To me it seems it seams like a clogged low speed jet. I’ve run into this type of situation before. I bought a hypersonic cleaner on ebay and they are amazing at cleaning jets. They clean passageways that are often to tight or too clogged to clean any other way. Professionals have these and you could have someone do this for you.
I’d also check other carb issues…
-upside down float (it happens to the best of us!)
-carb float height
-enough packing on the needles.
-correct jets install (on some models highs and lows can get mixed up)
-timing/throttle linkage connection
Other things to check…
-points timing (best to do with a multi-meter vs measured gap)
-compression needs to be good. a 70-80psi is generally needed. if low could be a stuck ring or badly damaged bore.
-magneto plate needs to be firm with no side to side play.
– make sure your timing cam is correctly installed.
-there is an outside chance that an exhaust or head gasket has dried out and is leaking water in the cylinder.
good luck!
A little information is a dangerous thing!
.
.