Hi Paul, there seem to be two main possibilities remaining: the decompressor is worn out or stuck, or there is a different problem not associated with the decompressor. Let's start with the easiest thing to check, despite it seeming unlikely as a cause: extremely premature ignition timing. This might explain both the refusal to crank and the tendency to eject burning gas through the carburetor, if it is kicking back on the starter. To check this possibility, just disconnect the spark plug lead from the spark plug but leave the spark plug installed, and try to crank the engine. If it will crank properly when there is no spark, but not when there is spark, the problem will be due to substantially advanced spark timing or perhaps incorrect camshaft timing. Just report the results of this test, please, rather than investigate this possibility further at this stage.

The other possibility is that the decompressor has stuck in the full compression position, or worn out completely.

Here is an illustration (from the manual) of the compression release on the Vanguard and Intek engines:
[Linked Image]

As you can see, the compression release consists of a stamped piece of steel, sitting on-edge, which protrudes slightly higher than the cam lobe at a point halfway through the compression stroke. Briggs indicates that there is a possibility of this component wearing out or sticking. Here is the instruction for inspecting the camshaft:
[Linked Image]

Before considering dismantling the engine to replace the camshaft, you should perform a simple check on the decompressor. Look closely at the rocker arms while slowly turning the engine by hand, with the spark plug removed so it is easy to turn. Watch for one of the arms moving momentarily during the compression stroke. If the clearance is correct and neither arm moves, the decompressor is kaput.