On 11 May 2005 10:40:01 -0700, "spongehead"
wrote:

I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.

It's the carbuerator diaphram, worn out most likely.
Yours is the plastic bodied carburator with the black or red primer
bulb yes?
If so you'll need to purchase a new diaphram and gasket for the carb,
about 4 bucks or so at a lawnmower parts shop.
The carb is secured with 5 philllips head screws. Take those out and
gently pry the plastic carb off the fuel tank.
Take care not to lose the diaphram spring( it'll be on the bottom side
of the carb.) or the mesh filter screen on the main fuel intake port.
Note whether the fiber gasket is on the carb side or the tank side(
varies with each model.) of the rubber diaphram.
Replace new ones in that order. Take care not to crease or wrinkle the
rubber diaphram whille tightening the screws down and you should be
good as new.
While apart use a good aerosol carb spray to clean out the carb and
tank resevoirs.(just about any brand but Gumout will do, I use Super
Tech brand found at Wal-mart. Autozone brand of carb spray is good as
well.)
Spray through the intake port(under mesh screen) to back flush the
supply pipe and the main port in the carb throat as well.
blow off any excess cleaner or allow to dry before replacing rubber
diaphram.(cleaner will attack the rubber shortening it's life.)
Any questions in particular just post back.
One of my many skills is small engine mechanic and I have years of
experience with lawnmowers.