black spark plug

[J-Nubs]

i live in michigan and its been freezing temps here and i installed a 42 jet and its been running fine but i know theres some other adjustments i need to make to adjust the air fuel ratio and stop fouling plugs but im not sure what it is I have to do or how to do it all help is greatly appreciated

[cals400ex]

you could also adjust your fuel skrew. the fuel skrew works with the pilot jet. the skrew is a fine tune adjustment for the pilot. this skrew is the bronze colored skrew on the BOTTOM of the carb. it is not on the side. i don't know your modifications, elevation, or temp, so its tough to suggest where to put the fuel skrew at. you could try around 2 turns out. by the way, your jetting may be off from 1/8 to 3/4 throttle or even from 3/4 to full throttle. the pilot jet/fuel skrew only really matters from idle to 1/8 throttle. these 400ex's can just be stubborn to start.

[400ex#18]

if your plug is black your running rich, and that means the main jet should go down, and with the air/fuel screw, you should be about 3-3and1/4 turns out. what sze main jet you running, and what exaust do you have?

[cals400ex]

3 turns out on the fuel skrew is too much in my opinion with a 42 pilot jet in. if you need to go out more than 3 turns, you need a larger pilot. however, i am not lean at 2 turns out with my 42 pilot on my 406.

[2004TRX450R]

To adjust your fuel screw get it fully warmed up to operateing temps and adjust it to the point it idles the highest then readjust your idle if need be. A black plug isn't necessrily a main jet. It could be a needle/main jet/or pilot circuit. You can't tell anything by just pulling a plug after running it either. You need to run it full throttle with a NEW plug in it for a few seconds then hit the kill switch before letting off the throttle. Pull the plug and see what it looks like. That will tell you what your main jet is doing. You can also do the same thing but only at 1/2 throttle with another new plug to check you mid range needle setting. If you really know what you are doing you can just ride it and be able to tell how it is running and adjust accordingly.

[J-Nubs]

i have a completely stock 400 except the 42 jet and a pro cirtcuit t-4 pipe. so it look like i need to do at least 2 full turns out on the fuel screw? is that screw on the outside on the bottom or do i need to open up the carb to get to it ?

[2004TRX450R]

no you don't have to take the carb apart. You might need to shorten a screwdriver to get to it though. It is directly in the midle of the carb from side to side and just in front of the float bowl. It is kind of up in a hole so it might be hard to see. You might use a mirror to find it so that you know where it is.

[cals400ex]

yep, its not one the side like air skrews.