Couple of quick questions. Vintage Edelbrock manifold, 3 2gc rochester carbs. Only port for vacuum, comes from base of center carb. With vac gauge connected, shows 16lb. Is this enough to run both auto and brake booster. Also, the 2 idle mixture screws on base of center carb. In or out for less gas.(or more air) Just smells a bit rich when idling in shed. Cheers
Manifold vacuum should be good for the brakes and to run the engine so long as you do not have a high lift cam which robs the engine of vacuum. Here is alink to a good article written by a GM engineer that was posted a few weeks ago you might find interesting and educational: http://www.jalopyjournal.com/forum/showthread.php?t=783034&highlight=timing+vacuum+advance+101 And if you tighten the idle screws = less air more fuel. Are you running progressive linkage? If so can you feel any air on the secondary carbs if you put your hand over the top of the carbs when idling?
The idle mixture control screws could really be renamed, as they do not control mixture. A good analogy (at least I think so) is to compare the idle circuit to an old-fashioned shower control with three knobs. The idle jets (idle tubes in the Rochester) compare to the hot water faucet. The idle airbleed and idle air bypass (pressed into the venturi cluster on the Rochester) compare to the cold water faucet. So the idle mixture in the carburetor is preset by fixed jets. You cannot change the mixture without disassembly of the carburetor and modification to either the idle tubes (fuel) or the airbleeds and bypass (air). The idle mixture control screws (which you can adjust) can be compared to the volume or pressure control of the shower. With the shower you adjust the water temperature to your liking (this step is already done for you with the carburetor); and then you control the volume of water by opening the pressure or volume control. With the carburetor, the idle mixture control screws control the volume of preset mixture into the throttle area of the carburetor. Thus on the Rochesters (this is NOT true with all carburetors), turning the screws in (clockwise) will give less volume, and turning the screws out (counter-clockwise) will result in more volume. It is important NOT to use the in-lean, out-rich as a given; as many carburetors have a different idle circuit when the idle mixture screw meters either fuel (rare) or air (much more common). In either of these cases, the mixture screw would actually effect the mixture. And on a historical note, some manufacturers have tried to use only one control screw on some two-barrel carbs using the screw for volume control. While more accurate for those without exhaust analyzers, these were generally very quickly obsoleted due to the "howl" setup by mechanics and self-annointed mechanics that hollered that the carb couldn't work without the second screw. The carb most on this forum would recognize using the single screw feature would be the Carter 2010s type WCD used on the 1953 Pontiac. The carb was redesigned quickly with 2 screws. Jon.
Thanks guy's. Bit the bullet and went for a test thrash. Pretty exciting when outer carbs get opened up. Brakes working good, and auto shifting fine.Still think i'm running a little rich, also hesitates a bit with initial pedal. Fuel pressure gauge at 3lb, think I'll step it up to 3 1/2. Running a bit warmer than before. May be slightly far advanced. cheers
I have experienced similar experience with Edelbrock manifold and 3 2GC Rochester carburetors. I was experiencing brake issues and determined I only had approximately 14" of vacuum at the under floor 7" single diaphragm brake booster. To get manifold vacuum, I had constructed 3 1" carburetor risers with tapped openings for booster, transmission, and distributor advance. I am using the carburetor base for the PVC vacuum source. I am using a Corvette filler tube with threaded bung and using PVC valve threaded into base of carburetor and vacuum line connected to filler tubeWith this configuration, I had gasoline in the booster vacuum line at the booster. I did not detect gas in the booster. My question, if I change to a dual diaphragm 7" booster, will my 14" vacuum be adequate? And how do I prevent gas at the booster? I have seen mention of check valves. I thought there was a check valve in the booster.
I'm sure 3 banjos has it figured out by now since its been almost 2 years now. You shouldn't T the brake booster, what's probably missing is the hollow carb studs that provide a vacuum source for the Trans and distributor.
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