Lawn Mower Engine
- Bad gas
- Bad spark plug
- Dirty / faulty carburetor
- Gas filter blocked
- Gas tank blocked
- Gas line blocked
- Gasket vacuum leak
- Manifold vacuum leak
- Governor control fault
Surging B&S Classic 450, 500 or 550
Tools You'll Need
This carburetor style is fitted to a few engines and is prone to gasket failure.
The job of replacing is simple and will solve the surging.
Remove the spark plug wire. This prevents the mower from starting.
Remove and clean the air filter and filter housing. Clean it using soapy water and when dry smear some engine oil over the surface of the foam this helps trap dirt.
Remove these two bolts. They hold the fuel tank to the engine.
With fuel tank bolts removed pull tank unit straight out gently and remove the governor control link.
Remove black rubber elbow crankcase breather pipe.
Remove the manifold seal and keeper ring.
Sometimes they will come loose and get stuck on the manifold pipe.
Remove all screws from carburetor and set aside.
Using a can of carburetor cleaner, clean all the ports on the surface of the fuel tank.
Empty the tank and rinse it out with fresh gas.
Pull the Siphon from carburetor, they can be stubborn.
Remove both gaskets and use carburetor cleaner to clean siphon metal filter and all ports of carburetor.
Check the primer bulb for damage, mice like to eat them.
Spray the carburetor with carb cleaner.
Careful of this spring it lives under the gaskets, it can drop off and be tricky to find, as I know only too well.
The gasket is a two part kit, the rubber type gasket faces the tank.
The Siphon pushes back in with a click. If you don't hear the click its not right - try again.
Refit the carburetor to the tank. Don't over tighten the screws, as this will distort the gasket.
Fit manifold seal and keeper. Smear a small amount of oil on the seal, it helps it seat.
Clean the intake manifold. The grey tube in this shot is the manifold. Inspect it for any signs of damage, they are prone to cracking. This will also cause surging.
To fully inspect the pipe you need to remove the pull assembly. I would only do this if:
There was obvious damage to the manifold or I had replaced the carburetor gasket and the engine was still surging.
Offer the carb/tank unit up to the manifold and attach the governor link.
Now push the unit firmly on to the manifold. Fit both bolts. Refit air filter and spark plug wire.
Use only fresh gas, make sure your gas can is clean. Gas older than three months is stale.
If after fitting the gaskets, you still have surging - Replace the Manifold.
Your good to go? Nice work!
The Surging Test
What's a Vacuum Leak?
Gaskets wear out and break down, that causes surging.
Finding a Vacuum Leak
Fixing A Vacuum Leak
For this test you will need a can of carburetor cleaner. Have a helper hold the bail lever, so the mower stays running.
To test, simply spray the carb cleaner around the carburetor gasket joints (where the carb connects with the engine).
Listen for an increase in engine idle speed - this indicates a vacuum leak.
All manufacturers will have a model number printed on a sticker placed on the body or on the engine. Have a poke around, you'll find it.
A new carburetor comes with new inlet gaskets.
I like to fit original parts, they fit and are guaranteed.