If the valves are not adjusted properly how would the engine behave?

Clicking, loss of compression in one cylinder... it can be tons of things... What are the symtoms?

To add to the above, you could get a backfire, not starting or bent valves or pushrods forget which.

And awesome low end torque. Just kidding.

Light ticking noise from under valve covers. Intake backfiring, exhaust popping/backfiring. Lopy idle / shaking engine. Misfiring. Hard starting.