85 Sportster XLX BAD Backfiring issue

[mord]

Ok im going to try to describe this as well as i can. I work around motorcycles but im not that knowledgeable in them and i cant figure this one out.

Im Having a backfiring issue through the exhaust. The plugs aren't wet and look good. It happens when im steadily increasing throttle from a stop usually or just cruising around at a steady low rpm the bike just loses all power and any attempt to give it more throttle just makes it bog further down usually if you let off on throttle down shift and then give it gas again it will clear up or backfire like a shotgun going off out the exhaust then clear up. it happens all the time now it used to just happen once in a while but its gotten worse.

The bike has a freshly rebuilt carb tuned by a good mechanic
The VOES does work
I run 93 octane
New plugs gaped correctly


Anyone have any idea what might be causing this?

thanks in advance

jon