Revision for anti-percolation holes in Webers

[1QuickS]

Long have we been drilling the top covers in our Webers to help lessen the chance of engine compartment fires due to boiling fuel in the float bowls of our Webers spilling out the vent tubes and down upon our hot exhaust headers. The remedy was created by Weber to drill some "venting holes" through internal webs in the top covers. This Weber remedy was popularized by PMO which has provided a jig for the convenient replication of this "venting" modification.

I have come to find that this solution was good to a point but is flawed in that the fuel vents through these drilled holes and then floods the air correction jet for the idle/progression circuit before being directed into the throttle bore.

This fuel path is expedient in that the fuel passes through the notch in the top cover which provides air for the air correction jet which is a required feature engineered into the carburetor.

The troubles with this expedient solution are twofold:

• when an engine is started shortly after shut-down, the fuel (if it has boiled or percolated) will add fuel into the idle/progression circuit via the air bleed hole which results in an effective enriching of the idle circuit causing a rich starting mixture (flooding)
• the second issue is for performance users to take note of; the sloshing fuel will flood the air correction jet so that during sustained high, lateral G-loading the idle/progression circuit is flooded which creates a "flat spot" during exiting of the cornering event...commonly assumed to be due to fuel starvation and not fuel enrichment

The photograph shows the PMO fixture and the location of typically located venting holes with the associated notch for supplying air to the idle air bleed jet. Also shown are the vent holes that are more centrally located and the additional notch in the top cover which spills percolating fuel down the throttle bore without flooding the idle circuit.

[trak ratt]

Great info, thanks!!!

[bodde911sc]

Paul, is the second drill bit that you are showing in picture in the spot that a "new" vent hole is being drilled and assuming that the notch above is also new. If this modification is done should the original "vent holes" be closed up to stop the fuel from spilling into the idle/progression circuits.

[1QuickS]

Yes, you can see the new hole if you look closely at the right side of the top cover. Holes located with PMO fixture should be closed off, either with some epoxy (JB Weld is fine since it is pretty much excluded from fuel contact) or you can fashion a tapered pin from brass, aluminum or stainless steel and drive it into the hole.

New hole is easy to locate if you let the body of the drill rest as it is shown in the picture; I recommend a 2mm to 3/32" dia hole; a Dremel is perfect for this; smaller drills will tend to "run-out" with the high RPMs of the Dremel.

[larrym]

fascinating!

my webers were "pmo'd" about 20 yrs ago & i still have that jig

i never had the hot-start problem described, but i have experienced a flat spot on corner exit acceleration, most notably on turns 3 & 4 at WSIR

THANKS

[DaddyGlenn]

I'll never need this trick, but this is the kind of info sharing that makes this place great!

[1QuickS]

The flat spot on corner exit may also be due to fuel sloshing away from the fuel pickups on the carb that is toward the inside of the turn, the outer carb will have sufficient fuel but the inner will starve. Fuel well baffle plates and "relieved mains" will fix that piece of the puzzle...see Bruce Anderson's book for a better description of the repair.

[kenikh]

Man, this was an easy mod to perform. Thanks, Paul!