I have a 355 sbc chevy in the "A" coupe (I know, I know). It has about 10.0 to 1 compression, Comp Cams 270 Magnum Camshaft, MSD ready-to-run Small body HEI style Dist with the MSD blaster coil. Initial timing at 10 degrees. Dual 500 cfm Edelbrock Carbs on a Edelbrock c-26 intake. I have rejetted Both carbs TWO stages Leaner than stock on both the primarys and secondarys. The car runs GREAT now, idles good, but real rich. It will put a terrible rich smell in your clothes if you are standing behind it. It does not foul plugs though, maybe due to the MSD ignition. I use a progressive linkage with only the rear carb working until about half throttle. It uses the idle mixture circuits on BOTH carbs though- It idles the best this way. All screws about 1 1/4 turns out. The idle speed is only adjusted on the rear carb. The front blades remain shut at idle.I tried closing the mixture screws on the front carb but then the idle quality turned to crap. My question, Would drilling very small holes in the front throttle blades lean it up at idle some? The car runs so good I really hate to screw it up. I know the cam overlap is causing some of this, but I have stood behind other rods with even wilder cams but with single carbs and they didnt idle this rich. Any ist hand help on this appreciated.
I have read that you must change those springs (on the top, under the Torx 'n plates) that control the metering rods. I don't remember why but an expert told me to use the lighter springs. Logic would sense stiffer springs though. I've always had Edelbrocks run rich (big cam) 'til I use I think the purple springs(eBay). I did it and it worked on single 4bbl aps.
How much base timing? Try setting the base timing up to about 12 to 14 btdc. Try hooking vacuum advance to manifold vacuum
My experiences with dual quad set-ups are if you run mixture screws on both carbs you must set idle speed with both carbs. Close down the idle speed on the rear carb as you open up the ilde speed on the front carb. I would not drill holes in the throttle blades. Just use the idle speed adjusting screws to balance the idle on both carbs. I've actually had the best success with not having any idle feed fuel on the front carbs, but you can run on both. Set both carbs up as close as possible as far as mixture screws and idle screws are concerened.
I don't think you are running enough ignition timming. I say bump it 2 degrees at a time until it cleans up then kill it and try to start it. If it starts hard, retard the timing slightly till it starts ok. Make sure you are setting the timing with the engine at the lowest idle rpm possible. Advancing the timing leans the motor out. changing the main jet should not have any effect on the Idle.With it running at idle losen one of the screws on the caps refered to just enough to see how far that metering rod is comming up in the dash pot well. It should be held down by the motor vacumn to a point that it is just about even with the top of the well or slightly above. If it is popping up hard then you need lighter springs. If the metering rods are sucked in too deep the Idle will be lean. once all this is set road test it. If the motor pings under acceleration,DON'T change the timing . Go the next step richer on the main jets until the ping is just slightly there or gone. This all ways going to be a balance situation on a street car. Most race cars run so much ingnition timing they have to be cranked with the ignition off until the motor has some cranking speed.
I agree. You're dumping fuel with out air to the front carb. You might also consider advancing you timing to a min of 12 degrees. it won't help your rich idle mix but even a stock small block runs @ 12. You exhaust won't get as hot either.
The Edelbrock site has a tech line, it is a great tool! The big block dual quad intake came with different "step-up" rods. That is most likely what you need (although they will be different in your application) and they will be more than willing to tell you exactly what to order from your favorite supplier.
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