Notation on carburation
Throttles, pilots, cutaways, needles, jets – Rick aims to resolve his fuelling fettling with white paint!
‘WORKSHOP MANUAL-TYPE TROUBLESHOOTING METHODS ALWAYS SEEM A BIT VAGUE’
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.