I have an 87 L98 that I just put a single plane intake and Holley 600 double pumper on. The engine has headers and 3" exhaust with a Flowmaster and no cat (other than that it is stock). I was using a vacuum gauge to set the idle-mixture on the carb, and the instructions I have say to use an idle speed of 850 RPM's and adjust the idle-mixture screws and idle speed screw to achieve highest possible vacuum @ 850 RPMs. Using an idle speed of 850 RPMs I can get a steady reading around 20" of manifold vacuum. The stock setting for idle speed on a L98 is either 500 or 600 RPMs. When I adjust the idle-mixture screws to get an idle apeed of 500 RPMs, manifold vacuum drops to a steady 15".

I know the engine is running good internally because the vacuum reading is always steady. My question is which idle speed and vacuum setting should I use: 20" @ 850 RPMs or 15" @ 500 RPMs? By the way my timing is set at 6* BTDC.

Thanks in advance!

 

since you don't have a radical cam, i'd go with around 600rpm or so. You don't need more then 15" I wouldn't think.

 

As long as you have the stock cam, 600-800 is fine. Play around with it a little and find a speed that the car runs the best at.