Norton 850 Mk 1 1974, Service Notes, Page: 23 / 48

GROUP 9:- Carburetors
CARBURETTORS: The carbs themselves are the traditional Amal deal: they work quite well but don't last
very long-the slides rattle about in the bodies, a fault exaggerated by the fact that the engine shakes
about anyway. Viton (synthetic rubber) tipped needles with brass bodies are better in the float chamber
than the nylon white ones originally fitted, but even so Amal will not guarantee that the carbs will not flood
when left standing So always turn your petrol off so that you don't fill the engine up with the precious
liquid-not only a waste of petrol but can also cause bent con-rods, blown head gaskets, and disastrous
fires when you next try to start the engine.
One thing which some owners do have difficulty with is synchronizing the carbs. Go about it this way:
1. Set the ignition up with nice clean points and a good ATD unit so that everything is as Uncle Joseph
intended.
2. Go for a ride of about 10 miles to warm everything up.
3. Screw out the throttle stop screws a bit on each side to make sure the slides aren't hanging on the
cable. As you do this the tickover should slow down and stop. Now screw the screws back in to get the
tickover right again, and try the effect of altering the pilot screws (the horizontal ones) about I turn at a
time each way to get the best (fastest) tickover on each cylinder. Get each cylinder pulling equally hard by
ensuring that the puffs of exhaust are about equal, and finally check that the machine stops in the same
time when you hold each set of points open with a screwdriver (A more civilized method than pulling the
plug-caps off alternately). If one cylinder carries on when you do this, but the other cylinder stops quickly
when you try that side, the first cylinder is obviously working harder, and needs the slide lowering a touch.
4. When all is to your satisfaction, switch off, park the bike in a draught so it cools off. and go and have a
Castletown Ale.
5. Now make sure that both slides open together-this is the difficult bit, but there is a dodge-if you put a
finger against the end of the slide-stop screws (the ones that angle upwards) you can feel the slide lift and
fall as the throttle is opened and closed. Go on, try it, but don't press too hard as it does need a certain
degree of sensitivity of the finger ends. I suppose those of you who are bricklayers may have difficulty,
but those who only wield a piece of chalk should find it no problem. The clever bit lies in the fact that it is
possible to put a finger and a thumb against both screws at the same time (so you can tell which carb is
opening first), by standing on the right of the machine and leaning over the tank with your left hand, reach
under the pair of carbs with your palm upwards, you should be able to rest a thumb on one screw and the
end of a finger on the other, leaving the other hand to open and close the throttle. Difficult with a hot
engine-hence the recommendation of the Castletown Ale. Property set up in this way the bike should stay
set up until the carbs are next disturbed. The screw adjuster in the cable(s) is nor a tickover adjuster but
to cut down cable inner/outer slack when run (pulled) in.
One last thing about carbs. If ever it is necessary to ride a bike on one cylinder (silence all you Model 18
owners I mean one that has mom than one cylinder to start with). the best procedure is to prevent both
sparks and petrol from reaching the dead cylinder. Don't just take off the plug lead, but remove one low
tension wire from the coil instead-this saves the coil from destroying itself-and take out the drain plug of
the carb and stuff bits of rag up inside to hold the float shut-thus preventing petrol getting into the cylinder.
Washing
Continues on page 20
21