If you make the ignition contacts bracket so that it is easier to adjust while the engine is running you can also advance and retard the timing which will also affect the speed at which the engine runs. This can often be done by replacing the short clamping screw with a longer one which will act as a lever for adjusting the position of the bracket.

Although not a 4-stroke take a look at this video from about 1min in where Nick is using the long lever to alter the timing and only making small adjustments to the carb but it has a marked effect on speed and if taken to extream allows the engine to run the opposite way.

The ability to slow run will also be affected by carburation. If the carb does not supply the correct mixture, you will never get a good stable low idle.

Mixture strength and ignition timing are inter related.

Carburetor designers have gone to a lot of trouble, on full size engines to ensure that mixture strength is maintained pretty closely, over the operating range. (Emulsion tubes, air bleeds, capacity wells etc and many hours of test bed time to obtain the correct jet and ignition settings ) This is a lot harder to do with a shop made device, which is going be fairly simple in comparison.

Also, induction tract temperature will have an effect. A good stable idle, after a long heavy load run, may change as the tract cools, and vapourisation deteriorates. The carburetor meters the fuel, much of it vapourises en route to the cylinder. If the carburetor is not supplied with enough heat, ice will form. (The latent heat of vapourisation will come from the carburetor rather than a hot spot or water jacket, and reduce the temperature, to below the dew point and then below freezing).

Sorry to complicate matters!

Howard