I had my 383 built a few months ago and the engine builder took my carb that I had off my old engine, a Holley 600cfm, and put it on the new engine. He ran the engine on the dyno and he did some tuning to the carb. Not sure if he used an A/F meter or what. I checked my float level the other day. When I pulled out the sight plug, fuel really came flowing out. I adjusted the level per Holley's recommendation of just below the edge of the sight. The car then proceeded to run really bad! It lost a ton of power. Fortunately he had marked the position of the adjustement screw and I was able to put it back. My question is did he likely do this to compensate for a lean condition? If so, would the right way to tune it been to swap out the jets/metering block? Also, I did not adjust the idle screws after I lowered the fuel level. Should I try lowering the fuel level again, but this time adjusting the idle screws as well?

i'd try adjusting things with the float level where it should be.

I would call the builder and ask him, you would think he would be the one that knows ??? Also, if there is any type of warranty, you messing with his settings might make it void. I would be answering differently if you built the engine yourself, but when you have one built for you, the builder is the first person you should bea asking before messing with it, especially when he dyno'd it and tuned it.

By the way, my cardinal rule is if it is working ok, dont mess with it ! :)