet. First turn the gas off of the carburetor, now look on the bottom of it and you'll see a 17mm cap get a socket and remove it. A little gas will pour out so keep a rag near by. Once removed, get a 6mm socket and feel around for the main jet. Remove that and put in the new jet. Once installed find a long open area (not the road) and get the quad warmed up to operating temperature. Now crack the throttle to 3/4 to full and hold it for about 5 seconds then shut it off. Once the engine has cooled down remove the spark plug and check the color of the electrode. If the jetting is right it will be a dark grey to light brown color. If it is black its way to rich go down a jet size or two, if it's white you need to keep going up on your jets until it's right.

Needle:
THis one's a little more tricky but is pretty easy to do if you have the time and patience. It will involve taking your plastics and gas tank off. You'll need a flat head and phillips screw driver, a 10mm socket and wrench, 7mm socket and an allen wrench (cant remember the size) to disassemble the quad. Once apart, look at the top of your carb there is a cover with 3 screws, gently remove these with the phillips screw driver. The honda screws are extremely soft and will strip so be easy on them. When you get the top cover off you'll see the slide inside of your carb. Take the bolt out of the arm thats holding it and then remove the 2 bolts from the slide themselves. Lift the arm up out of the way and pull the slide out. Attached to the bottom of it will be the needle, you can make it richer by lowering the clip on the needle or make it leaner by raising the clip. The needle controls 1/8 - 3/4 throttle, so to check the spark plug just hold it in that postition like you would have done to check the main jet. You may need to back the air/ fuel screw out to richen it a little bit, most of the time just back it out a 1/4 of a turn.
 

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Great find...
 
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