If the jet orifices are .250 down and engine idles smoothly, then either the needles are incorrectly positioned in the pistons, or the float levels are very low. I suspect float levels as the HIF is a bit fussy to set. It might even be float drop.
The bimetal main jet holder compensates for fuel viscosity as the temperature of the fuel changes. It will not be the problem.
The BBB needle is quite capable of supporting a modified B20 engine, and will provide more than enough for your engine.
Ignition timing is not the problem, unless the advance weights are sticking and you do not have something over 32 degreesa at 3500-4000 RPM.
The fix is external, as the engine sounds fine internally, and is therefore easy and simple. Just a matter of finding it, and that shouldn't take long.