in my limited experience with them, the Neptune singles and opposed twins were usually more reliable starters than the alternate-firing jobbies.
I've had a few of the Neptune/Muncie twins and they were all good for starting on the second pull. There was one that wasn't, but it turned out to have the pistons in backwards.
With any Tillotson, normal range to start with on needle jet settings was usually 1 to 1 1/4 turns out from closed on high speed, maybe one turn on the low speed.
FWIW I usually try and fire motors on Gibbs penetrating oil fed through the crankcase prior to feeding them any gas. If they fire on the Gibbs, they'll run on gas. If I can't get anything out of them on the Gibbs, chances are it will be equally frustrating (or even more so) to get them to start on gas/oil mix.
Where is your timing advance set? I recall starting them with the handle more or less centred over the carburetor.
Backfire - sounds like combination of crankcase flooding (or excessively rich mixture) and incorrect timing of spark. Flywheel in correct orientation? Last incident that I recall of back-firing only led to discovery of sheared flywheel key.
Hope it helps.