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MG MGB Technical - B won't start, but has spark and petrol.


I just bought an unfinished 1973 B in good condition a couple of days ago. The owner said it had a broken battery terminal so it wouldn't start, however today I changed the terminal and still no luck.

The batteries are charged well, the engine turns over quickly, all the spark plugs are firing, but I cannot hear anything igniting, just the crank turning. Just to check, the firing order is 1-3-4-2? (with 1 being the cylinder closest to the front of the car) and going round the distributor cap anti clockwise, 1 o'clock goes to cylinder 1, 11 o'clock goes to cylinder 3 etc. One thing I did realise is all the sparks were very weak- is this a weak ignition coil? Also one or two plugs were dry, while others were wet.

Also the alternator was getting hot just with the ignition on (a short in the wiring maybe?), and the ignition light would stay on until I rotated the key to get the fuel pumping, i.e when ignition was on, the light was off!! so I removed the alternator connector, the ignition light was completely dead, the alternator obviously did not get hot, but still wouldn't start.

I think it maybe a wiring issue, the previous owner recently changed the batteries- could they be wired the wrong way?

The previous owner did say he had it started recently and I do believe him, so hopefully its a simple fix.

Thanks for any help
RH Hodges

One thing you need to check is that the rotor arm is pointing at the correct plug lead when that cylinder is on the compression/firing stroke.

It is possible that someone has got the dissy 180 degrees out at sometime.

Turn the engine by hand until no 1 cylinder is at TDC and check that both valves for that cylinder are closed. Then look to see where the rotor arm is pointing.
Dave O'Neill 2

sorry quite new to this, when you say "Then look to see where the rotor arm is pointing." where should the arm be pointing and what/where is the rotor arm.
RH Hodges


The first thing to do is to check the battery polarity. Negative should go to chassis. If they are connected backwards that would explain the alternator getting hot.
If the are connected backwards, then there is a good chance that the alternator has been fried.

The rotor button is the little arm in the distributor, that goes around and "points" to the cylinder to be fired. You have actually done the correct thing and determined that it points to cylinder one. The arm normally points to #1 at around 2 o'clock, so maybe the timing is out a bit. Check.

A weak spark can be caused by a faulty capacitor, in the distributor, faulty or mis adjusted points, faulty coil or plug leads.

Herb Adler

You not only have to establish that it points to cylinder one, but on the correct stroke.
Dave O'Neill 2

Which plugs were wet and which dry? Maybe one carb is in trouble?
Stan Best

A reversed battery would cause a massive short and burnt wiring as well as burnt-out diodes in the alternator. Even if connected correctly now it's possible it has been reversed in the past for long enough to cause some diode damage, but not burnt wiring, and the damaged diodes are enough to cause a current drain and hot alternator. However it would be draining the battery all the time it was connected, not just when the ignition is on. A wiring fault (other than reversed batteries) wouldn't cause a hot alternator by itself. Ignition light on when the ignition was off does imply diode failure if not voltage regulator failure.

For the moment, unplug the alternator, it isn't needed for cranking and does remove some of your variables.

The rotor arm is under the distributor cap, and should be pointing to about 1 or 2 o'clock when you stand at the side of the car looking down at it. But I have to say that if you have to ask you are going to struggle.

Whilst the distributor can't be inserted 180 degrees out it can be dismantled and reassembled 180 degrees out. It is driven by a gear which itself can be inserted in one of several positions, so the rotor could be pointing anywhere if the engine hasn't been assembled correctly in the past.

As said how the plug leads are connected is determined by where the rotor arm is pointing when No. 1 piston is at the top of its *compression* stroke, it is also at the top on the exhaust stroke but this must be ignored. The easiest way to do this is remove the plugs, place your thumb over No.1 plug hole, and turn the engine by hand. When the compression lifts your thumb off the hole that is the compression stroke, when the piston is at the top of that it is in its firing position, so wherever the rotor is pointing that cap lead must go to No.1 plug. If that's not 1 or 2 o'clock it doesn't matter as long as all the leads reach, just connect up from there i.e. 1 3 4 2 anti-clockwise as you said.

A weak spark could be caused by a failed condenser, normally the spark should jump at least 1/4". If it barely jumps a plug gap it points to the condenser. Note that if you hold a lead off a plug near the block you are likely to get a shock, safer to ease the coil lead out of the coil and check the spark there.

After cranking for a bit with the choke out you will probably flood the engine and get a strong fuel smell around the car. If not remove the plugs and sniff them, there should be a strong fuel smell on them, if they are wet it is flooded, if no fuel smell no fuel is getting through. Did the pump click when you turned on the ignition? It should, but you could still have stuck float valves stopping fuel getting through.
PaulH Solihull

Thanks for all the help. I think it comes down to a carb problem (fuel pump is clicking), a wiring problem and condenser problem.
RH Hodges

Hi RH I live in Kingswood, if you are close by I can lend you a strobe, dwell meter and haynes manual. Or even take a quick look for you.
c cummins

Are you using fresh fuel? The car won't start with old fuel because the lighter particles will have evaporated.
Willem vd Veer

One trick I learned from an old hand was to have somebody crank the engine while I sprayed carb cleaner into the throat. It turned out that the air cylinders were sticking (sometimes).
Dan Robinson

C Cummins,
I am in Stoke Bishop, so it's a bit far.
RH Hodges

RH from the little that you told us it seems that you are new to this car repair stuff SOooo, I'm going to get a little basic. This car being totaly new to you and as you have never heard it run don't rule out anything without being sure.You've told us you have checked the fuel and spark but you seem to have left out compression. You have to have all three and no matter what the DPO has told you , until you check things yourself, you can't be sure what was done that you were not told about.
Get a good repair manual and go step by step without skiping a step, it's a simple car take your time you will find the problem. RIC

This thread was discussed between 06/02/2011 and 08/02/2011

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