too rich?? too lean?? help, i'm new at this

well, i have a flatead thats runnung, but not well. it had 42 jets, whitch i suspect was way to small even for here i am(colorado springs, colorado). what does an engine do when its too lean?? too rich?? i'm new at this whole engine thing. i'm more a body guy but i got into a project and i need to fine tune it now.any info would be great.
thanks, dave

 

check the plugs, If there black you're rich and if they are really light you're lean. Should be a chocalte brown you're after. That jet sounds a little small for your elevation.