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MG MGB Technical - Weber downdraft stalling issues

Hi -

I'm running an 80 B with a Weber DGAS downdraft carb, Cannon intake manifold, early cast exhaust manifold, no heat shield. I believe I'm having fuel vaporization within the carb - after running for a while in warm weather the engine gradually winds down and stalls. Pop off the fuel line from the carb, and a lot of pressurized fuel vapor escapes, and the car starts up and runs again (for a while at least) when the fuel line is immediately replaced. I wrapped the exhaust manifold but that didn't seem to help at all. Is this a missing heat shield issue or something else entirely? I think the fuel pump is putting out ample pressure.

Thanks for helping out with this puzzler, guys.

Peter Voigtsberger

Peter,

I may not be thinking right, but here is the thing that puzzles me. If the car dies because there is no liquid gasoline in the carb, how is the carb holding the pressure in the fuel line that you hear escaping? It seems that the float would not close the float valve if there were no liquid fuel in the carb, so no pressure could be developed.

Is it possible that you are hearing a vacuum rush rather than a pressure rush? In that case maybe it could be fuel tank ventilation issue?

Charley
C R Huff

Exactly so, which is why I'm very suspicious of claims of vapourisation, *except* when it happens in the jet pipe of an HS carb and there is a faulty heat-shield. Otherwise as soon as the level drops in the float chamber the valve will open, and any vapour in the delivery pipe will be pushed out of the vent, until fresh fuel flows in to fill the float chamber again. The heat would have to be capable of vapourising fuel at least as fast as the pump could move fuel out of the tank, which is a pretty tall order and tends to discount the possibility of *pressurised* vapoour in the fuel delivery pipe. I think you *can* get vapourisation inside a Weber, but it should normally only be an issue with heat-soak on a very warm day with the bonnet closed.

What fuel pump are you using? An SU has two one-way valves which stop fuel and to a certain extent air moving backwards through the pump, which is we you *do* get tank vacuum problems if the tank vent is blocked. If fuel/air could flow freely backwards then the a vacuum in the tank would equalise itself via the pump, float chamber, float valve open and vent, and we know that doesn't happen, i.e. it also tends to discount the possibility of *suction* back through the pipe top the tank.

So I'm not sure where that leave you with the symptoms you describe. I'd prove or disprove tank vacuum as a cause by removing the fuel filler cap when the engine starts to die. If the engine picks up again then it is indeed lack of tank venting, whatever the cause of the situation at the carb pipe.
Paul Hunt

Peter. The design of the intake manifold for the Weber DGV series carb does not require the use of a heat shield. Do not know why you are finding "a lot of pressurized fuel vapor" when you detach the fuel line from the carb. If the vent is functioning properly, the vapor should be pushed through the fuel line, into the fuel bowl, and out the fuel bowl vent.

There is no way that a vacuum, developed within the fuel tank because of a blocked tank vent which did not allow air to enter, replacing the fuel being pumped out through the fuel pump, could transfer itself through the fuel pump and up the hard, then flex lines, to the carb.

I think I would remove the idle mixture needle screw and spray some carb cleaner through the idle circuit, then see if I could blow it out with some low pressure air. This sounds more like crud in the idle circuit than some form of fuel vaporization issue.

But, it might be a good idea to study the routing of your fuel line from the hard line in the engine compartment to the carb. If the line gets too close to the exhaust manifold, it might be good to re-route the line.

Les
Les Bengtson

Thanks for the advice.

I agree with what you've suggested as areas to look at. I think I have to take a closer look at the carb float bowl and see what's going on in there. I'll also try the stuff you suggested Les. The car has the standard issue SU fuel pump and there's no seal on the gas cap whatsoever. The fuel line runs over by the master cylinder, so pretty far from the exhaust manifold. I tried to duplicate the stall today but it was pretty cool here, so unfortunately (!) the car ran fine for over 30 min, hood closed, fans running, sitting in my garage. There was nowhere near the pressure in the fuel line / carb inlet as when the car shuts down. It's supposed to heat up again this weekend so I'll put it through its paces then. I recently replaced the coil, plugs, coil and spark plug leads, fuel filter, thermostat, fan switch, distributor cap, installed a Bosch alternator, checked timing, checked and tightened gasket seals, added new fuel, did a coolant flush and fill, wrapped the exhaust manifold (all since having this problem). Car seems to be running a bit on the warm side @ 2/3 up the gauge but it's not overheating. I haven't checked the gauge accuracy.

I did see some comments on an Opel GT forum recommending phenolic spacers to keep the Weber insulated from the intake manifold and/or engine block. At its closest point my Cannon intake sits probably 3/4 inch off the center pipe of the exhaust manifold - does that sound right?
Peter Voigtsberger

This thread was discussed between 17/08/2008 and 19/08/2008

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