About Barrie Jones Expertise Any technical questions regarding MG T Series or MGB. I specialise in SU carburettors, Lucas wiring, suspension, steering, brakes.
Experience Owned TF1500 since 1966. Technical specialist for the TD and TF with the MG Car Club T Register. Owned 19 MGBs and currently own 2 MGBGT V8s and a TF1500. Written books on MGs.
Question QUESTION: My 74 B starts fine, I wait till it's warmed to about midpoint on temp gage to move, after about 5 minutes the car starts to sputter, loose power and finally stops. If I wait 5-10 minutes it starts again, runs about 1-2 minutes and the cycle repeats. I suspect a fuel issue, HSI4 SU carbs, I have a new fuel pump and hear it operate,installed new fuel filter, I've replaced the heat shield,the old one had lost it's asbestos liner. I thought it might be vapor lock. Now I'm wondering if it's the coil. Any ideas? Thank you, Peter Newbury, NH USA
ANSWER: Hi Peter. The fact that you splutter to a stop leads me to think fuel starvation. The pump is delivering enough fuel to fill the carburettors when stopped or running slowly, but it cannot deliver fuel fast enough to keep them full under load. After you stop, it refills the float chambers and the cycle then repeats itself.
A coil failure is normally sudden, and you seem to be having problems too soon for vapour lock. This normally only occurs with a really HOT engine after a run of at least 30 minutes, followed by a stop (such as when you fill the tank after a run).
How does the fuel pump sound when you switch the ignition on? Does it give a healthy Brrrt, Tick, Tick, Tick and then stop, or is it a slow chug-a, chug-a, chug-a sound? I have known brand new pumps to be faulty.
I would do a fuel delivery test with a jam jar. Pull the delivery pipe from the carburettor and put the end in a clean empty jam jar. Switch the ignition on for 30 seconds and switch off. How much fuel has been delivered? It should be at least 1/4 of a pint. If it is less than this, you need to check:
1) Voltage at the pump.
2) A good earth to the pump (run another one directly to the battery and see if it makes a difference).
3) A collapsed or kinked hose on the suction side of the pump.
4) A blocked filter inside the fuel tank, on the pickup end of the suction pipe (blow through the pipe to clear it).
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QUESTION: Hi Barrie, fuel pump sounds strong and crisp. Checked fuel flow, got about 1/3 of a pint in 30 seconds. Any other thoughts?
thanks Peter
ANSWER: Hi Peter
Thank you for running the test. That eliminates the fuel delivery side.
Yours is not the first MG that I have heard about with your symptoms. We are currently working on a fix, but we do not have an answer as yet.
I assume that you have a pair of SU HS4 carbs and no USA emissions equipment.
If you have a Vapour Separator Tank and Evaporative Loss Control System, then remove the VST tank and check that it is empty. If it has fuel in it, that could cause problems. Replace the carbon canister.
If you are using low grade fuel (especially if it has bio-ethanol in it) then please pay a bit more and buy 98 octane unleaded from a well-known gasoline manufacturer.
You must ensure that your ignition timing is spot on. If it is retarded, that will make things worse.
If the flexible fuel hoses under the bonnet (hood) are not encased in steel mesh, then fit new hoses or cover yours with aftermarket heat wrap.
If the hose below the heater across the bulkhead is not protected by its heat shield, then move it.
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Another thought - the last time I experienced anything like this, it was a problem with a brand new exchange fuel pump. The diaphragm had been set too tight. I re-set the diaphragm as specified in the SU manual, and the problem immediately disappeared.
If the problem persists, I would welcome any extra information that you can give me. For example, how hot are the float chambers when this occurs? Can you touch them comfortably? Is the coil getting very hot?
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QUESTION: Barrie, some additional info. Yes I have dual HS4 carbs, the the float chambers are warm but not hot, actually comfortable to the touch. I let the car run for about 1 hr at 2000 rpm, ran well no evidence of stalling, the coil gets pretty hot, doesn't burn your hand but I would not want to keep my hand on it for a long time. Also the car has had it's emission junk removed, air pump, gulp valve, all the feed lines for it from the manifold are plugged. I also checked the evaporative emission system and disassembled the carbon cannister, the filters were not obstructed nor was there any evidence of any liquid (gas) in the can. The lines to and from are unobstructed. I insulated the gas line as it passes under the heater, that may have helped. I also reset the timimg to 11 deg BTDC with the vac line to the distributor disconnected as per the manual. I'll try for a road test this weekend and let you know.
thanks for the tips, Peter
Answer Thanks, Peter.
You seem to be doing all the right things.
When you removed the emissions equipment, did you replace the fuel filler cap? The USA cars had a non-vented cap #BHH178. I can imagine a scenario where you run the engine for a while, and the fuel pump sucks fuel from the tank, creating a partial vacuum. Possibly this is enough to collapse the flexible hose between tank and pump. The engine splutters and dies. After a few minutes, the vacuum has partially subsided, and the engine will start again, only to stop a few minutes later. The problem would be worse with a full tank and a small air space above it.
Next time the car dies, immediately undo the cap and listen carefully. Does it suck air? The UK cars were fitted with a vented cap #18G8601.
