CLASSIC TECHNIQUES
During the build-up of the flat-track BSA (that series started back in RC160), I was at pains to praise the Amal Mk2 as a very worthy replacement for the old original Concentric, and promised to reveal how using a pair of them panned out in practice. In principle the Mk2 doesn’t differ massively in principle to the Concentric Mk1 (or in many respects even the old Monobloc), so I decided to combine a report on fitting the Mk2s with a look at tuning Amal carbs in general.
The principles involved in tuning any carb are the same – mixing air and fuel in the right ratio to get the best combustion. And the symptoms of getting it wrong are the same too. Settings that are too lean (not enough fuel) make the bike feel like it is gasping, throttle response is slow and in extreme cases you get a spit back through the carb. Running too lean is also a good way to seize engines or even melt pistons. Mmm.
Settings that are too rich (too much fuel) make the bike feel ‘burbly’, throttle response is stuttery and misfire-some, and in extreme cases the engine eight-strokes (like leaving your choke on too long). Running too rich is also a good way to create bore wash, which can dramatically increase wear in the bores and lead to piston rings chattering in enlarged grooves. Mmm again.
Despite these brief descriptions, accurately diagnosing fuel problems is an artdoubt I’m the only person who’s blipped the throttle a few times and told himself a bike is running lean, only to discover it’s actually running rich. The easiest way to work it out is by using simple logic – make the bike leaner or richer and see if it gets better or worse...
