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MG MGB Technical - lack of spark

Expat in France whose just realised how much he's forgotten. Sorry if this has been covered elsewhere, I have looked without success.
Problem is no spark at plugs but is present at HT lead into distributor. Does the fact that a spark exists at all indicate the the LV side of things is intact?
To my way of thinking if there is a spark going into the HT side of the distributor and not coming out then the fault has to be in the cap, rotor arm or plug HT leads? Trouble is these are all new. Am I completely wrong and a problem on the LT side could be causing the problem? Thanks
S Hill

It could be your coil HT lead. Check to see what sort of voltage youre getting to the LT windings in the coil. If its ballasted, around 6 - 8 volts, if not 12 or so volts. But you say you're getting a spark from the HT lead from the coil but its not leaving the distributor to the plugs. Id start by checking all leads, and make sure you have the right rotor arm.
Ross Kelly

You are quite right that the problem is somewhere in the dizzi. There have been a lot of reports of defective new rotor arms recentley and I think I would try it with the old one back in first.
Stan Best

Stan is correct. If you are getting a strong spark off of the king lead (coil lead), your coil and low tension circuits are good.

Possible problem areas:

Carbon bush on the distributor cap
Rotor
Spark plug wires

Check for spark at all of the spark plug wires, not only one. If you have no spark, either the dizzy cap or the rotor (or both) are bad. If you have spark on all except one spark plug lead, it is either a bad wire or a bad connection.

Les
Les Bengtson

Classic example of rotor or cap breaking down. The bad rotors mentioned have a round-head rivet very close to the centre of rotation of the rotor and hence the dizzie shaft. Good rotors either have no obvious rivet, or a flush rivet that is noticeably further out from the centre of rotation. This photo http://www.mgb-stuff.org.uk/images/rotors.jpg is actually comparing a certain Hall-effect rotor with a standard rotor, but also shows the offending rivet being much closer to the point of rotation of the rotor (and hence the dizzie shaft) than the flush rivet on the standard rotor.
Paul Hunt

Hi thanks for all the comments. I am totally baffled now! Have renewed everything. Even though a spark was present at inlet to dizzy I replaced the coil, to no effect. Is it possible that the dizzy has slipped somehow (even though still clamped tight) and the spark is occuring when the rotor arm is between cap contacts and so the spark is being lost? Steve
S Hill

How have you determined that spark is present on each of the spark plug leads?

What is the strength of the spark on the HT lead from the coil to the distributor cap? What is the strength of the spark from each of the spark plug leads to the block? You should have a strong, fat spark when the coil lead is held about 3/8" (10mm) away from the block. If not, the leads are deteriorated and cannot carry the necessary current to cause the spark plug to fire.

The timing is best checked by using a stroboscopic timing light and having an assistant turn the engine over as you observe where the strobe is flashing.

"Have renewed everything." What does this mean? What parts were replaced? Did you install the points correctly with the condenser and wire from the coil being insulated from the center post of the points set? You have to remove the nut and plastic spacer, install the two wire connectors onto the spacer, install the spacer onto the points set (ensuring it fits into the hole in the spring arm) and install the nut onto the stud. If you install the wires on top of the plastic spacer, they are a direct short to ground and you cannot get a spark.

More information would be of assistance.

Les
Les Bengtson

Yes, "everything" is a rather indertiminant word. I guess this includes dizzy rotor arm leads and plug caps but it would be nice to know. As Mr Hunt often notes here just because a part is new it does not mean it is working perfectly. Replacing one dud rotor with another will not cause the car to burst into life. If you want to check things out you can remove the plugs and dizz1 cap and rotate the engine CW viewed from the front (1 5/16 socket on the pulley but thats from memory or just pull it round using the fan belt), the rotor arm should go round ccw at 1/2 engine speed. You can see where it is when the points open(use a timing light to help here across the points it lights up when they open) and check where the rotor arm is relative to the plug lead terminals. The workshop manual suggests cutting a window in an old dizzi cap to carry out this sort of investigationbut you can get a very good idea by looking at the positions then offering up the cap. Check that when no 1 cylinder is on its firing stroke (both valves closed) the lead fom that contact is the one the rotor is pointing at.
Stan Best

The dizzie itself can't 'slip' so that the spark is occuring between cap contacts unless the points or trigger have moved a huge amount rotationally. If the distributor *does* slip in the block the spark will still be going out on the plug leads, it will simply be at the wrong time for the rest of the engine. I was sent an electronic trigger supplied with an MGB distributor that simply did not work, and found in that case the trigger head or magnets in the rotor was/were so far out that the spark *was* occuring between cap contacts and the engine simply wouldn't run. With the cap off turn the engine with the ignition on and note just where the rotor is pointing when you get a spark on the coil lead at each rotor position, then refit the cap and see how those points compare with the actual plug lead positions in the cap. You can do this approximately by cutting a hole in a piece of card and slotting it over the distributor body. But that scenario would almost certainly only be the case for new components, if it has started happening with existing components then if there is a spark going in but not coming out (as detected by a timing light flashing when clipped to the coil lead but not to any or all coil leads) then it is a cap or rotor problem. There are bad rotors out there, the ones with the round-headed rivet on top, which is positioned too close to the dizzie shaft when installed and can break through. The originals either had no rivet or a flush rivet positioned further out, see http://www.mgb-stuff.org.uk/images/rotors.jpg - the one on the left being of the dodgy type.

Even if the leads are on the wrong plugs you should still get a flash from each and every plug lead when cranking, it just won't be at the right time for the rest of the engine, and once you have No.1 right the others are numbered 3-4-2 anti-clockwise.
Paul Hunt

Hi. I'm a bit late to this but if you are still having problems you can check the rotor arm by taking the king lead from the centre of the cap and holding it about 1/4" from the rotor arm. Get somebody to spin the engine over and see if you get a spark.

If you do get a spark, the rotor arm is faulty and earthing out. No spark means the arm is ok and the fault is somewhere else.
Owen Frankland

*Don't* hold the distributor end of the coil lead or you will get a belt if the rotor *isn't* faulty. Same with the plug end of the plug leads. The HT spark is capable of jumping significantly more than the thickness of the HT insulation, at least 1/4" and up to 1/2" with Sport coils. It is only the relative narrownes of the plug gap that stops the HT lean insultation being breached as the HT voltage stops rising as soon as the plug fires. Increase the gap and the HT voltage goes up, which is why silly plug gaps often starts causing HT problems with leads, cap and rotor.

Clip a timimng light onto the coil lead and each plug lead in turn, and watch the flashing while the engine is being cranked. If the coil lead doesn't flash it is a coil or ignition LT problem. If the coil lead flashes but one or more plug leads don't then rotor or cap are at fault.
Paul Hunt

This thread was discussed between 27/06/2008 and 06/07/2008

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