I’m sorry that I didn’t see this at first, Sean, I’ve been a bit busy building the Ploughing section and the Garner Registers- I hope you like them!
Anyway, throttle settings on the Amal fitted to a Norton Big 4, here goes.
Use the closed butterfly as your starting point; set the twist grip throttle to the fully anticlockwise position, then you need to adjust the dog-leg at the end of the throttle rod so that it is offset towards the carburettor (you do this by undoing the clamp in the throttle rod joiner, adjust the dog-leg at the end, then tighten it all back up).
The carburettor link should now be pushed towards the carb by the dog leg, so that now all of the linkage should be in the tick-over position. After double-checking that the butterfly is in the fully closed position, tighten the clamp on the arm around the throttle spindle so that the whole lot is now tight and set at tick over.
The engine will not work at this setting with the butterfly fully closed because no air can be admitted, so we now need to open the butterfly slightly using the tick-over screw clamped to the throttle spindle- I use about 1/16″ gap as a starting point.
The main jet needs to be screwed in fully (gently!) then backed out by one turn. The tick-over mixture screw also needs to be turned fully home, then backed off by one turn. This should at least make the engine run so that you can tune it by ear.
Somewhere near 95% of the fuel goes through the main jet, so we set the main jet up by running the engine at about 1500 revs and turning the main jet in and out until smooth running is attained with no misfiring or banging, or excessive black smoke.
Turn the tick-over speed screw in a few turns so that when you drop the revs to about 1,000rpm the engine will be running on the tick-over screw; this will allow you a lot more control when you come to set up the slow-running jet.
You should now have the engine running at 1,000rpm with the screw touching the carb. barrel. Next you can slowly turn the revs down by unscrewing the tick-over screw slowly; as the speed drops then the slow-running jets alongside the butterfly start to take over and become more important. By using your ear to listen to the engine’s behaviour you should be able to adjust the slow-running jet at the front of the carb to achieve an even, balanced tick-over. Just remember that turning this jet outwards will admit more air to the tick-over mixture and thus weaken it down, turning it in richens it up; at this point do not touch the main jet, however tempting it may seem! If you do then you will have to start all over again.
When you get near to the correct speed for tick-over you will hear the clutch drum clanging like a bell; this is caused by the clutch shoes becoming disengaged. Adjust a little slower than this until you can see the end of the countershaft stop spinning.
You’ve done it!
It takes a lot longer to describe than it does to actually do the job, but be prepared to get it wrong a few times before you reach a setting that suits you and the machine.
Best of luck.
.
