I'd highly suggest one of these books, or another one,
educate yourself on how they work & what does what, when...
It could be a combination of all or just something simple
It could be just too fat/too big of main jets,
just that's not causing the soot thou, unless it's really fat/rich,
It could too big of a accel. pump cam &/or squirter/nozzle size,
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Just spit-balling here;
a what if, scenario
I don't know your build that well...
It could be reversion between the carb & manifold,
{blow by, bad intake valve to seat/sealing usually causes it}
usually too big of carb or retarded ignition or retarded camshaft,
I kind of doubt it's that {?}
If it diesels when it shuts off that could cause that too...
{usually runs hots & really lean, usually pre-ignition/detonation issues}
Possible wrong/retarded ignition/cam timing & it'd be sluggish
& hard starting too...
A leak-down test will tell you "IF" &/or where it leaks,
"if" it leaks by the rings or valves etc., you'll hear it there,
if you have any of those suspicions {?}
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It could be just idle circuits &/or air bleeds Fat/Rich on a Dominator
little jets on the top by the bores, generally bigger leans it out
opposite of the main jetting, anyway;
there's a ton of different variables it could possibly be
View attachment 410301
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it could need a tad bit more initial ignition timing too,
that will sometimes lean it out enough, if it's already really close
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for tuning purposes;
Have you read the plugs ?
What heat range are the plugs too ?
plugs "it's probably not cause of it",
but it helps to have the proper stuff to begin with
they will tell you if it's fat or lean etc.
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View attachment 410303
it's pretty hard to do accurate tuning over the internet
especially by only a couple of still photos,
good quality videos do help some too, but not like being there,
more information the better, when it comes to tuning issues
Someone here will get you close or steer you in the proper directions
Good Luck with the tuning