Thanks for the info. I am replacing all four idle mixture screws with new screws from Holley. I don't have a starting point for the old screws because they were not adjusted properly to begin with. I don't know if the small block carbs would be that much different than your big block carbs but previously I've been told several different settings for the mixture screws on the end carburetors ( anywhere from 1/8 to 1 1/2 full turns open). I know this setting is very precise and an 1/8 or 1/4 turn could make all the difference between a smooth idle and a terrible one. That is why I was asking for information on an untouched, original carburetor, hopefully to unlock this mystery. The problem is, if I get the setting wrong and the engine doesn't idle properly, I still have access to the front carburetor idle mixture screws and they can be adjusted. However, the only way to adjust the rear carb is to take it off and on, adjusting the needle setting each time until it is determined to be right. It is virtually impossible to adjust the rear carburetor with it installed. Does anyone know if there is a "spec" setting on the idle mixture screws for the 4789 and 4790 carbs?
Thanks
All 3 carbs - start at 1 5/8 turns out, stone stock jetting. This is what I ended up at on a T/A with a Comp 268 XE Hydraulic and stock exhaust. Get the best idle from the center, then the front, using the 'finger over the bleed hole' trick to tell if that bore is lean or rich. Get the front adjusted, then as suggested, swap front to back and tune the new front. Make sure everything is right first - linkage adjusted right, fast idle cam bend tab not hanging up and causing the motor to rev too high (DAMHIK..lol), vacuum lines correctly plumbed between the center and outboard carbs YMMV...don't worry about what the settings are out of the box (most likely 1/2 to 3/4 out on the end carbs) because it won't be close enough. It has to idle on all 3 carbs, not just the center one. I imstakenly thought for a long time I could get it to run well at idle just on the center carb and that was a mistake. Could never get it fat enough on just the center carb. Once I opened up the end carb idle screws it became much nicer and happier at idle, and cleaner too.
