Yeah Rob is right on. for those of you that like a good correct timing write-up look at
http://www.fordmuscle.com/archives/2000/03/timing/index.shtml
That article is correct and very good and easy to understand. If you read articles like I do in these magazines you will find incorrect information in many of them because darn few mechanics fully understand timing and its implications(both centrifugal and part throttle-vacuum) . The peak pressure of your cylinders is controlled by timing,,,,peak pressure = horsepower so when your timing is messed up you lose horsepower big time. The use of dual diaphragm distributors was for CO and some affect on HC for emissions at idle. It depends on the "calibration" of the engine. In any single model year you might have one, two or fifteen different calibrations released for a single car during that year, like 289/2v/auto. Thats why you see different carburetor numbers and different distributors, depending on the calibration level. Later in the 70's when we had driveability problems we would first release a "safe" calibration that we knew would pass 50,000 mile certification and then later we would work on fixes to make the car more driveable. So really the later in the year the better. Once catalysts came along things got a little better.
On using a dual diaphragm in a standard distr,,,,,, mmmm I am not 100% positive but I dont think you can because the plate has to rotate back from the standard position. I think the plate stops are different. If you take upper and lower plates with it ,,, yeah you can do it. You can run the single diaphragm and not have problems, I just like the dual one because it sets your timing back at idle and your not running 12 or 14 degrees at idle which isnt necessary and causes you to close down the idle system to control the idle. Your idle system in reality flows much longer then most people realize, fuel comes thru the transfer slot and idle slots and up to 30/40 mph (light load) your not really flowing much in the main system, so your idle system is calibrated to that. Most 2 barrels could use a slightly higher idle jet,,,,, but whats an idle jet steve??? When you pull out the booster assembly there are two long tubes sticking down into the idle fuel well. Those long tubes are double,,,,a perforated tube(emulsion tube) that allows air in and an inner tube that transfers fuel. That inner tube has a calibrated orifice at the tip ,,,around .028 inches,,,some smaller some bigger. If you like an initial offline surge, open that orifice up about .002 to .004 inches it will make a difference. shhhhhhh,,,dont tell anyone else about that,,,,, its my secret to make these 2 barrel carbs really run well.
