I have a Briggs and Stratton Classic 3.5hp push lawnmower that has the Pulsa-Prime carb mounted right on the gas tank. It starts very good, usually on the first pull after priming 3 times. It runs very good too, but then will eventually quit. I then immediately prime it twice and it will always start on the first or second pull. Again, it will run great, but it will quit again after 5 or 10 minutes. It seems the length of time that it will keep running is getting shorter. I think I used to go a half hour or even do my entire mowing job without it quitting. If I hear it starting to falter I can quickly hit the primer bulb and it will go for a few minutes more, but then quit. I checked the gas cap and it is not restricted. Diaphragm maybe? Any other possibilities? Thanks.

 

I had those same symptoms when I used up half a tank of gas and the longer pickup tube failed to take gas from the larger tank and put in the little sump from which the carburetor actually draws fuel . . . . or when I was mowing on a hill and fuel wasn't automatically added to the sump.

I'm no expert; but I'd say the screen on the longer of the two pickup tubes is clogged; and I don't know where your primer draws its gas from.

Is my theory consistent with what you're experiencing ?

 

So maybe I should try with a full tank of gas and see what happens?

 

Before you try the full tank test, try loosening the gas cap and see if that cures the problem. Sometimes the cap no longer vents as it should and the tank pulls a vacuum which stalls the mower.

 

Yes, as I stated, the gas cap was unrestricted. Also tried removing the gas cap. I just removed the gas tank and the o-ring on the intake tube may have been out of place. In any case it is in pretty bad shape and I'll replace it. Could a leaking o-ring on the intake tube cause my symptoms?

 

You need to install a new diaphram kit, it is Briggs part # 795083 or old # 495770 also Lowe's has a kit #5083 which has the same parts.
Here we go:

Remove air cleaner, there are 2 bolts that hold things together, one on the front of the tank 3/8" and one into the block 1/2", remove these bolts, now "slowly" slide the carb/tank off the intake tube and breather tube, now tilt the tank in to release the throttle linkage and waalaa the carb/tank is removed. Check the intake tube to see if the “O” ring seal and plastic retainer are still on the intake tube, if so remove them and re-install them into the carb. Remove the 5 screws from the carb/tank remove carb(don't loose the spring) now spray all holes, cracks and crevases in both the carb and tank surface with brake parts cleaner, remove the main screen(looks like a thimble), now with a small screwdriver pry out the main jet(carefully) and clean it, check the “O” ring on the main jet for damage, if it is damaged it must be replaced, reinstall the jet, it can be difficult some times to get it to snap back in place(I use the rounded end of a screwdriver handle). Remove and clean the fuel pickup stem(not necessary if the fuel tank was clean). Clean any junk/rust from the tank. Install the diaphram on the tank then the gasket(no goo or sealer) now carefully replace the carb(the spring will try to misalign the diaphram), tighten all screws a little at a time so as not to crimp the diaphram until they are all tight. Install the carb/tank in reverse order and you are done. If I missed something one of the real mechanics will correct me. Have a good one. Geo

 

Yep, I'll try a new diaphragm. I replaced that o-ring and it still would quit after 2-3 minutes.

 

Hey Vermont! You were right! I took off the carb to replace the diaphragm and I saw where the long pickup tube was clogged. So, I cleaned it up and put it back together and I haven't had a problem since! And it ran great even with barely any gas in the tank.

 

Thanks for the feedback. You described the behavior that suggested that was the problem.

I think those two-step Pulsa-Pumps were designed for a time when fuel was more pure and free of additives; and they typically have no filter. These days, I see a crystaline growth developing on these screens in less than a month of use. It's amazing that they last as long as they do. Most people just throw up their hands.

That's one place where I employ an old tooth brush and lacquer thinner for a free fix.