problem with idle new engine/new rebuilt carb

[cranky]

new to forum, i need some help. installed a brand new 350 cu.in. 295 hp. chevy engine in a 1980 corvette. carb the car came in on had the top warped very badly. replaced carb with a rebuilt one from koi auto parts. runs well, but won't idle unless curb idle is way up. mixture screws backed out 5 turns. will idle fairly well if hand is placed over airhorn. i figure i will have to drill out the idle tubes and idle channel restrictors and the idle mixture holes in the throttle plate, but i don't want to make the mistake of going too big. could you give me an idea of where to start size wise? it would be greatly appreciated, thanks mike   p.s. it has 11 in. vaccumn out of gear, and 7-8 in gear.

[omaha]

7 to 8 inches of vacuum seems kind of low. What compression ratio does the engine have? What cam does it have? this is important to know. cranking pressure would be a good thing to know also but not 100% necessary. The reason is you need to have a idea as to where you need to set your initial advance and how to set up your idle setup. Low compression (cranking pressure) and big cams means more idle fuel and more initial advance (sometimes idle bypass air too). This might increase your vacuum at idle. (a good thing). Yes you probably will need to increase the idle fuel and decrease the air bleeds DEPENDING on what the carb number is. I don't think you could just buy a carb from KOI and know for sure that it is even close unless you do some investigative work (taking it apart). Even just having the carb number wont tell for sure because you have a "rebuilt carb" you don't know what is in it.  I'll guess that it needs more initial advance but its just a guess.  I'd start at 12 degrees initial and go from there with experimenting. Then, I would folloe recipe one in the book (Cliffs book).

[cranky]

thanks omaha, i have experimented with the timing for more vaccumn. i believe the engine has 9:1 compression. i will take the carb apart to see what i have. just haven't had the time yet to do it. i will post up the number and my findings when i do. runs very strong, just having the idle issue with it............mike

[Shark Racer]

What's the cam spec?

Judging from what you say, you're correct that you'll have to go up on the idle circuit. The only way to know what to do is to measure what you have now. I'd recommend ordering a set of tubes from Cliff and then sizing everything (air bleeds, tubes, DCRs) and a decision can be made from there. You'll probably have to go up a few steps from stock though.