Percolation can happen when your engine gets hot, then you turn the engine off and the fuel boils over, sometimes over the top of the carburetor, or more often, simply evaporates. Back in the day we called this vapor lock, which simply meant that the fuel turned to vapor before getting to the engine.

This has been a more common problem these days because our gas has a lower boiling point than it used to. Yup you guessed it, ethanol.

This makes it hard to start the next time. You have to crank and crank, or pour a bit of gas down the carburetor to get the engine started.

What can you do:

• One of the best ways, but a lot of work, is to run a 3rd line from the fuel pump back to the gas tank. This extra line will help cool the fuel, allowing the fuel pump to pump gas instead of vapor. There are some fuel filters out there that include a return port. There are also return regulators available.
• Put heat wrap around the fuel line in the engine compartment.
• Put a spacer under the carburetor mount. These are hard to find so you will probably have to make one. Yup, a mill would be handy about now. And you will probably need to put longer studs in the manifold to accommodate the extra height.
• Instead of a spacer add extra mounting gaskets. A couple of 1/32 thick gaskets isn't going to do much. Hopefully thicker gaskets will be available for your engine.
• Add a helper electric fuel pump. Just before startup, you run the electric pump for a few seconds, then turn it off. Use only a low pressure pump. 7 lbs max. You may end up also adding a regulator because you don't want the pump to overrun your mechanical pump. Most 1 & 2 bbl carburetor run 4-4.5 lbs. Bigger carburetors might handle 5-6 lbs.
• Cool the engine with a cooler thermostat, bigger fan, or bigger radiator.
• Try different brands of gas. Different octanes won't help you here.
• Use non ethanol gas if you can find it.
• Remember when we used to put wooden clothes pins on the fuel line? The clothes pins act like a heat sync, drawing heat away from the fuel line and my wife says the wooden clothes pins are still available at the dollar store.

Anyhow, the basic answer here is to cool your engine and/or fuel.