i just cant get my jetting right for some reason

[drag101]

right now i have a yoshi slip on K&n filter with the lid on and a SRP rev box......i went form a 155 with a 40 pilot and that eat plugs so i tried a 152 with a 42 pilot and that wasnt giveing me the right amount of acceleration...and now i have in a 152main and 40 pilot with i though should be dead on but my plug is still black after about 40 minuts of rideing but its wierd because even though the plug is black the quad isnt spottering its still runing good ...what do u guys think i should do does it sound like my quad has the right jetting and its tuned right or whats off on it?

[Sparky_20]

have you tried adjusting the needle? To check for a main jet you need to get it warmed up and do a WOT run for at least 5 seconds. Shut it off with the clutch pulled in and the throttle open. Let it cool down and chop the threads off the spark plug and then go from there.

[wilkin250r]

The reason your carburetor has a main jet, and a pilot jet, and needle clips, is because those things affect DIFFERENT areas of your throttle. They need to be diagnosed and adjusted individually.

For example, it's certainly possible to be rich at 1/4 throttle, but lean at WOT. (Wide Open Throttle). And if you ARE rich at 1/4 throttle, decreasing the main jet size won't fix it.

Also, your spark plug can only tell you the LAST condition it saw inside your engine. Let's assume the above example (rich at 1/4, lean at WOT). If you race around at WOT, at a lean condition, and then putt-putt back to your garage at 1/4 throttle, your plug will be black as night. The LAST thing it saw was 1/4 throttle and a rich condition, so that's all it tells you. You don't get any indication of being lean at WOT until your engine melts.

So, you have some research to do. Do a search on this forum for "plug chop". You will learn how to get your spark plug to tell you what's happening at WOT for your main jet size, 1/2 throttle for your needle clip position, and idle for your pilot jet.

[JW450R1]

I think i had a 175 main jet and a 38 slow jet in my old 400ex.We are talking about a 400ex right.I also had a dome piston,cam too.