need a not more information about your setup..
what carb, what engine and jet sizes
have you checked your valve clearances?
also check your plug to see what colour it is..
are the backfires coming from out the exhaust or out the air cleaner...
check all your manifold gaskets and orings for leaks
is this a problem tht just showed up or are you fitting a new carb?
running lean down low, try adjusting the mixture screw and maybe go up a size in the pilot jet, do your valve clearances and check your exhaust gasket too
wet and muddy eh? tried draining the fuel out of the carby? good chance it has water in it... water wont mix with fuel, so it will sit on top (or is it underneath???) the fuel, and bike run like ****... try draining the carby first..
try and blow the electricals out including the CDI plug out with air and then give it a blast with WD40.
also make sure your air cleaner is clean, disconnect the kill switch just to eliminate that as the problem too..
and you didnt comment on checking your valve clearance
ok i had the same problem with mine i got the air compressor ....where the on/off petrol switch is on the bike disconnect the line at the switch to the carby get small adapter for compressor and put it inside the fuel line and let the air blow trouugh it fuel should come out the overflow line once fuel stops coming out connect it back up and start and see what happens hope this helps