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MG Car Repair/MGB 74 weird stall

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QUESTION: hi Howard - my MG stalls out at high speed -for no apparent reason. I checked and tightened wiring fuse connections. Problem gone it seems and it starts and runs fine for awhile only to happen again days later. This time I let it sit a few minutes, it starts and all normal again for another day or so. I have electronic ignition. Gas filter seems fine. Gas has been changed out twice. Any ideas on what to check or might be the cause?
thanks! P

ANSWER: Hi Paul,

By "Stalles Out" I am gussing you mean the engine quits?

The symptom of the engine quits narrowes the possible cause down to one thousand items. I learned while working in dealerships that "Symptoms" are pretty much useless in diagnoses. The only way to find a fault is by testing. The testing must be done while the car is in it's failed mode. It is no use testing when it is running ok.

This is not as difficult as it may sound. There are only 3 systems that make an engine run. Compression, Fire (Ignition) and Fuel. It is unlikely Compression because if Compression goes away, it does not return. So that leaves Fire and Fuel. Fire must be tested first. This is easy to test. Put a timing light on one of the plug wires like you are going to check timing but instead tape the timing light trigger down so the light flashes all the time and  run the wires out from under the hood at a rear corner and put the timing light under a wiper arm so you can see the light flashing while driving. If it is a bright sunny day you will need to tape a piece of cardbord on the windshield over the end of the light so you can see the flash in bright sunlight. When it dies and the engine is still spinning note if the flash has stopped and look at the tech to see if the tach had also died.

If you maintain a flash and a tach reading with the engine dead but still coasting down then your problem is fuel not fire.

If you loose flash on the timing light but maintain a tach re3ading then the problem is in the "Secondary" of the Iginition. (Coil, Cap or rotor) Don't toss money and parts at the problem until you test the coil, cap and rotor. If you lost the tach too then the problem is in the "Primary" of the ignition system. (Electronic ignition and/or the wiring) Wiring does not usually cool off and start working again but electronic control units do.

If you maintained flash and tach reading when the engine died then it is a Fuel problem. If it died all of a sudden then it was not a carburetor problem because you have two SU carburetors on a 74 MGB and it is unlikely two carburetors can die at the same time. If it first dropped two cylinders then it could be carburetors but not a sure thing. If you did maintain timing light flash and tach when it died you need to remove the timing light and get a low pressure fuel gauge and a long hose and a "T" fitting with hose clamps and remove the fuel line from the fuel tank and install the "T" fitting and the long hose and be sure to put clamps on all hose connections and run the fuel hose out from under the hood at the rear corner and place the low pressure gauge under a wiper blade so you can monitor fuel pressure while driving. Do not run the gauge inside the car.

Now take the car out and drive it until it quitsa and quickly note the fuel pressure to see if the pressure dropped first before the engine died. The pump is electric so it should run all of the time that the key is on and the pressure should never drop below 1.5 PSI to 3 PSI.

These tests will put the cause into a very small area so further tests will pin point the exact problem. Don't toss money or parts at the problem until the actual failed part is identified.

Let me know.

Howard  

---------- FOLLOW-UP ----------

QUESTION: thanks Howard! the logic chain is very helpful - I do have a timing light and look forward to giving it a try, although between stalls it tends to run fine for long highway distances.. not so easy to guess when it will fail. Very curious. In the meantime - I will listen to my fuel pump for familiar ticking and check the contacts etc..stay tuned.
P

ANSWER: That is the trouble with intermittent problems. You have to catch it in the faild mode. It is rarely usefull to test something that is working. That is why you need monitors on Fire and Fuel at the time of the failure. It is too expensive to just toss parts at a problem.

Working in dealerships we were paid by commission so we needed a 100% fix rate. Monitioring the Systems first and then move down to testing the individual parts last. Ignition must be tested first then fuel last.

That is why manufactures finally put computers with memories in them and readers to take the codes out when a computer spotted a failure. Before cars had computers with memories (like MG's) we had to know what section failed when it failed. Thus the timing light and later the fuel pressure gauge. To be able to spot a fault when it happened. Timing lights and fuel pressure gauges are not expensive and can be used on most cars.

Howard

---------- FOLLOW-UP ----------

QUESTION: Howard thanks - engine finally stalled while the timing light was on, and tach was good, so it points me now to 'fuel' per your logic chain. I checked the fuel pump and noticed a loose electrical connector. However still no joy starting, until I gave the pump a couple of knocks, and no problems since. It would seem to point to the pump although I will test further ( luckily the previous Owner gave me a new pump as a spare when I bought the car )
regards P

Answer
Yes, after seeing the flash continue after the engine die, the fuel pump is the next suspect especially if you find a loose connection.

The SU pump and many of the aftermarket replacements operate by a solenoid that is pulled against a spring. The solenoid retracts the diophragm and when it reaches all it's travel a set of contact points breaks the circuit and the solenoid magnetic field drops the diaphargm and the spring pushes the fuel toward the carburetors.

This works fine for a while but as with all magnetic fields. (car ignition coils too) There is some current left over in the coil windings and it tries to go to ground or any place it can and that is across the contact points which archs and burns the points.

Part of my job at the MG dealersips that I worked at was to ocassionally have to go out to a customer broke down and see if we could get his MG running and most of the time it was a fuel pump had stopped. We had a standard procedure and that was to have the customer turn on the ignition and we would listen for the "Clicking" of the pump in the back of the car and if we didn't hear it we first would ask for his knock off hammer (if it was a wire wheel car) I would lay down next to the right rear wheel and use the handle of his hammer to bump the pump and it would usually start clicking so I knew it would run at least for a while before quiting again. I admit that I played with the customer a little when I heard the pump running I would hand the hammer back and say "It will start now so follow me back to the shop" ha! Most didn't know they had a pump back there as all American cars back then had mechanical pumps on the engine. Usually the customer would stand there a minute with his hammer in his hand and scratch his head. ha! I explained it later. ha!

Later SU started to put a condenser on the contact points to slow down the arching of the points making the pump last longer. There are still rebuild kits for SU pumps. The later MGB's put the pump electrical end through the wall of the wheel well so you could go into the trunk and bump it.

When it is working normally when you turn the key to "IGN" you should hear the pump make some fast clicks and then slow down to a very slow rate of click. That means the pump has built fuel pressure in the carburetor float chambers. Then it is ok to go to "Start". This procedure over time lengthens the life of the starter system. It also lengthens the life of the pump because in the "Start" poaition there is not 12 volts available to the fuel pump. (only from 9 to 11v are available in the "Start" position. So you wnat the fuel pressure in the float chambers up to par before the starter is engaged.

By use of all the little items like that I have a 1966 MGB I bought off of the showroom floor in 1966 and I put 189,000 miles on that car up to 1981 and it still pumped 80 PSI oil pressure and had over 175 PSI on a compression test on all cylinders. And I never had the head off. I only retired the car in 1981 due to rust and I still have that car and plan to restore it some day. So all the little items will extend the life of any car.

Howard

MG Car Repair

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Howard M. Fitzcharles III

Expertise

MG from 1956 (USA versions only) up and Engine theory.

Experience

Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Organizations
Associate member SAE EAA member

Publications
Import Car magazine

Education/Credentials
ASE Master Auto with L-1 certification up to 2000

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