Yes, it is the right test. I use breakeclean. What your doing when spraying is that engine will suck in the gass in the spraycan, witch in turn removes oxygen and the enigne will stall.

Is your +4 engine used, or is it brand new? Sometimes, engines is abit out of tolerance, so the hole were crank rotates is a bit to big. That in turn will make your engine suck in "unmessured" air, ie, it does not go through thr carb. Your engine will be difficult to tune and unstable, even with new bearings. Bearings is not airtight :-). Onroad engines f.eks. use steel shield bearings, but perform very well. As i said, the seal is between the crankcase and crank, not in the bearing itself. I would say your engines is "worn" but not so worn it is broken.

That is why the BUKU cap is sooooo nice. I would suggest you buy and test. It will help with tune and how stable it is. I am not shure if it helps to use RTV to seal the wacumehole under the carb, but you could try. Engines that i have used Buku on, does not stall like your engine does.

Reason the wacumhole under the carb, is to have a dry front bearing. To many people screems and complaines when the fornt bearing is moist but in reality, it does not meen anything at all, regards in what shape your front bearing is.

Test and you will see the light.