The only way to check high speed mixture correctly is to do a road test with a 4-gas exhaust analyzer connected. Swapping main jets and/or power valves, if your carb has them, is how high-speed mixture is adjusted. Nothing you do to the mixture screws will have any effect above idle speed. You can do a WAG by looking at spark plugs or the color of the tailpipe. A too-lean mixture will have a tendency to detonate ("spark knock") and be short on power climbing hills or accelerating. It will also run hotter than normal. Really lean, and the plugs will turn a white color, with glassy-looking deposits and/or powdery-looking shiny stuff on them as the pistons melt. Fuel is cheaper than internal engine parts, so don't get it too lean.
If anyone tells you he can change high speed mixture by adjusting the idle jet(s), one of two things is going on:
1. He's too dumb to be tinkering with your carburetor.
2. He's lying and knows it, and he's about to get into your pocket in a big way by charging you for a procedure he knows won't help.
In either case, RUN, do not walk away, and find someone who knows what he's talking about.
Jerry
