MGB & GT Forum

I have been chasing an issue with my 67GT and I have decided that I must be doing something wrong. From what I have read, the fuel flow at the carbs should be something like 16-20 ounces in 30 seconds. I get 12 ounces in 30 seconds. I have replaced the gas tank and the line to the pump. The pump is a new Hardi 9912M, and I have access to another new pump of the same type which I have switched out with mine to confirm that part. I suspected that the line from the pump to the carbs might have an obstruction, so I ran a temporary line from the pump to the carbs. That did not change the flow rate. I have removed the filter for now just to eliminate that as a problem. I checked the flow into the pump and it was good. I checked the output of the pump and it was very strong. I got 20 ounces in about 15 seconds. The problem is that the car seems to run out of gas under long periods of high RPM, like going up a long hill. Can anyone suggest what I am doing wrong here? Thanks

Are you saying that eliminating the filter brought the flow up to specs but the engine is still acting like it's running out of fuel?

No, I was just saying that I eliminated the filter early on to eliminate that as a problem. The flow did come up to spec a while back and the car ran great. The problem came back after about 25 miles. That was what led me to replacing the tank. I thought that the internal filter was the issue. The new tank is much nicer than the ld Moss special, but the flow problem is still there.

Do you have a sealed cap on your tank (which should be vented)? Do you have have a vented cap that has become clogged? Sounds to me like your pump is fighting a vacuum in the tank. It may be partially vented and that is sufficient under low load conditions, but under high demand not enough air enters the tank to replace the fuel being used.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

For one thing, I believe the Hardi should be able to pass more like 36 ounces in half a minute at zero pressure. Rate goes down as pressure rises. Measured at the pump, not at the carbs, you should have upwards of three times more flow than you are seeing. But that is an absolute sort of number, not necessarily a real world one.

You would want to know the pressure available at the engine bay. Even 12 ounces is enough to run your engine at full throttle, but if it measures 12 ounces free flow, what does it make with 2 or 3 psi of resistance?

I would be looking at the pump. What is the measured voltage available at the pump key on, and then key on engine on? Compare that voltage to what you have at the battery or the fuse block, looking for voltage drop. If you are not seeing essentially the same voltage at the pump as you have elsewhere, your pump is not running at rated volume or pressure. Look for bad wiring, corroded connectors, bad ground, etc.

As an alternative, test the pump on the bench with a fully charged battery. Compare performance between bench and installed. If it doesn't make factory spec flow on the bench, something is wrong with the pump. If it does flow on the bench, something is wrong with the car.


GMc

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Wait for me at the gate Tuffy, because without you they're never gonna let me in.

SDCH WTCH-X Twin Oaks Tuff Nutt
2008 - 2022

You have been fighting that for a long time.
I would call John Twist at University Motors 1-2 eastern time Monday thru Friday.616-307-6737

Maybe install a facet pump as a second pump near the underhood filter and when running out of fuel turn it on and see if you get enough fuel. Other than that, that hill might be a mystery spot with extra gravity issues.It is near the San Andreas fault.

Rich, you've been fighting this issue for too long now. I suggest you rig up a temporary fuel supply; in the past I have used a gas can on the passenger's side floor with a separate fuel pump there and a fuel line running to the carburetors. This will bypass the entire fuel supply system in the car and help you to isolate the problem. If the problem persists, then you know it has to be in the carburetor/induction area...

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

Thanks everyone, I really appreciate the great advice. I did think of the vent in the cap. Just to be sure, I did the test with the cap removed and that did not help. I also thought about the voltage at the pump and I will check that today. I did check the output at the pump and it was like a fire hose. Given that I have replaced the entire fuel system up to the carbs, I am about ready to think that the problem is in the carbs. The thing that keeps bothering me is that the fuel flow is low and when it is up to spec, the problem goes away. I believe that I am at the point where I need a fresh set of eyes on the problem. I suspect that it is something that I am missing.

Richard, do you have gross jets or needle valves in your carbs? I found the gross jets sometimes would not flow fully and required higher pressure when I was testing.

Thanks Lewis, I am not a sure. I am comfortable rebuilding a Holley, but I am an idiot when it comes to the SU's.

Easy to check if you're using HS4’S. remove float bowl lid. Then remove the float. Unscrew the needle seat assembly.
The attached photo, (not mine) shows the difference. Many problems have been reported with the grose jets, however if fuel delivery to the carbs is the problem, then I wouldn't expect the type of needle and seat to be the problem. I would try Dick's suggestion of a separate tank and pump.

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Ben
1966 MGB Tourer
1973 MGB Tourer

SU carbs are build idiot proof , so don't be afraid of not being capable ...

Personally, I think a Holley is more technical than the SU.


GMc

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Wait for me at the gate Tuffy, because without you they're never gonna let me in.

SDCH WTCH-X Twin Oaks Tuff Nutt
2008 - 2022

Wow this problem is persistent! you are one verrrrry patient chap.

Do as Dick suggests. All these carburetters need is fuel in the bowls. I have crosssed halfway Canada with a gravity feed tank under the bonnet!

SU™ carburetters are probably the simplest of simplest carburetter design.

B

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Life's most persistent and urgent question is, "What are you doing for others?"