3 1/2 hp B&S. Starts, then immediately stops.

Once I prime my lawn mower 3-5 times to start, it starts right away. But once the primed gas is used up, the mower dies. I have taken the gas tank off, removed all the old fuel and cleaned it out with paper towels. I also took the carb off the tank and blew compressed air through all the ports I could see, which seemed to be open. I also installed a new spark plug. To recap, it only runs on the primed gas. Any help would be greatly appreciated, as I have been out of work for the last 6 1/2 months, and am trying to save $$$ by doing these things myself. Thank you in advance. Jack

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frollard8 years ago
A plugged fuel line or fuel filter comes to mind - the primer bulb has enough oomph to overcome that problem, but the carb can't suck hard enough to get gas.
notmyfault7 years ago
I know this is a late post but others might be looking for similar solutions. I found out that on some lawn mowers if the oil is low they will stop running or be hard to keep started.
BigNateMI8 years ago
If the fuel line/filter is plugged, it's likely from rust in the gas tank. This is a common problem when machines are stored outdoors. Water gets int the tank, falls to the bottom (higher specific gravity than gas), and rusts the tank. Try removing the carb assembly, removing the tank, and de-rusting the bottom using a wire brush, fine steel-wool, or a very gentle sand-blasting. Clean out the "fuel filter" which, on your lawnmower probably consists of some fine mesh over the dip tube attached to the carb assembly. Then reassemble, and see what happens. Good luck.
psymansays8 years ago
It sounds like the diaphragm in the carbeurator isn't working, to pump up fresh gas into the carb from the tank. I had one of those Briggs-n-Straton motors on my minibike, same HP, and the carb on that had a rubber diaphragm that opened and closed two valves, to pump up the fuel. They're under a small cover on the side of the carb.
NachoMahma8 years ago
. My first guess is the same a frollard's, but you seemed to have covered most of the bases. Could be crud in the carburetor bowl/float/valve. . I'd double check the tank outlet. If you're sure it's clear, then check at the carb inlet. If you have free-flowing gas there, it sounds like it's time for a carb rebuild.