Hesitation?

I recently put a White brothers E-series slip on pipe on my '99 400 ex. I have a 155 main jet in it now and it works good. It has good response and added low to mid power. The only problem I have is that when im in a higher gear say third or fourth and im just giving it not more than 1/4 throttle it cuts out, hesitates, and will eventually stall out. Also when using engine braking it pops and backfires slighlty. Is this problem in the pilot jet? if so what would your recommandation on pilot size be to solve this problem. OR if it is another adjustment on the carb please reply.
By the way all components are stock on quad except for the 155 main jet and the e-series slip on.

[cals400ex]

if you are at 1/4 throttle or less then the pilot jet should be the problem. the stock pilot jet in a 400ex is a 38. i would probally try a 40 or 42. make sure if you go to a dealer and pick up a new pilot jet you get a 21 series. they may ask you what series the 400ex has and it is a 21 series.

[440exnacsracer]

i would look at the main jet, make sure its clean, and if thats not it, get your valves adjusted. are your plugs fowling? the same thing happened to me last week and it was my valves.