I have a 6.5 Briggs on a Scotts Mower that's really difficult to diagnose. I have to continue to press the primer bulb to keep it running until it warms up. It then does not have full power. Have rebuilt the carb several times paying particular attention to the High speed jet and blowing it clean with compressed air. No luck! I have rebuilt hundreds of carbs in the past, so I know what I'm doing. Have finally replaced the carb with a factory new replacement. Still the same problem! One Noteworthy occurance. Just about the time that this problem started, our area was targeted for the new "Environmentally Friendly Fuel" with MTBE added. Anyone know of an additive that I can use to reverse the problem and determine if this is simply a fuel additive problem? Maybe trying a tank full of Coleman fuel?

Take the primmer itself out clean hole leading through back of air filter box from primmer to carb and replace with new primmer bulb.

B&S makes a special tool to do tiis with, it looks like a plastic mushroom - or you can use a 3/4" socket and a hammee.
If you take or have taken the air filter box off of the carb,then replace thet little gasket there.The pimmer wont work with a gasket that is damaged.

Already did that. replaced the carb with a new factory carb and put a new primer bulb on at the same time. Interesting to note that this Carb has zero adjustments. The Primer bulb works great. It's the only way that I can get it to start. Then I have to continue to pump the bulb for 5 to 10 minutes till the engine warms up. This has to be some problem other than Carb. That's why I suspect the fuel change in our area. You know ya gotta use that environmentally friendly fuel, but this engine doesn't like it. This problem started two years ago, and hasn't gotten worse or better, no matter what I do.