My W.A.G. on this would be float level. Figuring out what it should be set at is only half the battle, then you have to know how to set it/where to make that measurement. It is not particularly obvious IMHO. I've rebuilt two Q-jets now, the original to my '76 and the Edelbrock Q-jet that is on there now. The first one had lots of weird symptoms and I'm pretty sure now it was a wrong float level that was causing them. The E-brock unit (not a typical E-brock that's like a Carter AFB, this looks just like a q-jet and uses the same parts) that I'm running now runs great no problems whatsoever. I eventually plan to rebuild the original the right way and switch back to that, but it will be spendy because I want to get it re-plated first.
There are at least two good books out there on Rochesters/Q-jets, one by Doug Roe, the other by Cliff Ruggles. I'd recommend getting at least one, preferably both because they compliment each other well. Rebuild parts and kits should be purchased from outfits that specialize in these carbs, one that I know of is Cliff's High Performance. The rubber parts that come in kits from the auto parts store may not be compatible with today's fuels.
Getting a Q-jet to work right is not an impossible task! You just need to be very organized and methodical and have good references and parts. You may surprised - you may be able to do this better than the mechanic you typically use to change your oil or replace your oxygen sensor on your modern car. (Although the best bet would be to send it to a shop that specializes, but that wasn't an economical option for me). Good luck!
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1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.