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Q & A: Runs Only on Full Choke

Hello John!

Thanks in advance for any help you can offer.

I have a 1979 MGB Roadster with approximately 124,000 miles on the original motor. I acquired the car after it had been sitting for 8 years in a garage in fairly poor condition. It is equipped with the original Stromberg carburettor (I know, I know, Webbers are cheap but they're on strike right now and I hate to pay the inflated price while availability is poor). For the driving I do the Stromberg performs adequately and there is no immediate reason to change.
The cast iron head was replaced about 3,000 miles ago with a complete, new, bolt-on head. The rings and cylinder walls appear to be very good for the miles on the motor. All emissions equipment has been removed. The timing is right on the nose, and the condition of the plugs and wires appears to be perfect and delivering a strong spark.

The car has always run well and smoothly. The head was replaced because I purchased the car in fairly poor condition and while running it for evaluation had major overheating problems due to fans that didn't work properly and a radiator that was nearly completely clogged. The head had many cracks that showed up during magnafluxing and was replaced. There are no overheating issues remaining. Oil pressure is good (60-70lbs) at speed for an original motor. The car does not appear to be burning excessive oil, and, for once, I have a british car that doesn't leak much, either!

At present, the car seems to be always starved for gas. The condition came on fairly slowly. I noticed that as I drove the car it felt like it was being held back by dragging brake shoes or a poorly adjusted emergency brake, but when I richened the mixture with the choke it broke away and ran smoothly and powerfully like it should. It starts a bit hard but idles fairly well, but when a demand is placed on the engine it shows all of the signs of a very lean mixture by hesitating, backfiring and stalling while at speed or bringing the rpms up while stopped. The only way to operate the car is at almost full choke all of the time.

The carburettor has been completely rebuilt and appears to be within reasonable spec for a carb 25 years old. The floats are good, and the needle, jet, diaphragm and all gaskets have been replaced. The water choke was replaced with a manual choke during the refurbishing three years ago. The carb has been set using the proper adjusting tool. Damper oil is properly installed.

The fuel filter is correct and new. The fuel pump is delivering 4+ lbs of fuel pressure at all loads. The fuel lines are in good condition and do not appear to be clogged. A smoke test on the motor does not reveal any obvious vacuum leaks. A clamp applied to the brake vacuum assist line causes no change in performance.

At idle I am getting between 3 and 5% CO through the exhaust as I adjust the carb, within reasonable spec. At about 35% (yikes!) it runs smoothly because it seems to be getting the fuel it needs. At idle at about 4% I am pulling about 18 inches of vacuum, but when I attempt to accelerate the motor the vacuum drops to 8 inches. Spraying starter fluid directly into the empty air cleaner casing brings the car to a smooth run at any rpm and brings the vacuum level right back to 18 inches until the fluid is burned and the motor returns to delivered fuel.

The only area I haven't gone to that offers a strong possibility is the head gasket. Even though I see nothing externally, perhaps there is a failure in one or more of the barriers between the cylinders? I was meticulous in prepping the block for the new gasket when I replaced the head but who knows? Maybe flooding the engine with extra fuel provides so much combustion that any breach is overcome?

I'm running out of things to check, and I'm almost out of hair to pull out. Before I start really tearing things apart, I thought I would ask your sage advice.Any insight is welcomed!

Thanks!

Steve

Steve!

Offhand, I'd say you were trying to run the car without the air cleaner completely fitted, as it runs very, very lean without the restrictions offered by that unit. But if that's not the problem:

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