Why did someone put a 142 main jet in when the stock main jet is 148 can anyone tell me why they did this? they changed it to a 142 when they put in the K&N power kit.
I put in a 155 main jet does it sound right for ->

K&N Power kit, Supertrapp pipe (12 discs) and a 3" airbox hole?

 

redDog400ex, how do you like your supertrapp exhaust? Is it an ids2? What does it sound like? Did it add much power? I would get a white bros e-series but they cost over $800 in canada. the supertrapp ids2 costs $325. I have no idea why anyone would put a 142mj on a 400ex. Maybe they didn't know what they were doing.

 

I like it. It gives really good power, and I didn't notice the power that it should have given me till today when i jetted up. It Works really good, but their contraption to attach it is really cheap so you will want to think of something as soon as you get it.

 

My stock main was a 150. Did your dealer put the 142 in or was that in when you bought the 400? What is the elevation where your at? It must of ran like garbage with a 142, thats too lean. I have talked to a few guys who toss the lid and use a 170 main, i think if its on then a 155 will do but i am not sure about it when you drill the hole.

 

I previously talked to Tom Miller Motorsports in Clarksburg, TN (You may have heard of them. They have done engine work all over the states, and advertise in atv mags) about the stock jets and what they recommend. Tom said most 400ex's come with a 142 or 148 main jet, depending on where they were assembled. They don't recommend removing the airlid or drilling holes in it either. We are about 300' or so above sea level, and they recommend a 152 or 155 for a all around main jet at our elevation.

 

Most all of the 99's came with a 142, & 00's have a 148.

 

The Jet that came stock was a 148 because they put it back in the box when they installed the K&N filter. But why would they put a #142 in after putting the K&N on? They should be jetting up not down. The #155 and the #160 that I put in seems to be running good, but i havn't been to the dealer to get some new plugs to check the jetting.
Can someone tell me what a lean plug looks like, and what a rich plug looks like, so I can jet it correctly. Thanks for all the help!

 

..... WHAT PILOT JET DO YOU RECOMMEND A 40 or 42 I am at same sea level

 

We used to call it coffee with a little cream to get the right color! Light gray to white is very lean, and black is to rich. A good run at 3/4 throttle then cut the engine then check plug color. Do not let engine return to idle before shutting off to take a plug reading. That is how we did it in my day....oh yea, I am 70