by Tim Buja, TKBuja@insightbb.com
When setting the idle speed on a Stag equipped with the Borg Warner automatic transmission, you must strike a balance between an idle speed high enough for sufficient oil pressure, but low enough to permit smooth gear selection. Ideally, the BW automatic prefers its idle speed to be around 500-600 rpm in neutral, but it is rare to find a Stag that will idle that low nowadays due to worn carb and distributor parts. An idle speed higher than 900-1000 rpm in neutral will cause the transmission to engage Reverse or Drive with a lurch. If you can't get the idle speed low enough, you may need to check the carb throttle shaft seals and/or the carb pedestal-to-manifold o-ring for leaks, and for proper actuation of the vacuum retard mechanism.
It is common for the breaker point baseplate assembly to wear to the point where the proper point gap can not be reliably set and maintained. Since this affects both dwell and timing, it will also affect the idle quality and speed.
Another wear point to check is where the vacuum capsule actuating rod connects to the breaker point baseplate.
The left vacuum capsule in this picture is a NOS part (Triumph p/n 520131, Lucas p/n 54424241), while the right capsule is a worn version of the same part from a 1973 MkII Stag (US market). Note the elongation of the hole in the actuating rod of the old capsule. Together with wear in the pin that connects the actuating rod to the baseplate, there was so much slop that the vacuum capsule could not reliably retard the timing to its proper setting. This resulted in an occasionally higher than normal idle speed, causing annoying vibration or shaking at idle with both Drive and brakes engaged (when you're stuck at a traffic light, for example). If you encounter this shaking, try moving the gear selector from Drive to Neutral. If the idle comes up to 1300 rpm (or higher), your vacuum retard is probably not functioning properly. Try slightly opening the throttle to raise the engine speed up to 2000-2500 rpm, then quickly release it so that the throttle snaps closed. If your idle then returns to 800-900 rpm, there's a good chance that your distributor's vacuum retard system parts need attention. Remember that if the coolant temperature is above approximately 210F, the vacuum retard is disabled by a thermostatic vacuum switch mounted just under the upper hose connection to the "hot" tank of the radiator. Disabling the vacuum retard causes the spark timing to advance, which will raise the idle speed and increase the coolant flow through the radiator. If the idle speed remains at 1300 rpm or so but only while the coolant temperature is above normal, the vacuum retard is working properly.