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.

[targatuesday]

Always appreciate your posts!

[gled49]

Paul, just a Quick question, are you using 260 or 260GTX. GTX is non Alcohol, 260 is ethanol. Gordon

[John912]

I had this problem with my 914-6, and solved it by installing phenolic insulating spacers between the carbs and the intake manifolds. On hot days fuel was spilling out top cover vent, not good -- no longer the case. I chose top of manifold because it was easier to replace (longer) studs with manifold on the bench instead of trying to remove studs from the heads.

[1QuickS]

I noticed this on my car with standard pump gas and performed the test on a spare Weber body I have.

Typical percolation issue and resolution have been fuel spilling out the vent tubes due to fuel in the float bowls boiling, the "PMO" drilling in addition to installing insulators between heads and manifolds help. I do not like the "PMO" fix as it spills gas into the slow speed circuit causing hot restart issues. I have developed a different top cover modification than the "PMO" fix. The "PMO" fix was actually developed by Ferrari for the triple Webers used on the 512BB.

My "discovery" was fuel percolation in the galleries feeding the accelerator pump jets. There is quite a lot of fuel in those lines and when it boils it is then is delivered to the cylinders. When I was heating the galleries the fuel was squirting in a continuous stream until the galleries were purged of fuel.

My car has insulators and when I noticed fuel on the throttle valves without any fuel coming out of the top covers or vented via my "anti-percolation" features I became very interested in discovering how fuel was dumping into the throttle bores. I have had previous queries as to why fuel was coming out of the squirter nozzles but I did not understand how this could be since the galleries are sealed by a gasket/valve arrangement in the inner accelerator pump body. My "discovery" was fuel percolation within the galleries that purged the gas into the throttle bores.

Grady Clay mentioned he had three sets of insulators on his track car, he lived in Denver so high altitude was an additional mitigating issue for percolation problems.