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MG MGB GT V8 Factory Originals Technical - Carburetor problem?

Hey guys, I wonder if anyone has had a similar problem, or has any ideas on what I should check for. My car is a 79B with a box stock Olds 215 and a Carter AFB with a dropped base air cleaner and K&N filter. The engine runs smoothly, with plenty of power and torque as long as I give it 1/2 throttle or less. If I floor the accelerator, the engine starts missing badly, the car will actually start to slow down, and I have to back off the gas, and things return to normal.

When this happens, it feels like the car is running out of gas. I don't have any problem maintaining 75 mph on the highway, as long as I don't floor it. I was thinking possibly the fuel pump (stock SU) isn't keeping up, or low level in the fuel bowl.

Any suggestions? Thanks.


For starters, I would start by changing the fuel filter and checking the fuel line that you have not flattened a section - then I would worry about the pump and its electrical supply - the coil perhaps and ignition amplifyer.
If you don't have a fuel filter - I suggest you start with the carb.

Finally, if you are losing water consider a head gasket.

Good luck


My first V8 was a 215 Olds w/ Buick heads & mild cam. A stock SU fuel pump worked fine for over 200,00 miles including some auto crossing & ventures onto the track.

It was not quite enough for a 4.2 under full pedal.

I would suspect your float level or fuel supply, assuming a good fuel pump.

You didn't say which carb, 400 or 500 cfm. If new, either should run right out of the box, although the 500 really should be re-jetted. If rebuilt or used, buy a rebuild kit for the gaskets & instructions, & do a little disassembly.

Jim Stuart

Had the same problem. I replaced fuel pump with a higher volume (Purlator brand from Advance Auto) and it took care of the problem.

I also removed the resistive wire on the positive side of the coil, which also helped with overall power. I am running a pertronix ignition system and it will take a full 12 volts without a problem.
Jim Miller

My vote would be fuel pump also -Im using a Holley Red and no problems --but I have a 302 and a 1406 Edelbrock carb jetted down.
Both Jims suggestions are good as well
All the best --Gil
Gil Price

I have a 302 MGC GT with a 600cfm AFB and it would stumble on full power with a single SU pump. It was fine on light throttle. I found one of those old double-ended SU's used in early V-12 E-types and the problem was completely solved. However, a more modern electric pump is probably the answer. A fuel filter intended for a V8 is also essential to not act as a restricter.
Once you can determine if fuel starvation is the problem, the other likelihood is weak spark under higher loads. Bob.
Bob Elwin

A standard MGB pump works fine with the V8 and was a factory fitment. They should deliver at least 1pt a minute at the carbs, but the later ones were about half that.
Paul Hunt

Going toi show my ignorance here, but isn't the AFB supposed to run at 5-7 psi like most 'merkin carbs, while the SU pump delivers 1-3 psi? If flow rate's high enough, it should be OK, but it's something to consider
greg fast

Ah, my ignorance-

Boldly displayed in the mis-spelled second word of the prior post!
greg fast

It could also be that the secondary section of the carb is popping full open too early. If it is vacuum operated then get some different springs for the actuator and see what happens with a stiffer spring. Float level is also suspect, especially on the secondary side.
Bill Young

Tell us about the carb - which AFB is it? What did it come off of or what is it supposed to fit. You are talking about a 4-barrel carb?

If I had to guess, I'd say that the problem is with the secondary side of the carb. Carter carb's have a progressive mechanical secondary that don't start to open until about 65% of the primary is opened. Airflow through the secondaries is controlled by a weighted flapper-type air door (called the auxiliary air valve), which prevents the carb from bogging down when the accelerator is floored at low speeds. As air velocity through the carb increases, the valve is pulled open, allowing the secondaries to operate fully.

Maybe this air valve is flopping open and killing the engine. Or maybe the carb's just too big.

Wayne Pearson

This thread was discussed between 05/07/2004 and 09/07/2004

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