I know I'm a bit late here, but have you tried using fresh gasoline?
That and clogged air filters are about the only difficult-running issues
I've met with small engines. Someone suggested a carb rebuild -- that
might be necessary if old gas varnished up the passages in your carb.
I think the "thingy" is probably the governor vane. The faster the
engine runs, the more air the cooling fan blows. This vane is spring
loaded to open the throttle and the air from the cooling fan causes it
to close. If the spring is off or disconnected, you might get this
behaviour too. (Now that I'm explaining it, I do remember that I had a
Lawn-Boy that did that once.) With the engine stopped, the throttle
should be wide open and you should be able to close it by moving this
vane thingy. (Take off the air cleaner and look down the carb throat to
see what's happening.)
> lawnmower having issues. does not like to idle. will rev up and down then
>spontaneously die. playing with a thingy (tm) on the carb makes it not die.
>the thingy(tm), if I had to guess, seems to control what would be the slide,
>were it on a carb on one of my bikes. it seems to be spring-loaded, and if I
>hold it back, the engine will rev. throttle pinned the whole time, fwiw.
> can these things possibly be cv carbs?
> this is a tecumseh flat-head, and it's four hours old (to me--the old one
>splatted this weekend right after I asked about the light fixture. this one
>was close and cheap, and a nice mower except for that pesky not-running
>issue.) sears craftsman 6.5 horse, if it means anything.
> help me out here--valve job on a desmo ducati: no problem. old leaf-blower:
>stumped. cam chain tensioner, cbr 600: cake. craftsman lawnmower: stymied.
>I so want to join the ranks of those to whom these things come easy, and I've
>watched the mouth-breather here fix these things with an adjustable wrench, a
>flat-blade screwdriver and a can of wd-40. how hard can it be?
> somebody take a guess--there's only six moving parts on the thing, eventually
>you'll get it. I'm just terrified because last time I want through this (I'd
>just ported and rebuilt an rz350 engine...how bad could briggs do me?) I ended
>up having to drag said briggs to the local mower shop and pay $120 to watch
>the mouth-breather assemble it in 5 minutes. with an
>cardboard-and-aluminum-foil head-gasket, I think. I can't go back.