I'm new to Small Engine repair and I really love it but I'm having a problem with a Briggs 10A902 (I think it's a 3.5HP) and I'm not schooled enough in diagnosing the problem that I'm having. The engine starts right up on the first pull and runs great for about 5 seconds and then it revs up and down. The Carb has no adjustments, just springs. looking in the manual I got from the Briggs web site, it seems that the springs on the throttle are in the right place. HELP!

the carb needs cleaned, did you ever leave gas in it over the off season or use gas more than a month old?

I Drained the gas out of it last fall but it's always rev'ed up and down since I aquired it. I put fresh gas in it and it still does it. Believe it or not I picked this mower out of one of my neighbors trash last spring and it's in GREAT shape!. He said that he could not get it started. I put a fresh plug in it and fresh gas and it started on the fourth pull. Now it starts on the first now that I messed with it a bit. If I could get this idle issue figured out it will a fairly new mower. I guess my one neighbor is not mechanicaly inclined.

rebuild the carb, IE new diaprahm and new springs if they show they are stretched out. oh and clean it good, it is most likely gummed and when its gummed just replacing the old gas won't help it.

Your engine is surging. This is most certainly a governor problem.