Craig, if you recall the days when cars had carburetors, whether they had a manual or automatic choke, they always ran at well above the normal idle speed when on choke. There was a mechanical link between choke and throttle so that as the choke was reduced, so was the engine speed - but the choke was wide open somewhat before the idle speed reduced to normal. The reason was that you tend to get cold-running lean-mixture malfunctions (sags, spits or stalls) if you don't elevate the idle speed at first. You could also rich-load the engine rather easily if you over-choked it without raising the idle speed. (You probably know that rich-loading engines is bad for their health, since the excess petrol tends to wash the oil off the cylinder wall.) As soon as the engine will run smoothly without either increased idle speed or choke, you can dispense with them both and reduce the idle speed to normal.
The objective is just to get a clean, smooth warm up, and the brief fast idle immediately after the choke is opened will help to achieve this, especially in cold weather. Remember, the maximum governed speed for that engine (a little over 3,000 rpm) is less than twice its idle speed (1,750 rpm) - it's not as if you were revving it hard, as you easily can with a car engine for instance.
Last edited by grumpy; 02/11/13 10:05 PM. Reason: Add detail
