Weber fuel percolation discovery

[1QuickS]

I just completed an experiment where I heated a Weber body with fuel in the float bowl that feeds the accelerator pump. I was seeking confirmation of the source of fuel in the throttle bores after a hot shut down. When I heated the fuel gallery feeding the accelerator jet for each of the outer throttle bores a steady stream of fuel was emitted from the jet until all fuel in the gallery was purged.

This is the first mode of hot restart flooding and the second is when the fuel float bowl reaches a high enough temperature for it to boil and escape via the emulsion tube well followed by escaping out the vent in the top cover.

I have no solution for this except to use insulator(s) between heads and the manifolds and to use race gas which has a higher boiling temperature (Sunoco 260 boils at 247 degrees) than ethanol (boils as low as 130 degrees).

A discrete fuel pump kill switch can be effective for the float bowl percolation if you kill the fuel pump a block or so from your destination then this lowers the amount of fuel in the float bowl. It also serves as a good theft deterrent.

[Steam Driver]

Yes, this was a well-known and readily available fix back in the ‘70’s. I had the insulators on my 914-6. It cured the percolation most of the time.

To me percolation was only an issue with hot starts after sitting for a short time. Primarily during the summer months, in the K.C. area.

[1QuickS]

Yes, I discovered issue on my car which has insulators. What I "discovered" is not gas escaping via vent pipes but from accelerator circuit due to percolation within the galleries. When I noticed this issue there was no gas being purged from top of carb which indicates the first mode of percolation is via accelerator pump jets.

Killing fuel pump and running carbs dry of fuel while stabbing throttle pedal will purge fuel so percolation is completely eliminated. Obviously a re-start requires re-filling carbs and accelerator circuit. Using non-ethanol gas and installing insulators help as well. Grady Clay used a stack of three insulators to control percolation on his race car, he lived in Denver so high altitude compounded his issues for percolation ills.

Controlling this spilling of fuel into the throttle bores is recommended as fuel draining past closed throttle valves and piston rings will dilute the engine oil over time.