Dur­ing Odgie’s ear­lier ad­ven­tures with his A65 flat-tracker, he got to grips with the Amal’s mod­ern Mk2 car­bu­ret­tor. Here are his top tips for fit­ting and fet­tling both the Mk2 and ear­lier Con­centrics…

Dur­ing Odgie’s ear­lier ad­ven­tures with his A65 flat-tracker, he got to grips with the Amal’s mod­ern Mk2 car­bu­ret­tor. Here are his top tips for fit­ting and fet­tling both the Mk2 and ear­lier Con­centrics…

Real Classic - - What Lies Within -

Dur­ing the build-up of the flat­track 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 art in it­self. I

Pho­tos by Odgie him­self

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