I need "HELP"

[oldfartracer]

Please can someone give some ideas!
I just built up the engine in my sons 300ex
it ran good stock but we are racing MX and needed more power
build up
.040 over 11:1 Wiseco piston
ported head by Janssen Motorsport +1 intakes
Curtis Sparks X5 exhaust
400ex carb with choke plate removed
Megacycle 166-20 cam .370 lift/272 duration/107 lobe center
The problem is this:
The engine starts and idles fine, when I first put it together it seemed to run fine, not as fast as i thought it should be, but faster than stock.
It had a little miss at the top rpm's but figured it was jetting
We were at a race so after practice tried rejetting but no luck the miss got worse with everything I did.
My son took it out for the first moto and about half way through it started missing as soon as you tried to give it any gas.
I changed the CDI, the spark plug, closed the gap on the plug, jetting changes, back to the 300ex carb, but nothing helped.
This last week I took the top end back apart, no leak down past the valves, cam was in right as per the service manual, check the bore looked good, .002 piston to wall within 1/10th.
I re-honed for ring breakin, put it back together and it is still doing the same thing.
It starts fine, idles good, if you give it very little gas it will rev OK very slowly. If you try to accelerate at all it starts to miss real bad.
Does anyone have amy ideas??????????
Thank's in advance for any "HELP"

[Mx_523]

Double check the timing. Is your jetting fine? Check to see what color the spark plug is. Black-Rich, Grey-Lean, Tan-Perfect. To me it dosen't sound like you have a electrial problem sounds like a intake. Clean out your carb and air filter also. Also, does it smoke at all?

[zeppelin]

^^^ well if it is missing as soon as you give it gas then there is no way to get a true plug reading... i think it is the jettig.. you seem to know the at least the basics of it but this link may help you out some..http://www.exriders.com/vbb/showthread.php?s=&threadid=221872

[Mx_523]

My bad, I missread it worng.

[kyleschonert]

This probably isnt the problem. But you shoudl have gotten the X-4 exhaust. The X-5 and X-6 are for bigger bore bikes. They dont even reccomend the X-5 for 330's.

[oldfartracer]

It does not smoke.
Thanks for the link on carb tuning, i printed it and gave it to all the tech's that work for me.
I was hoping that someone would have something like, O YA the 300ex is known for shearing the fly wheel key or something like that.
The first of next week I will bring it to work and run it on our dyno with a O2 sniffer, then I will know if it is jetting.
I have a pretty good understanding of how to jet a carb, I have been running and tuning Harleys on our dyno for 8 years.
We just started racing the 300ex about 6 weeks ago, my son finished 4th in the GNC MX National 70 Production class in 05 on a Cobra but he is now to big for it.
I talked to the guy's at Curtis Sparks before I ordered the x-5 and they said for a National level rider that the X-5 would make more peak HP.
Thank's for all the responce, i will let you all know when I figure it out.

[Mx_523]

Alright, good luck.:)