timing/tune up problems

How big is the engine, camshaft and heads? Is it a quench engine?

Put the 18 bushing on the distributor. I wouldn't run the 25* bushing on my grannies valiant! :) If you reach back, twist the distributor a bit CCW and it picks up idle rpm, the engine wants the timing.

40* is usually a bit much total.

Like 67 mentioned, make sure the carb isn't dripping fuel. It's a common issue with the taxi cab carbs and too much fuel pressure. :)