To demonstrate how important exhaust measurements are, my buddy has a race-spec 350 BSA Goldstar, and out of slow corners he uses his boot-toe to block the tailpipe [no silencer], until the revs has climbed
a bit and the engine has gotten into the powerband. Below the powerband the bike is dead.
His boot blocking the tailpipe provides some back-pressure, and keep more of the mixture in the chamber.