ok so my friend finally got his new 400EX today. well new to him anyways. its got a white bros. e-series full exhaust, K&N and no airbox lid. the guy who sold it to him said he can never seem to get the jetting right. i asked him what jet was in it and he said a 165 main! i have a pipe, filter, no lid, no choke, and a stage 2 cam and i am only running a 158! so i was like man thats really rich! so i went to give my friend who baught it my spare size 160 main, but first i pulled the plug and when i looked at it it was REALLY white. its like it was running really lean! so after that i was afraid to put the 160 in it and i didn't really feel like tearing the whole carb apart. i would expect it to be running rich with a 165 with only full exhaust and filter as the only mods but the plug says otherwise. anybody have any advice?

 

juggalo -

Plug reading can be almost impossible with today's gas additives, especially on a four stroke.

A fail-safe way to jet is to purposely put a large main in (say 180). Ride it around a few minutes to make sure it's blubbering (cutting out when you open the throttle). Then drop down to a 178 and repeat. Do this until it stops (i.e., it runs smoothly from idle to wide open throttle). Then drop one more jet size down (since it will still be borderline rich). You will need a handful of jets, but it never fails.

You've been around awhile juggalo, so I know you have some insight into jetting. I think you'll find this method to be the quickest, easiest way to find what that motor wants.

I ran my 400ex jetted borderline rich a few summers ago - but the plug showed white/grey. I knew it was in the ballpark cause I jetted it like I said above. How did I know it was borderline rich? First, if I put the next size up main jet in it, it would blubber. Second, when I tore it down for some mods, the piston dome was carboned up. That's why I tell folks to go one jet size down more after it stops blubbering.

Also, you don't need to tear apart the carb. Just remove the main jet access bolt on the bottom of the floatbowl. I've described the procedure in detail before, just search my user name. I can change my main jet in about 5-7 minutes (superfast!).

Finally, if you aren't convinced that this way is easier and quicker, just think of all the blisters you'll avoid by not having to put your hands anywhere near those hot fins to get the plug out!

ogre

Edit: juggalo, make sure you get a big-enough main jet to make it blubber, this is the only way you will find out the reference point. Don't assume that a 165, 170, 175 is big enough (if it doesn't blubber).

One day I was screwing around and took my lid off just to see what the main jet would end up at. It was starting to get a blubber at 180. Every motor really is different.

 

ogre thanks a lot for the reply, at least somebody cared enough to help me out. thats a good idea about jetting and thats the way i would do it except i am poor so i don't have the money to buy a ton of jets (they are 5 bucks a peice at all the dealers around here!) so i have to get it as close as i can the first time. some people at another forum said the jet may have been a dyno jet, and they also said the gas additive thing. i found he had a big exhaust leak and i think that may have had something to do with it too. i'm just gonna put the 160 in there for now and see if it runs better which it should until he wants to buy the jets for me to put in for him.

 

juggalo,

One thing you can do is put the airbox lid back on your friend's ex. That way it'll give you a better idea as to if it is lean or rich right now.

Have fun!

ogre

 

Look for a air leak around the carb boot. that could be why it is white. Pull the jet and make shure that it is a 165. The guy could be wrong.