You could attack the problem scientifically by inserting an oxygen sensor attached to an A/F guage. Thats how I tune my holley. In all honesty, my plugs always look white even when I am running super rich, so I would heartily recommend a sensor instead of the plug reading method.
A 625 cfm seems a bit low in my opinion but I dont think its really hurting you that much. If the car runs well and doesnt overheat, then I dont think there is much of a problem..heck, you are saving yourself some gas. What jets/powervalve/shooter/cam are you using? I am using 68/70's,8.5 pv, .28 shooter and the base cam on my 670 street avenger vacuum secondary to run my little 302.
I love holleys for their adjustability. I will never fuel inject again. I also had a lean situation that was affecting idle and I alleviated this by spray carb cleaner into the fuelbowl and letting the carb clean itself from the inside.
Good luck!
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91 GT Carb conversion, holley 600 double pumper, edelbrock performer intake, FMS "C" drop springs, march 1000 underdrive pullies, crane 1.7 roller rockers, GT-40P headers, bassani x pipe, american thunder catback, FMS 4.56's, msd aL6, trunkmount battery, A/C eliminator kit, 3000 stall tci streetfighter, AOD with transgo kit, A+ servo, 300M hardened lockup shaft, kevlar bands and 28,000 gvw trans cooler, 3 core radiator, 300 lbs stripped with a full interior
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