CARBURATION
The Technical Advice' questions on carburetion, at times, seem endless, along with the strange ideas
some people have on the subject. A few basic facts, based on the Mk I Concentric; the main jet size is not
that important and does not wear out! It starts to have an effect from 90mph plus, so if you don't cruise
above 80mph (which is illegal anyway) then an error of 10 per cent, or possibly more, will not be noticed.
The 220 or ex-works setting can he left alone. If you must fiddle then err on the larger, richer side. Now
the needle jet and needle. This is about the most important on a Norton, especially as the needle jet has
a very short life for accurate carburetion, often less than 10,000 miles. Starting point is 106 with the
needle position as ex-works settings, to be checked 60-70 with the engine pulling, i.e. slightly uphill for at
least ½ mile then engine and plugs checked. Sooty, richness, lower needle. or replace needle jet and
start again, white, hot, weakness, raise needle. The slide does not seem to wear so as to affect the
mixture greatly, usually just rattles, hut can cause erratic running just above tick-over. Slide to be checked
at 40mph with engine pulling for ½ mile as above. Richness, fit a higher number slide ( ½ sizes):
weakness, lower number to richen it. Pilot air screw for tick-over mixture only, and possibly pick up too.
Unfortunately the N7Y plug (suitable for all riding, up to hard racing) tends to look reasonably correct
even if the carburetion is a bit out. Other symptoms to look for; 1, Good acceleration in a particular hand
but rough cruising, which clears the instant the throttle is opened - Richness, finally leads to fouled plugs.
2, Poor acceleration, spitting back, pinking (sounds like small ball bearings rushing around on top of
piston) and overheating - weakness. Soon melts plugs and holes pistons. Do check ignition timing. if
points, and correct it before checking carburetion. Removal of air filter will weaken mixture, also noisier
silencers can weaken it. The Viton tipped brass float needles do wear out, causing intermittent flooding.
also caused by a damaged float-bowl-to-body gasket. Every third needle jet replace the needles as well.
If you must remove the chokes do plug up the hole in the carb top, there is a screw available, this falls out
and gets lost, chewing gum, electrical tape over a piece of wood. plasting padding, but a nut and bolt
might drop into the carb and engine. ! ?? urgh.
AMAL CONCENTRIC SETTINGS FOR NORTON COMMANDOS,
EX-WORKS
Year Model Carb. No. Main jet Slide
Needle
pos'n,
Needle
jet
1969 Commando 930/30-31 220 3 2 106
1970 Commando 930/46-47 180 3 2 ) 06
197/
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