It's in the archives, and I didn't invent it anyway. It applies to B&S engines with the Pulsa-Jet carburetor, which essentially means those with a 2 or a 9 as the third group in the model number. The first one or two digits are the capacity in cubic inches, the next digit is the basic design series, and the one after that tells you about the crankshaft, carburetor and governor. If the number is a 2, it means horizontal crankshaft and Pulsa-Jet carburetor. If it is a 9, it means vertical crankshaft and Pulsa-Jet. The Pulsa-Jet was the premium carburetor for the small and medium-sized engines in the third basic design series (first series was 0, second was 1, third was 2). So, the classic Briggs engines up to say 250 cc of the 70s and 80s had either Pulsa-Jet or Vacu-Jet carburetors, the Pulsa being the more desirable one.

Here is a basic diagram of how the Pulsa worked:
[Linked Image]

The carburetor is on top, the fuel tank below. The Pulsa used a two stage fuel pumping process, to ensure constant air/fuel ratio. The left hand vertical tube dropping from the carburetor sucked up fuel and pumped it into the small chamber in the top of the fuel tank, on the right side of the diagram. The pump had much more capacity than necessary so the small tank was always full. The right hand vertical tube, connected to the carburetor venturi, drew fuel up through the jet and it was discharged into the intake air stream.

When the mower has been standing for days, weeks or months, chances are the small chamber will not be completely full. The mower will always start first pull, though, if it is full, so to make it start immediately, we need to fill that chamber. We can easily do that for a push mower by tilting the mower toward the cylinder head (which means tipping it toward the small chamber, but you can't see that when you look at the mower). Fuel then runs under gravity up the left hand tube, through the diaphragm pump, into the small chamber. Result: instant start.

In practice, the same tilting process is likely to help to some extent with the Vacu-Jet and some other carburetors, just because it makes fuel run up into the venturi and prime the engine.

This secret starting technique is sometimes practised by overhaulers and sellers of Briggs mowers: nothing impresses the customer as much as an instant start from a demonstrably cold engine, without priming or other visible interventions - the tilt trick can be done hours ahead of time. (Mind you, much the same thing could be achieved with a bulb-primed engine just by priming it before the customer arrived, but that only works if you know which mower he or she will want demonstrated. Frequent priming of mowers that are not subsequently started will flood them and keep them from starting.)