450 backfiring help

Look my 450 has been backfiring on decel. Thought this was jetting issue but not so sure now. It also hessitates at WOT and toping out at 40 on speedo. What I've done. I thought I was flooding out so I dropped 2 main sizes. Then I was able to pull choke for a little more fuel and it ran good and didn't backfire. So I went up one jet size and problems returned. Every time I pull plug it's dry and black. I swapped my cdi back with stock and seemed worse so I put dynatec back in. Could my coil be week or is there some fine adjustment I can do with my carb. Like I said the best it has ran was with 140 main and choke half way out. Now I put 142.5 back in. Any help or advice would be great thanks!!!

I would check for an exhaust leak

Ok couldn't find any exhaust leaking. Changed fuel filter and seemed to help a little but still have some backfiring. Plug still looking a little black. Which way would I go with my air/fuel screw to lean it just a little and would this affect me anywhere other than top end?

how old is your fuel, and the air screw only effects off idle punch,

Just filled it up. I run 89 ethanol free with a little octane boost cause I have an 11:1 piston and it's 10 over. I've been running rich since it was rebuilt. Just added snorkels and that leaned it up a little. Dropped one main size from 145 to 142.5 and still get the backfiring on decel and a black plug. Dropped to a 140 and it seemed lean so I pulled choke half way out and it ran good and did not backfire. So I went back to the 142.5 main.

Mine started the backfire on long downhills right before I traded it.  No engine mods at all, and had never made any adjustments on the carb ever.  I figured the carb needed cleaned out good or maybe rebuilt.  Used no oil, and had no other problems.  It was a little slugish until I rode 10 miles or so. Then it would run great, but still backfire.  I just kept running the hell out of it.

Yeh that's about where I'm at lol. I'm ganna go through the exhaust one more time and check it out. Like I said main reason I think it's fuel related is I drop down a jet size and run with the choke out it doesn't backfire. Also going to try and get hands on a air/fuel gauge.

Clembone, Talk with James, he had an A/F gauge he was lending out. Used it on my 450 when I opened up the air box and exhaust. Fine tuned the bike nicely. If you messed with the air intake that might be causing some of the issues.