I'm partially referring to you, Skunky... "Kick up that main jet to a 160." Your main jet has nothing to do with a decelerating backfire. Futher, putting the airbox lid on or drilling holes in it is not the correct way of jetting in your machine! Maybe that person WANTS their airbox lid off. Why don't you read the 100's of letters in the tech connection - it will tell you first to do your modifications, then rejet. First go up on your main until it skips, then drop it back one. Then do a midrange throttle chop to check jetting for your needle by reading your plug. If you're getting a backfire, increase your pilot. Your heart is in the right place, and I'm certainly not trying to anger you; its just nice for these folks to have the right info. You are certainly correct that what dynojet's instructions tell you to do aren't always correct, often they are way off!

ogre

 

so the air/fuel mixture screw richens the jetting when turned out?

 

O well I suck cockarocha

 

That's right cg400exmaster, but only for your idle circuit (0 - 1/8 throttle). Here's what you want to do to richen your idle circuit: turn the screw clockwise to tighten it(remember that when you tighten upside down your instinct is to turn it the wrong way, best is to visualize it first). As you tighten it, count how many 1/2 turns you make, most of the time the factory setting is about 1 1/2 turns out. When you feel resistance, stop, you don't want to overtighten. This first step is to just get a baseline of how far it was out initially - so you can set it back if needed. After you know how far the factory setting is, advance the screw 1/2 turn past the factory setting (example: factory setting - 1 1/2 turns out, after adjustment - 2 turns out). Then test to see if it clears up the backfire. If not, repeat. Do this until you have gone out 4 turns from tight. If, at this point you still have a backfire, then you'll need to get a one size larger pilot jet. it is usually positioned right next to the main jet, inside your float bowl. Once replaced, repeat above procedure.

Even with modifications on my machine, I didn't need to replace my pilot, I just turned it out about 1/2 turn. Not because it was backfiring, just to help it start easier.

Even stock 400ex's are hard starters, especially in colder weather. It REALLY helps to turn it out a little, it will start so much easier - just a little tip.

One other thing, some carbs have the pilot screw under the float bowl, not in front of it - then it becomes more tedious to adjust.

From what I've observed, the idle curcuit (0 - 1/8 throttle) and low throttle curcuit (1/8 - 1/4) are a blurry region; but the pilot's size pertains more to the 1/8 - 1/4 region, whereas the idle curcuit is controled more by the air/fuel screw. Either way, they are so closely linked that once you get the screw adjusted correctly, the low throttle region appears to be sound. By the time you are at 1/4 throttle, the needle is lifted enough to actively participate in fuel delivery. At that time, the needle's height/taper/length is dictating the mixture anyway.

Regarding the idle to 1/4 throttle "blurry" region I described, I sure would like to know if I'm barking up the right tree, let me know.

ogre