poor idle / strong hesitation at WOT

Mistakenly put this in the engine thread. Cross-posting.

Symptoms

Idle: Takes a good 10 minutes to warm up to the point where I don't need to use the choke - mid-summer. This has gotten progressively worse over the last two seasons.

Throttle: When I twist to WOT, power drops out/hesitates then picks up after a couple seconds. Typically in the 4K-6K RPM range.

Exhaust: Little rich. Getting small backfires after letting off the throttle at higher RPMs.

All of these symptoms have been progressively worsening over the last 2 years.

Status

I have the carb off now to check for debris. It's looking fairly clean. I also needed to check my jetting because I didn't know where I was at. Previous owner put on a 4-1 so I wanted to make sure jetting was up at least a step. It is. I believe 102.5 mains are stock. I'm at 105. Pilots are at 15, which is stock.

Last year I had a mechanic set the valve clearances. I also asked them to fix this hesitation issue I have. No luck. Asked another mechanic to take a look at it this year too. No luck. Pretty bummed about that.

Electrical is good. I have a Dyna ignition and coils. Good solid spark. I'm pretty sure fuel from the tank and through the petcock is coming in fine.

Air filter is maybe a tad dusty but not so bad. I sprayed a little carb cleaner into the air box with the filter out and it had no real effect.

Next steps

I'm cleaning the carb. I can't see any obvious reason to change the jetting. I haven't checked the floats yet so that's on my trouble shooting list.

Any ideas guys? Gone through two mechanics and I've been stumped on this for a couple years now.

Cheers,
-A

Check for vacuum leaks around the carb holders