I have a letter dated July 12, 1968 from Carter Carburetor Division which was in response to the question I asked about the carb CFM rating. Letter said; the 3506S Carter (R-1) has a rating of 637 CFM @ 2" mercury. Avanti, Jim Frakes CredoVette@aol.com March 2001 Carter Carbs Specs for R - Engines R1 - Carter AFB 1 7/16" primaries, 1-1 1/16" secondries model 3506s flow 625 CFM (+-10) Replacement = Carter CRT-9635 or Edelbrock 1406 Jets: .089" (120-159) primaries; .058" & .069 (16-99) metering rods. Secondary jets were .077" (120-177) R2 - Carter AFB 1-7/16" primaries, 1-11/16" secondries Calibrated and sealed Stude# 1561641 model AFB-3606 flow 625 CFM (+-10) Replacement Carter CRT-9635 or Edelbrock 1406 ( Use Procharger bonnet to adapt supercharger hose) R3 - Carter AFB 1-7/16" primaries, 1-11/16" secondries Competition model Stude# 1558637 Standard model 1563050 model AFB-3808S flow 625 CFM (+-10) Primary jet # 120-159 Size (.089) Secondary Jet # 120-166 size (.0935) Primary metering rod # 16-103 size (.068 X .060) float setting 13/32" R4 - Carter AFB 1-9/16" primaries, 1-11/16" secondries Front Carb Standard model Stude# 1563051 AFB-3810S flow 675 CFM (+-10) Primary jet # 120-162 Size (0-1015) Secondary Jet # 120-176 size (.0635) Primary metering rod # 16-124 size (.069 X .0635) float setting 5/16" Rear Carb Standard model Stude# 1563051 AFB-3811S flow 675 CFM (+-10) Primary jet # 120-162 Size (0-1015) Secondary Jet # 120-122 size (.070) Primary metering rod # 16-204 size (.073 X .063) float setting 5/16" Front, without choke Stude# 1563051 Rear, with choke 1563052 ================================== Replacing any of the above - 1406 (electric choke) - 600cfm ////You will need an aftermarket air cleaner though.. it's easier than using an adapter- and the Stude air cleaner doesn't flow enough through that small snorkel... -------- I have a 1405 manual choke that I used on a 259 and 289 with stock intake and an adapter plate.... for a while I used a home made wooden adapter riser plate that worked just fine... FYI ------------------ I had better performance with the 500 than I did the 600. An engine is an air pump. Fuel burns at a steady rate at a certain air/fuel ratio (about 17:1 ifrc) Get it to flow more air (emphasis on flow as in total volume in and out) and you can add more fuel. Burn more fuel and you make more power. (Hence the supercharger..just 'shove more air in and add more fuel). On a carb engine you have to create a lower air pressure at the venturi to allow the greater air pressure above it to 'push' the fuel into the throats of the carb ( That Bernoulli guy again...).... If the throttle butterfly's are balanced to the air requirements of the engine at that particular point in time and rpm, then the max power will be made in the burn process in that cylinder. If the airflow is way more than the engine needs, then the ratio goes way lean and the power drops off dramatically. If the carb bores are too big, then the 'signal' is smaller and less fuel is sent to the engine. Hence the proverbial bog right off idle. Too much air too soon (too big a carb) AFB's (and Q-Jets) have a secondary air valve that opens by having airflow overcome a counterweight. This is how they get around the secondary bog at a fast wide open throttle condition. Regulate the airflow for just a split second and let the flows match up better. You can put a bigger carb on and it will work, but it will be sluggish off idle and at shift points. Too small a carb limits total airflow (top end, rpm wise), but will have better low rpm performance. Where do you spend most of your time, rpm wise? Sometimes smaller is better.. Jeff ( Just don't ask my wife ) Rice ------------------------