Ive been swapping carbs, on my 540 and running them through my lm-1 A/F meter. I decided to bench the 1000cfm proform carb, the a/f at cruise rpm was awesome with it, right at 14.7 to 15. The throttle response and wot is fantastic from the 870 but the cruise a/f is pig rich(10 to 12). Will adjusting the four corner idle screws help lean the engine at cruise. Any suggestions on how to improve the part throttle A/F ratio.

Thanks Rich

The jets, in a Holley, control cruise mixture. Start going down in 2 number increments. If, once that is set you are lean at WOT, you can drill out the PVCR (or is it PVRC). That will richen the mixture at WOT with out effecting the part throttle mixture.

On mine, I did as Pat said and started reducing jet size just till it started to surge at cruise. Then went back up one size. That's all it took. I did look at plug color after a couple of weeks to be sure, all was good.

I had a chance to play with the carb, I put a vacuum gauge on the carb vacuum port and got 9 to 10 inches. I was able to lean the idle a/f ratio to 14 by adjusting the 4 idle mixture screws but as soon as I began cruising the a/f went to 11.8 to 12.8. Since I believe there is 6.5 power valves front and rear does this sound correct for this level of vacuum? And could the power valves be open while cruising, Is there any other way to lean the cruise a/f with out having to drill.

you don't have to drill, just put two size smaller jets in it and try it. If your meter is quickenough make some full power runs from 2500-5000 and see what the power mix ratio is. It's probably rich too. then install the 2 sizes smaller jets and see where it is, both power and cruise. My bet is that will lean out some and give better power too. For perf engine I wouldn't run the cruise any leaner than about 14:1 or so. Try it and see, jets are cheap.

Tom