QuestionQUESTION: 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: Hi Barrie, tried for a road test this weekend, warmed up fine but no power in trying to move. Definately felt like fuel starvation. So I rechecked the fuel flow by bypassing the fuel filter. Not any difference about 1/4 pint in 30 sec. Checked the spec for flow on the SU AUF 3000 and found it called for a min of 18gal/hr (US), this equates to about 19 fl oz in 30 seconds, so I figured I was way low. I removed the fuel pump to check the input filter, almost clean just a little junk, less than 5% of the area was blocked. I noticed that the diaphram had a tear near the edge between two of the mounting screws. I can't be certain that I didn't cause it during disassembly, although I was very carefull in seperating it. I've ordered a replacement as well as a back up "solid state" SU, figuring to avoid future contact issues. Unfortunetly it's on back order through Moss, scheduled to arrive in 2-3 weeks. Meanwhile I'll reassemble the old pump after I get the new diaphram. I also blew out the feed and outlet lines, some very fine sediment on the outlet line, can't really tell on the feed as it cleared into the tank. The pump chambers were free of any particles.
I'll advise as I progress, Peter
AnswerHi Peter. I think you are getting there. A partially split diaphragm would be enough to give the symptoms you are experiencing, although the AUF300 pump is also used by the V8 so it has plenty of spare capacity.
When you have fitted the new pump, check that it stops pumping once the float chambers are full. If it keeps on ticking (more than once every 10 seconds), then you need to take a look at the floats in the carb float chambers. Check that they are correctly set, check that the needle valves have no wear ridge, and look for any sign for fuel inside the float.
