No­ta­tion on car­bu­ra­tion

Throt­tles, pi­lots, cut­aways, nee­dles, jets – Rick aims to re­solve his fu­elling fet­tling with white paint!

Classic Bike (UK) - - WORKSHOP -


I’ve been play­ing about with car­bu­ret­tors quite a bit lately. I don’t know about you, but to me the con­ven­tional, work­shop man­ual-type troubleshooting meth­ods al­ways seem a bit vague; re­al­ity sel­dom throws up text­book faults. If you’re lucky, set­ting the carb up as per maker’s rec­om­men­da­tions will work fine – but these set­tings were for new ma­chines, run­ning on the fuel of the day, so you may need to ex­per­i­ment.

Man­u­als ex­plain that the pilot sys­tem op­er­ates from closed to 1/8 throt­tle, slide cut­away height takes over from 1/8 to ¼. From there to ¾ is the nee­dle jet range, with the main jet only tack­ling the fi­nal quar­ter. Sim­ple, ex­cept there’s al­ways some over­lap and, since most rid­ing is done in the nee­dle jet range, what’s the dif­fer­ence be­tween al­ter­ing jet size and nee­dle po­si­tion? The slide nee­dle is par­al­lel for half its length, ta­per­ing nearer the bot­tom. The ta­per only en­ters the jet at around one third throt­tle; be­fore then nee­dle jet size con­trols how much fuel gets through. Al­ter­ing nee­dle po­si­tion with the op­tional clip grooves is more about tim­ing; ad­just­ing the point at which the ta­per be­gins open­ing out the hole. My Norvin had been tick­ing over per­fectly, but was rich low down on the front cylin­der. A weaker slide made it spit back, a smaller nee­dle jet led to more spit­ting and lumpy ac­cel­er­a­tion; but drop­ping the nee­dle one notch sorted it, the ta­per must have been open­ing just slightly too early.

Some past owner has elab­o­rately marked the twist­grip on my ’35 In­ter Nor­ton, di­vid­ing open­ing into four. In fu­ture I’ll do the same – just a tem­po­rary dot of white paint would do to re­veal ex­actly where prob­lems oc­cur; I know it works - the In­ter has al­ways run beau­ti­fully.

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