slight backfire, please help

[bsmith]

I have an 04 400ex, I just got it about 1 month ago. I had a few minor kinks to work out of it, and I got them taken care of. Now I have another slight issue. It runs great at idle, mid, and high range, but if I find a good long straight where I can lay into it really hard, and stay in it from 1st through around 4th gear, I let up off the throttle, then get back in it, there is a slight pop and poor throttle response. After that one time, it is fine until I do it all over again.
BTW, It is suppose to have a 40 over kit in it, it has a white bros. exhaust, 38pilot jet, 152 main, and the needle is clipped on the forth from the top.
Im also wondering if it has a 40over kit in it, does that make it a 440cc, or is that .40 over bored, like a car?...seemd to me like .40 over bored would be alot for the thin cylinder sleeves.

[Here 2 help]

A 40 over bore would make it a calculated 406.7839344295082cc.

.040 inch = 1mm approx. It started life with an 85mm bore, it should now be 86mm - a 440 kit would take it to an 89mm bore and to be bored that far, the cylinder should be re-sleeved.

As far as the backfire - it sounds to me as though it is loading up during deceleration. Four strokes are known for this. First thing you might do is turn the idle up. Next try adjusting the fuel/air screw in in 1/4 turn increments. Loading up during deceleration is caused by an overly rich condition at idle. Overly rich is caused by too much fuel or not enough air.

Good luck

[bsmith]

Thanks for the info. I figured it was a bit rich due to a slight bit of black smoke, but I failed to mention that in my original post. I will check out what you have suggested. And thanks for clearing up the 440 kit for me. I have been an auto tech for about 15years, but I dont have much knowledge on this type of toy. It is my first 4stroke. I had a few 2stroke bikes, but never got into the 4strokes until this one. I love it, after I got the few minor carb issues worked out, it runs like a scalded dog.