How can I fix my Briggs & Stratton 3.5 Classic push mower?

This sounds as though it is a spark plug problem. My advice would be to purchase a new plug. As for de-coking the engine, I would suggest putting a very small amount of diesel in the tank, this does the job as well as Redex that you can purchase at any decent garage or diy shop. The amount to put in is about a thimble full, and then top the tank up with petrol and make sure it is well mixed. I would also suggest that you do an oil change, as this can also make the engine run very ruff.

Richard Shannon
August 2008

Replace the carb diaphragm - costs pennies, takes 20 mins, and is an almost guaranteed fix .;-

1) Remove air filter
2) Remove 2 bolts securing carb/tank assy to engine. One at front of engine,
and one just in front of the tank filler. On non-electrc start engines there
wll be a spacer - don't lose it.
3) Manoever the whole assy off the engine, unhooking the throttle from the
govenor vane as you go.
4) Unscrew the carb from the tank (5 or 6 screws, depending on the model.
Seperate the carb and tank.
5) CLEAN the tank surface. Place the new gasket and diaphragm in position on
the inverted carb, making sure that the fuel pump spring and cup are in
position.
6) Position the tank on the carb. turn the whole assy right side up and
insert the screws. Tighten them evenly. Don't overdo it - the carb is
plastic.
7) Install the assy onto the engine, making sure that you hook up the
throttle lnk as you go. DON'T twist the govenor spring - check that the
throttle moves freely. If it does not, FIX IT NOW - otherwise the engine
will attain some really impressive RPM's before self-destructimg. Most likly
cause of problems is a twisted spring.
8) Re-fuel, and check that the primer squirts fuel into the engine (you can
see it by looking down the air intake). If OK, fit air filter and test. If
not, either the diaphragm was damaged during fitting, or there is a stuck
check valve in the carb. If a sharp tap on the carb body doesn't fix it, a
replacement carb will be needed (not likely, but possible).

Phil Saunders
August 2008

I've just found simular problems with mine. The first thing is to remove the spark plug and look at it - if it is covered in muck then the quick fix is to clean it with a brass wire brush or replace it (about £5) this gets the grass cut this week. You then need to look on the side of the mower for the serial number - mine is just above the exhaust but hard to read as it is all painted in black. Go to the Briggs and Stratton Web site look under "Maintenance and Repair" then type in your model number and they will have a PDF of the manual which will show how to set your carburettor correctly to stop this happening. The bad news is that you do need to de-coke your engine and that is too complicated for me to go through here but a simple manual or book can make it a very "do-able" and strangely satisfying task.

Chris G
August 2008