I can also point out this from my recent run-ins with my RB30 engine rebuild, particularly the cylinder head.
Worn hydraulic lifters(mechanical tappets in your case I reckon) & worn cam lobes as well as excessively machined cylinder heads can cause cam timing issues & backfire/misfire etc as a result. Once or if you rule out electrical and other mechanical issues, start investigating those ones I pointed out.
I'm not sure how critical it is to adjust tappets in EFI TB42 model(I dunno whether EFI model comes with hydraulic lifters or not), but for a carby TB42, it is an important factor on the smooth running of the engine.