Reply To: Norton Big 4 Spark Plugs

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When I set up my engine after cleaning the carb I set both jets to 1 1/2 turns out. That should be sufficient to start it, then once it’s running you can adjust firstly the main jet (engine speed needs to be 1500>2000 revs approx.), then drop the revs down slowly so that it drops to tick over. The best way of doing this is to turn the slow running adjustment screw in a few turns to open up the butterfly, then you can slow the engine down in a controlled way with gradual adjustments to this screw and if it looks like it’s about to stop you can adjust the slow mixture screw accordingly. This gives you plenty of time to make the adjustments without having to rush or continually revving it up and down. As a final check, with the engine on tick over just increase the throttle as if you were going to drive away- if it splutters, hesitates and then picks up you need more fuel, so richen up the slow running mixture screw.
The slow running mixture screw not only keeps the engine running when on tick over, it also assists the engine to increase speed ( a little like an accelerator pump ); it is often overlooked when adjusting the carb, but it is the most important. If you look in the venturi you will see that there are three holes spaced along the inside, roughly where the butterfly comes to rest. It is these holes that control the tick over and acceleration of the engine, and they are adjusted with the slow running mixture jet.
One other thing to check is the timing, which is crucial. Somewhere near, in my experience, is not good enough. It needs to be set at 3/8″ BTDC with the points just opening, anything else and it just doesn’t run correctly.
I hope that helps!