Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - 74 BGT won't start when cold

I have a 1974 MGB GT that wont start when cold,I have to use a piece of rough sand paper and clean the points,it will then start unwillingly.Some history about the car,I acquired it 2 yrs ago and have not been able to get it running properly.Before that it has sat in an underground carpark a non runner for 2 1/2 yrs.I recently got it running after taking it to a mechanic, he fitted second hand carbs,clean & setup points,replace condenser,adjust timing. It started fine when I went to pick it up,until a week ago.All I had to do was clean the points and it would start, once warmed up it will always start.The next day if I go to start the car I have to clean the points.I also noticed that the hose that runs to the rear carb from the distributor is not present.On checking the carb,there is no connection for the hose.The distributor is a 25D,that is what I saw on the side of it.The carbs are HIF4 FZX-1229(wk34-f & wk35-r,I suspect that something is causing the points to foul-up quickly.Can anyone help? If I have missed out any vital information needed, please ask.

You could be getting a mist of oil comming up along the distributor shaft but that would affect it when hot also. Depending on the weather you may be getting condensation under the distrubutor cap if it is cold and damp I have had this happen. Water dripping form the heater valve if it is leaking may also be getting into the distributor.
John H

The points could be a red herring, it may be that whereas a normal car will start and run fine with 'used' points yours has some other problem which means that the points have to be tip-top to work round it. OTOH if the points are *burning* then it's possible you have the wrong coil. Coils for chrome bumper cars work at 12v and measure 3 ohms across the primary (spade terminals). Coils for rubber bumper cars work at 6v and measure 1.5 ohms. They need an external ballast resistance (contained within the harness) to drop the voltage from 12v to 6v. Running a 6v coil at 12 will burn the points.

On a UK 74 the rear carb should have the vacuum advance port, they should be HIF AUD 616s, the FZX 1229s were only used from September 77. Having said that the later carb shouldn't make much difference to starting, at least. 77 and later cars had the vacuum pipe connected to the inlet manifold instead of the carb. Connected to neither the effect will be slightly worse fuel consumption and reduced part-throttle acceleration, but it won't affect starting.

Given the history and the fact you haven't been able to get it running properly in the 2 years you have had it, I think you need to go through the whole setting-up procedure from first principles, there could be any number of things wrong, in terms of incorrect and faulty parts. This is valve clearances, plug condition and gap, points condition and dwell/gap, condenser condition, distributor cap, rotor and HT leads condition, timing including centrifugal and vacuum advance function. Then you move onto fuel and fuel pump delivery, carb jets and needle and type and condition, air flow and mixture balance, throttle and choke cable adjustment, and that is after ensuring there are no intake gasket air leaks.

Paul Hunt 2

Thanks for the help guys. John,I'll check for damp in the cap,or if water is getting into it. Thanks. A couple of things you mentioned Paul has brought somethings back to mind. When I was trying to get it running previously, I tried lining up the timing to Number 1 piston and the timing marks were not exactly matching up. The engine number is 18V780F-H2412.This puzzled me a bit, when I checked the manual it was even more confusing. Also the the rotor was not exactly lining up on the number 1 plug. I checked the position of the distributor on my 69B and compared it to the 74BGT,the 69B was correct according to the manual.In regards to the voltage drop with the coil, I think I heard the mechanic say something about a voltage drop, but I am not sure. Also in the work sheet, the mechanic mentioned that the ignition barrel was worn and would need replacing. I am not sure what he mean't by that,he is the mechanic that we have been taking all our cars to,to be repaired. I was expecting him to sort out what problem there was and let me know, I have lost faith in him and will be doing all the work myself from now on.I did not have time to do the repairs on this car which is why I took it to him.I have a 69 Roadster and do all the work on that myself. The situation is where do I start and what tools do I need. Electrics is not my best side, so it will be difficult. Any suggestions on starting point or books that would be of help. I have to say that the Haynes manual is more confusing at time because they don't go into a lot of detail.There are a couple of things that I replaced when I was trying to start the car initially. The plug leads,rotor, and I have a new rotor cap.One thing I did not mention was the starter was not working properly. When you turned the ignition key, it would click but the engine would not turn over. I fitted my starter from my 69B and it works fine. Your help would be much appreciated, Thanks a million guys.

The rotor can point to a different cap contact when No.1 piston is at TDC after its compression stroke if the distributor drive gear has been inserted incorrectly (it can go back in one of 6 or 8 different positions) or the distributor has been dismantled and reassembled 180 degrees out, but getting round (short of correcting the mistake) that is simply a matter of moving the plug leads round to the correct positions.

Regardless of the above the timing marks should still line up with No.1 piston at TDC, regardless of whether it is after its compression or exhaust stroke. If it isn't then the crank pulley could be delaminating, it is a metal-rubber-metal sandwich to damp vibrations and the outer part (with the timing marks) can slip relative to the inner part (keyed to the crank). The timing chain incorrectly fitted i.e. the gears not being correctly aligned first can also cause it.

To check the ignition you will need a multimeter with ranges capable of showing 12v and resistance down to 1 ohm or lower. To check the resistance of the coil remove all the wires from the spades (noting where they come from) and measure between the two spades. For a chrome bumper car you should see about 3 ohms, or about 2.4 ohms if it is a Sport coil. If you see about 1.5 ohms or lower it is the wrong coil.

To check the circuit reconnect the white wire(s) to the coil and turn on the ignition. Connect the voltmeter between the +ve terminal (white wire) and a good earth and you should see 12v. Now connect a good earth to the *other* terminal of the coil (no wires) and the voltmeter should still show 12v. It may drop a tenth or two, but any more shows a problem. If it drops to about 6v then someone has added a ballast resistance to the cars wiring for some reason. If this is associated with a 6v coil then the efefct is much the same, but if you have a 12v coil with an added ballast resistance you will get weak sparking. If you get more than a couple of tenths of voltdrop but not as much as 5v or 6v then there are bad connections somewhere, which *could* be a defective ignition switch. Measure the white wires either side of the multi-way conenctor by the steering column, the white and brown wires on the switch, and the brown wires in the multi-way plugs to see if you can find where the low voltage suddenly becomes the 12v it should be. You must do these tests with the ignition and the earth wire connected to the coil -ve, but don't leave it like this for any longer than you need to or the coil could overheat.

If the voltage on the coil +ve is correct then disconnect your earth and reconnect the white/black wires. Now with the ignition on and the voltmeter connected between the coil -ve and a good earth, slowly turn the engine so the points open and close. With the points open you should see 12v. With them closed ideally you should see 0v, but a couple of tenths above this is OK. If you see more than that then you have bad connections through the points and the earth wire inside the distributor, so with the voltmeter test both sides of the points to see what voltage you get.

When that is all OK you can check the strength of the HT. With the coil lead disconnected from the distributor can and connected to a spark plug laying on the block, and the ignition on, flick the points open and closed by hand. You should get a good sharp 'crack' from the plug. Now remove the lead from the coil, moving it further and further away from the coil centre connection as you continue to open and close the points by hand. You *should* be able to draw out a spark up to about 1/4" in length, and putting the big gap between the lead and the coil instead of the plug and the block *should* mean you won't get a belt! If the spark will barely jump more than a plug gap, and is yellow and fizzes, then probably the condenser is faulty. Another way of checking the condenser is to compare the spark at the points and the plug by breaking the circuit at the points on the one hand, and by disconnecting and reconnecting the white/black wire at the coil -ve (points closed) on the other. By greaking at the points the points shouldn't apark much and you should get a strong HT spark. By breaking the circuit at the coil terminal you will get lots af sparking there but a weak HT spark. If both are the same the condenser is faulty.

One thing to check is correct assembly of the points. Earlier distributors have separated wires for condenser and coil and it is easy to get these assembled incorrectly, and two ways of doing it wrong takes the condenser out of circuit. First thing on the threaded stud should be a shouldered washer, with the shoulder facing upwards. Then the points with the shoulder of the washer located in the hole in the points spring. Then the condenser and coil wires, in any order. Then the 2nd shouldered washer with the shoulder pointing downwards and going through the holes in the tags for the condenser and coil wires. Finally the nut.

That should be enough to be going on with.
Paul Hunt 2

6th para "By greaking at the points " should read "By breaking at the points"
Paul Hunt 2

Thanks Paul, that will keep me busy for awhile.
G Thompson

This thread was discussed between 19/11/2007 and 21/11/2007

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.