Here we go again! With the fuel overflow - Rolls-Royce and Bentley Forums
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post #1 of 16 (permalink) Old 05-09-2017, 02:17 PM Thread Starter
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Angry Here we go again! With the fuel overflow

Hi All,


Once again Gracie decided to pour gas out of the over flow on to the garage floor, she always makes a liar out of me though, last time this happened she went back to the shop, she never did it while there and as stated in a different post, the shop left her outside for several days during a rainy spell, so not only did they find nothing happening with the carbs and overflowing but I got a car back with soaked front rugs...UGH! Here's the story:


So I went out yesterday to start her up and it poured gas, I shut it off, then started it back up a few minutes later fully expecting it to pour gas everywhere and NOTHING! Dry as a bone...this may drive me insane, I called the mechanic who rebuilt the carbs, he says he's not sure what is going on but assures me that the floats were replaced when the carbs were rebuilt but obviously something is happening, not all the time so I'm sure if I bring it in it wont do it and I'll look like a nut case and if it rains I'll have a disaster, incidentally I did apply flow able silicone to the windshield but who knows what that may have done. So very frustrated, so, so very frustrated.


Any wisdom is appreciated


Jon

"Gracie"
1979 SSII
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post #2 of 16 (permalink) Old 05-09-2017, 03:46 PM
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Jon,

The only thing that causes this is overflow from the carbs. The most likely reason by far is a sticking float. It's highly unlikely that both carbs are doing this. When it next happens, and it will, check the overflow hose from each carb. Only one will be wet with fuel (unless the nearly impossible is occurring). Then at least you'll have the offending carb identified.

Brian

The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
~ Niels Bohr


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post #3 of 16 (permalink) Old 05-09-2017, 07:16 PM
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Had this overflow problem a couple of days ago, not with the Silver Shadow 11 but with the Triumph Stag which has twin Zenith Stromberg carbs that have a similar operation. I removed the fuel inlet pipes and squirted some WD 40 into the inlet ports, I also squirted some carb cleaner into the ports, reattached the inlet pipes and tried to start the engine again. It took a few attempts but it eventually did the trick and fuel is no longer overflowing. I think that the fuel was somewhat stale and that modern fuels are more inclined to gum the float needle valves. The problem tends to occur if cars are not driven regularly to keep the float needle valves moving. The float bowl level drops as fuel evaporates after a while, the floats move downwards and the needle valves open, when left in that position for a while the needle valves stick. When the ignition is switched on the fuel is delivered by the electronic pump and the floats try to rise but because the needle valve is gummed and stuck open the floats are obstructed from rising far enough and fuel keeps on being delivered. You hear the fuel pump continuing to tick even though the engine hasn't been started, that's because the stuck needle valve is allowing fuel to continuously flow into the float chambers and the fuel then overflows through the overflow pipes. If you can't get it to clear using some encouragement with easing or cleaning agents squirted into the fuel inlet ports the only remedy is to remove the carb assembly, open the float bowls and free or replace the needle valves. When holding the needle valves in your hand and turning them upside down the needle should fall or rise freely by gravitational pull, they should not have any hesitation in falling or rising if they're clean and free to move.
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post #4 of 16 (permalink) Old 05-09-2017, 07:54 PM Thread Starter
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Hi All,

I have with the help of our local Rolls Royce guru come up with another theory regarding this problem, follow along with me as I may ramble on.

The fuel pump in my particular rolls is the later type, I am theorizing that this pump runs constantly therefore it has a return line to complete the circuit or else the pump would just send too much pressure and fuel to the carbs, if this is in fact the case then on the return line there would be a check valve of sorts to keep the fuel flow going in the proper direction and not gravity feeding on the return side from the tank. If this check valve is not opening or gets stuck or is gummed up, whatever, then the return flow of fuel would be stopped causing the pump to over load the carb or carbs therefore forcing an overflow situation as the fuel that would normally go through in the return line to essentially stop.
So when I shut the engine down and the pump stops the pressure is stopped, the check valve if it exists gets un-stuck and when I restart the engine all is well with the flow and no more leak.

That's my latest theory, now if there is no return on the pump then it's just plain old stuck needle valves, I am wanting to doubt that because the carbs including floats were rebuilt. Anything is possible. And with this Ethanol excuse for gasoline anything is possible, **** corn squeezin's!!!

Jon

"Gracie"
1979 SSII
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post #5 of 16 (permalink) Old 05-09-2017, 09:00 PM
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Hi Jon, My 1979 Silver Shadow has twin SU fuel pumps. The carb overflow pipes lead down to the lower front of the engine and fuel dumps out on the ground if the float needle valves stick open. When the carbs are working properly the SU pumps stop pumping when the needle valves close because back pressure builds up and the pumps are designed to cut off when back pressure reaches a certain level. However when the float needles are stuck open the back pressure can't build up as fuel continues to escape out through the carb overflow pipes. If I leave the ignition switched on while the float needle valves are stuck open the fuel continues to flow out through the overflow pipes and I can hear the pumps ticking away until the ignition is turned off. Being a USA designated car your fuel pumps and overflow may be of a different design, I think the overflow is routed back to the fuel tank but in any event I don't think there should be any overflow once the float needles are free to close fully when the float rises to the top of the float chamber, the metal lever on the float presses the needle closed. Turning the ignition on and off could free the float needles if they're stuck open or closed but not fully seized, If they're stuck open the rising float could close them and if they're stuck closed while the float chamber isn't full the pressure from the fuel pump could open them but the chances are that they will stick again at some later stage because of the gum building up while they are stationery. I've noticed this problem a lot more lately and I think it's caused by the composition of modern fuels as you suggest.
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post #6 of 16 (permalink) Old 05-09-2017, 09:55 PM Thread Starter
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Hi Shadow II,

what we were thinking is that the fuel pump is a constant run type, maybe not, but its the single "non Serviceable" type...but in that case there would be a return to the tank. My car does in fact have the overflow tube under the front and that is where it pours out of. We plan to further investigate this problem tomorrow with the help of yet another very experienced person, it may just be sh*tty gas, quite frankly. This new fuel with ethanol that is nearly 99% of what is sold here is just garbage and goes bad quickly, I think the fuel gums up very quickly and I am going to start putting an additive like "star tron" in the gas to see if that helps, it claims to be an ethanol treatment of some type. These cars don't seem to like this fuel but its all that is readily available around here. Maybe the treatment will help to slow the gumming process, especially since the car is not driven much, it can sit for 3 weeks without being run which is enough time for this fuel to turn to trash.

The saga continues

Jon

"Gracie"
1979 SSII
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post #7 of 16 (permalink) Old 05-09-2017, 10:34 PM
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Jon,

With regard to ethanol, i seriously doubt it's contributing in any way to this problem.

If your car has Pierburg rotary vane pump it does then introduce a return valve that opens when the carb floats close fuel flow to themselves.

If that is erratically staying open you'd still have to have some involvement at the floats.

Inconsistent and intermittent malfunctions are the grandest of PITAs.

Brian

The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
~ Niels Bohr


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post #8 of 16 (permalink) Old 05-09-2017, 10:39 PM
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Hi Jon

My car is a series 1 but since I'm finding this discussion interesting I checked out the series II workshop manual. Section K4 (Fuel lines and ancillary equipment) will be of great interest to you. Some models do indeed have a fuel recirc valve fitted to the outlet of the bank A carb. It is simply a fitting with a 1.88mm hole through which the fuel on the return side passes to maintain the inlet pressure. It may be this hole is getting blocked, as you surmise.

Another point of interest is the Pierburg pump has a pressure relief valve on the outlet. If pressure goes above 7.2 lb/sq in the valve opens and redirects the fuel back into the inlet port. If this is faulty the increased pressure down the line will overcome the force of the float in the carb and cause flooding. I would get your guru to pressure test the fuel line after you have checked the recirc valve.

I really recommend you check the manual. It's particularly good on this topic.

Geoff

Last edited by Dounraey; 05-09-2017 at 10:41 PM.
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post #9 of 16 (permalink) Old 05-09-2017, 10:47 PM
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Hi Brian

I concur. When the floats close and stop the fuel flow, the pressure goes above 7.2 lb/sq inch and the recirc valve on the pump opens. This redirects the fuel at increased pressure back into the inlet port of the pump. If this is sticking the line pressure could easily increase sufficiently to overcome the resistance of the float.

Geoff
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post #10 of 16 (permalink) Old 05-11-2017, 03:36 PM
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Here is a site that can help you find ethanol free gas in the US and Canada.

http://www.pure-gas.org/

Mark
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