Can anyone give me advice on a lawnmower engine?

[Yota Up]

I've got a Briggs and Stratton 3.5HP classic mower.  The engine revs up and down, up and down.  Only runs right for a few seconds on startup after priming.  And then there it goes, up and down.  Took off the aircleaner and can see the lever for the throttle control going, pulling on springs and then going right back, over and over.

Is that it?  Is it toast?  Manual says no adjustment to carb is available, and I have no idea what else to tweak.  (New plug, clean airfilter, everything ok.)

If by chance one of ya'll knows and can help out, thanks a million.

[Duffil]

sounds like a vacuum leak.  It gets lean, revs up, pulls on the governor, revs down, gets lean, etc, etc.

[79coyotefrg]

my 18 horse was doing the same thing,  bowl on the carb  was  rusted  and had  some crud in it,  cleaned it  out,  blew the jets out with carb cleaner  runs great

every now and then it  idles funny,  i peck on the carb and it fixes it

[BLACKDOG]

kick it