More than half the Tecumseh engined mowers I see that don't start, or
won't stay running have gummed up carbs. Float bowl carbs (B&S
too) seem to be a lot more prone to gumming than the B&S Pulsajet (carb
on gas tank) carbs. Here's some things to do when the gas smells
or looks like varnish:
Ugh, smells bad, burns your hands, and doesn't run the lawnmower.
So you have to pull the carb off. Easiest way is to loosen the
two screws holding the manifold to the engine. Then you have to remove
the two links to the governor lever without bending them or stretching
the spring. Pay attention to how they're hooked up. The top
lever has a wire that connects to the spring, The lower one actually
moves the carb butterfly. They cross over so the butterfly link is
on the top on the governor lever. Don't worry, the thirtieth time
you do this it will be easy and make sense. You remember how they
were hooked up, right?
This is what a gummed up carb looks like. Those aren't lichens
growing on the float and the inside of the bowl. It needs to come apart
(careful!) and be cleaned. If the carb was leaking as well before
it became inoperable, it can mean that either
the float has sunk or the needle seat is bad - something that I didn't
mess with here. Do check the float by shaking it (after it's out
and clean) - if it rattles it has gasoline inside it and is leaky and needs
to be repaired or replaced. WalMart has great prices on carb cleaner
spray, the spray is VERY flammable, and makes a great engine starting fluid
This is what the carb body looks like cleaner. Notice the drilled
passage here on the center tube, sometimes they are plugged with gum.
If the carb is gummed up, the carb butterfly probably is stuck too.
That's what the screwdriver blade is pointing at. Usually a little
carb cleaner on the inside and outside will get it moving freely (it has
a light spring load) again.
Here's a gummed up main jet. That's right, it does more than hold
the bowl on and there is a small drilling on the unthreaded part of the
jet. There are some slightly different versions, one which has large
holes in the side and the main jet is drilled down from the top, but this
is the usual type. This is the main frustration point, because if
nobody TELLS you there's supposed to be a hole there, how would you know?
See the little dimple in the picture?
Cleaner. I have a small copper wire sticking out the main jet!
THAT'S the main reason for it not running because they gum over completely.
Everything I've read says to NEVER use a wire to poke out jets, but I haven't
found anything else that will clear a plugged orifice. Voodoo doesn't
Some of the newer carbs have EVEN MORE JETS to block up. You can
tell it's one of these carbs by the extra "lump" running up the side of
main jet holder in the middle of the carb. See it? These carbs
have an extra passage that runs from the main jet to the back of the carb
by the butterfly. See the extra jet drill with the wire in it in
this picture? This isn't the main jet, instead the main jet is drilled
vertically into the body of the jet and that little area by the larger
horizontal drillings trap and HOLD gum and gook to replug the carb. There
is also a small drilling in the threads of the jet holder that needs to
be cleaned (see the plug about half way up the outside? The drill
is on the INSIDE!) Then there is a diagonal passage (see that diagonal
part of the casting?) that runs to the back of the carb behind the butterfly.
You need to remove the carb from the manifold to get to that.
One other thing to watch out for on these carbs is a rubber o-ring sandwiched up at the top of the main jet holder. These can go bad and close up. I found one with a tiny plastic sleeve in the o-ring to keep it open. When I assembled it with a new o-ring and omitted the plastic sleeve, and found it would not start.
Everything's reasonably clean and ready to go back together. You
checked the float, you hope that the needle seat is good, and you're going
to reassemble it JUST like it came apart. The flat part on the bowl
goes toward the float pivot. The float should be just about flat when assembled
and you remember how those governor links hooked up, right?
Go wash up, your hands stink and burn from the old gas!