Rubber parts fail. If you have the carb apart anyway, replace every damn
rubber part in it... seals and washers, diaphragm and everything. B&S
will sell you a "rebuild kit" that has everything in it. If there are any
rubber parts in the thing that haven't failed yet, they are on their way
to failing anyway so just change it beforehand.
And keep the old diode as a spare!
"C\'est un Nagra. C\'est suisse, et tres, tres precis."
The diaphragm was always good. You never had any symptoms that indicated a bad
When you fill the tank all the way to the top the diaphragm isn't even needed
because on this style carb it is only really needed when the fuel starts to drop
Your problem is the choke is not closing. There is a spring that closes the
choke. The spring may be broken or missing. The diaphragm pulls the choke open
once the engine is running. If you had holes in the diaphragm your symptoms
would be the opposite - you would be getting too much gas as it would be sucking
gas in thru the holes.
One tube (the longer) goes down to the bottom of the tank the other goes in a
small reservoir. Part of the diaphragm is the fuel pump that pumps gas up into
this reservoir (there are two flaps that act as pump valves). the gas in the
reservoir stays at a constant level because what ever extra is pumped in just
overflows and runs back into the tank. When the tank is full then the reservoir
is also full so no pumping is needed until the fuel level drops down.
The small tube feeds the main jet which controls the air fuel mixture. That
mixture is adjustable with the threaded needle valve.
The longer one pumps too much gas. The flow of gas keeps a
small basin (in the fuel tank) full. The short tube supplies
the gas to the carb. By keeping the small basin (at top of
the tank) full, the engine always has the same distance of
lift for the fuel, even when the tank is nearly empty.
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