Weber 3c ida problem

[ghornbostel]

I'm new to this forum and have a slightly off topic application of Weber 3c ida carbs. The carbs are being used on a Inglese intake manifold for a V6 Buick engine. The engine is in a 1957 TR3 Triumph. I know, trouble enough but never the less here is the problem. The carbs siphon the gas out of the float bowl that supplies the accelerator pumps after the engine is warm and shut off. At one time I thought that it was a fuel boiling problem caused by heat from the engine. I have all the isolation gaskets plus a phenolic heat shield plate between the intake manifold and carburetors. I believe I have all the parts inside the carbs in the correct locations from parts diagrams. My fuel pressure is at 3psi and with pump on but engine not running the float valves hold that pressure. Somehow I feel that I am missing some sort of check valve or balls.
Thanks for the replies

Greg

[1QuickS]

I believe there is still a lot of heat entering the carbs after shut down which can boil the fuel in the bowls. Once this happens the fuel will come up the vent pipes unless you have the "PMO" anti-percolation holes drilled into the top cover. In this case the fuel will flood the cavity for the idle air bleed jet and drain into the cylinder until the float bowl is evacuated.

The accelerator circuit has a disc valve in the pump body that positively seals fuel flow from any possible siphoning event.

I performed an experiment some years ago where I put a plexiglas cover onto the top of a throttle housing, mounted the housing onto a large plate of aluminum, filled the float bowls with gas and then heated the aluminum plate with an acetylene torch. What I noticed was one float bowl boiled MUCH more quickly than the other. After everything cooled down I saw the reason for the early boiling, the bowl was clean but had pitting from corrosion. The sharp details of the pitting created points for bubbles to form which advanced the boiling response to heat. I assume dirt in the bowl could also create this effect. Possibly the inlet check valve for the accelerator circuit creates a "point" for boiling to initiate.

An issue with the "PMO" anti-percolation repair (actually developed by Ferrari for their 365BB which used these Webers) is the flooding of the idle air correction jet which makes hot re-starts very difficult due to a flooded idle/progression circuit. Another issue is that during high-G cornering loads fuel will flood the idle/progression circuit which causes a stumble in power delivery during the exit phase of a corner. I developed a fix that alleviates both of these issues.

Since a TR3 has a closed bonnet vs. the open lid on a 911 (air grill) you will have more heat trapped in the engine bay than a 911 resulting in a more difficult heat soaking issue to resolve. You could try installing even more insulators between the Weber and the manifold if you have the headroom. This should help a bit. Beyond that I do not have much to offer. If you had vapor lock issues then there are things that can be done to help but boiling fuel in the carb is mostly keeping the carb cool, clean and making sure all items inside are installed correctly which sounds like you have achieved.

[ghornbostel]

Paul, thank you for your response. The bonnet is well perforated with louvers over the carburetor area which helped a lot with under bonnet heat. I haven't seen gas percolate out the vent tubes nor is there indication of this happening however it could be a bad gasket surface which I will check on a surface plate and high spot blue. Perhaps I need to go to the next step and install a auxillary pump that will circulate coolant after the engine is shut down.
I have used your tech articles to set these carbs up and refer back to them whenever they need to be retuned which isn't very often. Thank you for them, they are the best.
Greg Hornbostel

[1QuickS]

Thanks Greg! I was thinking about a method to drain the float bowls to tank after shut-down which would positively fix the issue. You would need to allow pumps to re-charge fuel bowls upon restart with a small chance of floats sticking in the open position until engine vibrations settle them out. Recirculating the fuel only addresses the fuel in the lines themselves but if they get heat soaked then they could over-pressure the needle valves and flood the carbs in that fashion. Recirculating back to tank or draining fuel lines to tank after shut-down could help this issue.

Perhaps a fan in the engine bay to blow after engine shut down? Kind of gross but you could remotely install fan behind a headlamp and duct cooling air to the Webers.

Send a photo or two of your Triumph!

[ghornbostel]

Paul, I have done the fuel pump switch thing. Is it my carbs or do the idle jets draw gas from only one fuel bowl, the same one that the accelerator pump draws from? Anyway, I still see fuel dripping from the accelerator jets in the bores and I'm going to rebuild the pumps because of this. The percolation problem has persuaded me to modify my carbs.
I have owned this automobile for 40 years now and it has evolved into the unkempt beast you see here. 1957 TR3 Triumph, 231 ci Buick, 10:1 pistons, 290* .500 lift camshaft, matched and ported heads, Jim Inglese intake with 40mm 3c ida carbs, headers, 14# flywheel, Saganaw 4sp and 2.9:1 10 bolt GM rear end. It is what it is and I enjoy driving it very much.

[1QuickS]

Thanks for sharing photos of your toy, I bet it is really fun to hop in & go! The engine looks just fine in there also. Although it is in a garage it is obviously a driver, my kind of car!

So, fuel comes out the squirter nozzles? This seems improbable since there is a disc valve that is loaded by a spring that should seal fuel from coming from the float bowl. Since siphoning requires the discharge to be below liquid level and the squirter nozzles are above the liquid level then I would rule that out. This leaves pressurized fuel as the method for gas traveling from the fuel bowl, up and out the squirter nozzles. Since this path requires fuel to defeat the disc valve seal I would think the fuel would have an easier path to escape the vent pipes. Possibly fuel is boiling in the fuel galleries between the disc valve and the squirter nozzles. This would suggest the middle nozzle does not pas as much fuel as the outer two would due to the decreased gallery volume supplying the middle cylinder.

Fuel delivery design of the Webers:

• fuel for accelerator nozzles and for one cylinder comes from the float bowl nearest the throttle lever arm
• fuel for the other two cylinders comes from the second float bowl

The fuel for any cylinder first passes through the main jet and then it has two paths into the engine:

• the fuel travels up the emulsion tube and into the engine via the auxiliary venturi
• the bottom of the emulsion tube well has a port that allows fuel to pass up to the idle jet and then from there it goes down to the idle mixture screw after passing past the progression ports drilled into the side of the throttle body

Please keep me informed, I am very interested in your findings.

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