Weber anti-percolation revisited

[1QuickS]

This is my third iteration of "anti-percolation" resolutions for Weber carbs (Zeniths also suffer this issue). Classic solution to fuel boiling is to install insulators beneath intake manifolds and drill the top covers ala "PMO" solution which was originated by Ferrari for their 512BBs. I then revised the top cover drilling with my own that spilled fuel into the throats without passing over the idle air bleeds for the idle circuit. The PMO solution spilled fuel into the throats but enriched the idle circuit causing difficult hot restarts. Both top cover solutions spill fuel into the cylinders which can flood the cylinders and cause fire or at least difficult starting. Another issue with the PMO solution that was resolved by second iteration was the idle circuit would be flooded during road race turns (only outer bank of cylinders unless it was a series of esses).

Third iteration takes advantage of un-tapped ports in the top covers. I tapped these ports and plugged the vent tube holes. Into the tapped ports I plumbed some banjo fittings (old Solex 40PI banjos) and now all fuel is discharged to a location away from exhaust headers, cylinders and idle circuit jetting. Venting the float bowls would be through the discharge line.

Picture shows the banjo fittings and breather line ready for installation. Both carbs would have their breather lines connected with a tee fitting and one line would then pass through forward sheet metal to allow routing of discharge line forward of the heat exchangers.



Photo shows interior of top cover of Weber with un-utilized ports in ceiling of top cover located opposite from the fuel inlets..

[Fleming]

Paul,
this is so cool.

You are still finding engineered improvements to Weber problems that have been bugging us for years.

Maybe I can get rid of the garage gas smell after parking the car after a drive.

Good for you...and us.


Thank you.

[SCrescue]

Nice mod Paul...what about routing the line to the charcoal canister or fuel return line rather than dumping overboard?

[BURN-BROS]

Bravissimo Paul!

[1QuickS]

I haven't installed on my car and don't really have a percolation issue to deal with. I do plan to perform some experiments on one of my test "mules" to see if any fuel does escape into the barrels from the emulsion tube wells.

Haven't considered the charcoal canister as a solution, I'd be worrying about flooding it for a hot climate car.

[KTL]

No disrespect intended towards Paul's cool modification to the float bowl venting. That is a fantastic idea to collect the fuel on the clear side of the top covers and send it to a collection container. He certainly knows his craft very well and provides a great service to Weber owners.

Originally Posted by Fleming

Maybe I can get rid of the garage gas smell after parking the car after a drive.

.

If it's a constant smell regardless of temperature, it is likely not fuel percolation. Percolation occurs when the heads get hot enough to transfer heat into the carb bodies, which then boils the gasoline.

So if it's stinky when cold, take a look at your accelerator pump area for leakage. This area can go unnoticed for leakage due to their location being so close to the heads. I had this on my USA made Weber 46 carbs. The source of the leak is the pump body and pump cover are usually not very flat and therefore the diaphragm seal is poor.

[chris_seven]

Really nice idea to solve the percolation but I would worry about what to do with the fuel.

If you put a T piece into the fuel line between the fuel pump and Tank. If you use a controlled orifice of say 1mm in this line the fuel pump should scavenge the rail and not back feed - at least in theory.

Might be worth a go?

[911pcars]

Originally Posted by chris_seven

Really nice idea to solve the percolation but I would worry about what to do with the fuel.

If you put a T piece into the fuel line between the fuel pump and Tank. If you use a controlled orifice of say 1mm in this line the fuel pump should scavenge the rail and not back feed - at least in theory.

Might be worth a go?

... or tap into the fuel return line (if a return line is available). However, not sure of how much residual pressure exists downstream of the FP regulator. Probably depends on pressure and volume output from the fuel pump.

Perhaps a collection tank, timer and adapted emissions purge valve routes fuel/fuel vapors into the return line after shutdown.



How much of a continuing issue of percolation is there given adequate insulation between heads > manifold > carb ? I suspect more after-effects upon engine shut down, yes? If so, perhaps a well-place (under grill), thermostatically-controlled cooling fan energizes after engine shut down?

Sherwood

[haasad]

That is a very nice modification well done. I agree with other posters it would be nice to return any percolated fuel to the tank. But an overflow maybe far simpler, maybe need to be careful not to inadvertently pressurise the "percolation line" by connecting it to the fuel return if that line does indeed carry any pressure, suppose it depends on individual installations.

Love to hear how this develops.

Andy.

PS Paul your advice regarding rebushing my webers was invaluable many thanks.

[BURN-BROS]

What this mod really helps with is a stuck float. When stuck, the bowl fills and pours out of the vent tube and onto the top of the motor. The fuel then trickles down and makes contact with the heat exchangers and you can end up with a fire.
I would not run the line to a catch can or charcoal canister as the volume of fuel can overwhelm a catch can pretty quick.

Plumbing the fuel away from the engine will reduce the chance of an engine fire.

[1QuickS]

I'm pretty much an environmentalist and would like to catch spilled fuel but the amount of risk and real time running these cars see allows me to (a little guiltily so) let a little fuel drain onto the pavement.

A stuck float would still force fuel through the main jet and into the cylinders via the emulsion tube well but the path through the drain hose is larger than the main jet orifice so the majority of fuel would go through the drain.

I don't know about about LWB cars but the SWBs have a structural tube that supports the forward end of the rear bumpers, this tube passes vertically through the longitudinal frame on each side of the engine bay. Being hollow it would be easy to pass the drain hose into the tube and let the fuel spill somewhat far from the engine. Alternately the drains could be tee'd into one line and terminated near the nose of the gearbox.

Spilled fuel indicates an issue to be addressed and hopefully be corrected. The drain is like the canary in the miner's cave, an early warning system.

I'm currently acquiring Cohline fittings to create a drain system, let me know if there is interest.

[911pcars]

Paul,
The vent routing seems well thought out. However, dumping raw fuel on the ground seems incomplete and in the right conditions, equiv. to BBQ starter, esp. in a track scenario. Maybe a float sensor in a remote dump tank (outside fender well) simply cuts power to the fuel pump instead.

Sherwood