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A good and simple check for crankcase suction is as follows:- Petrol turned OFF, plug leads taped metal-to-metal onto crankcase to earth them, spark plugs removed, put into top gear, back wheel off the floor, throttle wide open, air lever wide open, then by hand turn the back wheel smartly forwards. If all is well you should hear a distinct ‘plop’ noise, twice per engine revolution, rather like the sound of a cork being pulled from a bottle.
I hope that you are successful ! If not, there are several possible culprits, and first of all check that any crankcase drain plugs are in place ! (Easily overlooked in a rebuild). The next possibility is that the engine has been assembled without cylinder base rings, but removal or lifting partially of the cylinders is needed to check that item.
I presume that you are following the correct cold start procedure ? Oil tap on, petrol on, flood carburettor generously (if Amal type), until fuel pees out of the float chamber vent hole and drips onto the flywheel. With throttle and air lever closed kick engine over a couple of times in order to draw some mixture into the crankcase. Then open throttle about one third, and give the proverbial ‘long swinging kick’ on the kickstart. If it doesn’t show any life at all, try switching the plug leads over. Usually if the leads are on the wrong side, you will get some pops and bangs in the exhaust, and nothing at all is not a good omen…..
Finally, well firstly really, make sure that your petrol is absolutely fresh, as this modern stuff containing ethanol, benzene, etc., has a ‘shelf-life’ of only about three weeks in your tank before it loses its low-boiling aromatics that are vital for easy starting.
Good luck !!