Performance preparation of carburettors

Many familiar standard and racing motorcycle slide type carburettors from past and present makers based around the world give greater output relative to respective size than the butterfly types found on lots of cars and petrol engined commercial vehicles – the reason some past performance car engineers have used single or banks of motorcycle carburettors.

Even better, many slide type motorcycle carburettors are of simple design, construction and operation. Thus after performance preparing our motorcycle engines using routes suggested in the previous features of this series, simply preparing and setting up a standard carburettor following the maker’s ‘tuning’ guidelines makes a huge difference and if one then experiments with main jet size and needle/needle jet variations, standard instruments can often (but not always!) make a decent performance instrument for little outlay. Of course, an appropriate racing carburettor will do the job better, but at extra cost.

Taking British Amal carburettors as an example

(Picture 1) Amal’s favoured TT carburettors (a development of those offered earlier by AMAC and others) were famous in pre-Second World War days and developed from track instruments. While intended for racing and associated performance roles, many enthusiasts favoured – and still favour – these for fast road use and while at their best in the upper performance range, they are controllable at slower speeds, too. And thanks to the side-mounted air slide, they have good cold-starting abilities and offer the virtue of easier tuning but unfortunately the central needle does restrict air flow marginally, which racing engineers don’t like.

Despite its central needle, the compact TT offers excellent performance relative to its size. Although Amal recommended mounting at a downdraught angle of no more than 15 degrees, I’ve mounted them more steeply using a remote float chamber and others have set them at as much as 45 degrees and, with a flange converter,