400ex carb problems!!! HELP!!!

[SNO_DAY]

I have a 1999 400 ex . the carb has been jetted with a dyno jet kit and installed the dno190 needle on the third slot and dj170 jet for up to 3000 ft elevation.it has a k&n filter and aftermarket exhaust. the bike worked awesome when we first got it. but then it started to break down in the lower gears when starting out, still had lots of power in high gears. My son took it for a run and said it was now popping and missing when starting out and couldn't get any speed or power. So I took off the carb to clean, and it wasn't that bad, but now its hard to start and misses and pops on low speed acceleration and when you get the rpms up it will break down, bog out, and sometimes flames from the muffler.the filter was cleaned and tried 2 new ngk plugs. but still the same problems. this has me stumped because sometimes it seems its not getting enough gas and other times it seems like to much. also the jettig needle was at 3 1/2 turns out when I cleaned the carb, so I left it there. no better so I set it at 2 1/2 turns out and still had the same luck. If you can help please advise.
Thanks Darrel,

[buster024]

Hmm...toughy. The float needle could be sticking, which could account for the hard start, and a few of the other symptoms. Obviously, this could be a great opportunity to upgrade to the 450 carb. $160 or so brand new.
That being said, I hate giving up on something too. I had what I thought was a major carb issue, and it ended up being the stinkin' Outerwears cover was clogged. Not sure if you have one installed or not, but that one stumped me for a week or more. I assume your K&N is clean. Make sure your gas is new, your jets are clean etc.

Good Luck.... (once you get it figured out, get a 450 carb anyway) :D

[SNO_DAY]

HOW SMALL IS THE OPENING IN THE SLOW JET? I WAS RECLEANING AND CHECKING THE CARB AGAIN TODAY AND WHEN I HELD THE JET UP TO A LIGHT IT LOOKED LIKE IT WAS CONSTRICTED SO I TRIED TO PASS SOME SMALL FISHING LINE THROUGH IT AND IT WOULD NOT GO. IS THIS NORMAL? I SPRAY SOME CARB CLEANER IN IT AND BLEW IT OUT WITH AIR HOSE BUT STILL THE SAME. IS THE PASSAGE REALLY THAT SMALL? THANKS AGAIN
DARREL.

[Snipe]

If you spray cleaner through it and it blows through it and not back at you its clean. You should be able to see light through the hole.

[SNO_DAY]

I WAS TALKING TO A BUDDY TODAY, AND HE THINKS IT MAY BE A DEFECTIVE CDI MODULE. HE SAYS IT MAY BE THROWING THE TIMING OFF OR CAUSING A WEAK SPARK.WOULD A BAD CDI CAUSE THE PROBLEMS DESCRIPED ABOVE?

[matt14c]

Did the bike backfire or anything before it started running bad? A buddy of mines backfired really bad and it jumped the timing and had similar problems to what your having. His was on a 300ex though. How old is it? Timing chain or tensioner ever been replaced?