I'm posting this in hopes that it may help others.

modification history:

Last year (june 2000), I bored to a 416, put a 10.8:1 je piston in, and put on an IDS2 supertrapp pipe on with 7 disks (k&n air filter as well). I ended up with a 162 main jet, needle raised one clip, and pilot jet remained stock.

This really woke up the 400ex, more power everywhere, but still lacked top end power (it did help a little, but tended to fall off the last 1/4 of RPM range).

This year (june 2001):

Replaced the stock cam with a webcam 450/451, hard-faced rockers and stiffer springs (not sure if this was needed). Also a rev limiter from LRD.

With the same jetting as last year, it was running really lean on the pilot jet. Also, when the throttle was opened up from off idle, it would act as if the main jet was too rich (it stuttered around 3000 RPM, then cleaned out as the RPMs increased).

Discussing this problem with Gabe (86atc250r), he mentioned that aftermarket cams like lots of air flow; further, constricted air flow causes the jetting to be difficult to dial in.

With that advice, as well as some posts that I went on, I ordered 40, 42, 45, and 48 sized pilots.

First, I put in the 48 pilot jet (one step at a time) - this was WAY rich, and the machine refused to idle after warmed up, no matter how the air/fuel mixture screw was adjusted.

I dropped to a 45 and was able to get the machine to idle ok. At this time, I also lifted the airbox cover's rear by flipping the rear tabs closed then putting the cover on. This was to see if the stutter would go away. To keep the machine from becoming too lean, I put a 172 main jet in. After testing this, it confirmed Gabe's idea of aftermarket cams creating non-linear fuel needs when air flow is more constricted. It ran MUCH better, although not perfectly.

With this knowledge, I drilled two 1 3/8" holes in the rear of the airbox lid, then cut out the bridge between the two holes (I actually tested air flow after each hole to make sure I wasn't cutting out too much).

After cutting airbox lid, testing confirmed that it was still a little rich on the main jet. I brought it into the correct range with a 168 main jet.

At this time, I noticed that warm engine starting was still difficult without giving it some gas. Dropped to a 42 pilot jet. This helped, but still not perfect. Also, I figured this might help the machine, since it still gave me a fraction of a stutter when I stabbed the throttle open.

It didn't.

I thought that maybe the needle was running too lean (felt a little like a lean bog). Raised the needle all the way (lowered clip to the bottom of needle). This gave me a definite rich condition mid throttle). Put the needle back the way it was, then I got to thinking... the more quickly I stabbed the throttle, the more bog I would get. A quick look at the throttle cable pull (under black cover on side of carb) and I could see what was happening. The accelerator pump was giving the machine an extra shot of gas when I stabbed the gas.

I pulled off the linkage that operates the pump, thereby disabled the pump.

Went out to test, and sure enough, the short bog was gone.

It now revs quickly and smoothly all the way to the top. Power gains are VERY impressive, especially midrange through top end. Two stroke like power and great bottom end.

Thanks to all the helped me, I would've gone crazy without it. Also, I want to thank Powroll, they spent a good amount of time with me ironing out the setup at first. Unfortunately, they were out of the supplies I needed at the time, and had to buy elsewhere. Their customer service and tech help are A-1, I therefore go there first for parts/service.

ogre