25hp Evinrude backfiring problem.

[KenWeaver]

I have a 1978 25hp. I'm having trouble when you first start the motor it will backfire and shut off. After the 3rd try you can get going and after the motor warms up it will start on the first try. I have rebuilt the carbs, fuel pump, electrical system and both cylinders are#1 173/lbs #2 171/lbs. Any ideas???? Thanks for any help!!