B21A, Oh dear... Are you in Europe or US? Those Carb engines were common in Europe but rare in US, Canada maybe.

Assuming your compression and blow-by is ok, you might wanna try renewing bits in your electrical system (very limited):

-Rotor
-Rotorcap
-Plug wires
-Plugs
-Contact point and condensator (unless electronic)

First thing though: does your vac and more inportantly speed advance work? The centifugal system tends to get stuck.

Then the Carb. If you have and SU, change out to Solex (get from junk yard). Check the membrane and oil level. The needle on the SU might be worn.

Vacuum check should be extended to all gasgets and vac pipes, renew the gasget between manifold and carb.

Finally check your crankcase breather. Too much blow-by will drop the idle Vac, little you can do other than engine overhaul.

I changed a B19A in a 240 once. The engine was new overhauled and came from a Volvo 340. Engine was soo sweet even with the bonnet (hood) open you could not hear it idle. Power was too little though, never got it to go faster than 70mph, after a good 5 minutes of flat out.

Good luck

Jorn, Oslo Norway.