i have a 50k mile motor thats running a lil lean. will runnin a lil lean make me not pass smog.

 

It can, but... 'how much' is rather important in answering your question (how lean) as well as which state you're in (standards vary) and possibly MMY (older .vs. newer, OBD1 .vs. OBD2, etc).

Here in Washington, you pay for the first test ($15) and the next re-test is free.

You could go anytime and check... or perhaps buy a beer from a local shop that's emissions certified and ask them to check the parameters most affected by running lean?

WA doesn't test NOX in the Portland-Vancouver area (for example) and NOX tends to go up with leaner/hotter burn.

Good luck