JOIN THE JET SET
Roger Martin asks what size main jet he should fit in the Amal Premier carburettor he’s bought for his 1960 BSA A7SS Shooting Star, because the 200 jet supplied seems too small.
This is the problem with fitting more modern carbs to old bikes – you can’t use the standard settings, but I would imagine Burlen, who supply these carbs, would have set up the carb correctly. Originally the A7SS had a 270 jet, hence Roger’s concern, but that was for the Amal Monobloc; the Concentric (upon which the Premier is based) always used a smaller main jet, generally reduced by a factor of roughly 1.3 by my reckoning.
The ‘Rosetta Stone’ here is Amal’s 1967-68 fitting list. It covers the changeover years from Monobloc to Concentric and my factor comes from averaging the jets of say, a 1967 Velocette Venom (Monobloc) with a 1968 model (Concentric). My 1.3 is an average to give a starting point. The A7SS was long out of production by 1967, but taking the example of the A50 Royal Star: fitted with a 376 Monobloc it used a 260 main jet, while next year’s Concentric had a 200; bang on 1.3 in that case. Multiplying Roger’s 200 jet by 1.3 gives 260 (or dividing 270 by 1.3 gives 207) so I’d say 200 isn’t far out. Interestingly, the twincarb A65 Hornet’s Monoblocs had 270 mains, but reduced to 190 for the Concentrics. This is probably because the carbs were bigger (389 and 930) and a larger choke size usually requires a relatively smaller, not bigger, jet.
So I would try it as it is, but making sure I had a 190 and a 210 as well, just to try. Fitting a new carb to an old bike is always going to require a bit of fettling.