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MG MGB Technical - dizzy leads in wrong order

Hi,

I'm sure I've seen something threads about this kind of thin before, but can't find any words that access anything useful.

Helping a mate with his 1970 mgb I noticed that leads coming from the distributor to the plugs were in a funny order at the dizzy. Decided to have a further look. When the valves on piston chamber 1 fully closed. I then took the dizzy cap off and found that the rotor was pointing straight down.

How do I correct this to get the rotor to point to no 1 cylinder when both valves are shut?

Or: can you remind me of a thread I've seen.

Ta

Iwan
Iwan Jones

Iwan. Remove the valve cover and turn the engine over by hand. Watch the number one intake valve (second valve from the front) open, then close. Watch the timing marks and rotate the engine until the timing mark is at top dead center. At that point, the rotor is pointing at the distributor cap terminal that should be connected to cylinder number one. Does not matter where this terminal is located, it is cylinder number one. Then, attach, in counter clockwise fashion, the leads to cylinder three, then cylinder four, then cylinder two.

If the distributor drive cam is not oriented correctly, the distributor will not show the typical "number one at two O'clock" position. Correction of that problem would require removing the distributor from the engine, the distributor housing from the block, the distributor drive cam from the block, correcting the alignment of the drive cam, putting everything back together again, and retiming the engine.

Might be easier to simply work from what you currently have, even if it seems wrong.

Les
Les Bengtson

Iwan. If the valves on #1 are fully closed, check to see where the piston & timing mark is. Piston should be at TDC with valves closed & the timing mark should be in the 0 -10 Degree range. Then look at the position of the rotor. That is where #1 lead needs to be. Going COUNTER clockwise from there, it is #3, #4, then #2. Hence the firing order is 1-3-4-2 COUNTER CLOCKWISE.

Cheers

Gary
79 MGB
gary hansen

Hi Iwan,
I changed the dizzy to an electronic version on a mark 2 not long after purchasing and it would not then fire. After a bit of hair pulling I found the PO had installed the dizzy drive tube 180 deg out. Due to time I simple put the leads 180 deg out on the dizzy cap as they must have been before. No 1 firing from no 4 position. Appears this fault is not uncommon.

Ken
K Stuckey

Iwan,

I presume the car doesn't run now and you are trying to get it running?

If so, please ask if the distributor has been dismantled during repairs.

In the past I've managed to assemble the distributor cam 180 degrees out on a 25D distributor, and not the complete tube as in the answer above. That has the same effect as you describe; cylinder no.1 (the one nearest to the radiator) on TDC, timing mark on TDC and the rotor pointing downwards. Took me a while to figure out what I had done wrong....

Willem
Willem vd Veer

Easy way to see if you're on #1 firing or #4 is to put the timing mark at TDC and stick a finger down the oil fill hole and see if you can wiggle the #1 intake valve rocker. If it's tight (no movement) then you're on #4. If you can feel a little lash then you're on #1. OTOH, if you do pull the valve cover it's a good opportunity to adjust the valve lash....
Rob Edwards

As Willem says the distributor can be dismantled and reassembled 180 degrees out, there are no match marks I'm aware of. The drive gear can also be inserted in any one of n positions, where n is the number of teeth the gear has. Also if you follow the Haynes instructions for the timing gears and dizzie gear you will end up with it 180 degrees out. The Workshop Manual is correct, because it tells you to turn the crank 360 degrees after fitting the timing gear which puts No.4 cylinder at TDC on its compression stroke, before inserting the drive gear with No.1 at TDC on its compression stroke. My Haynes misses this step out. Either the dots on the cam gear have always been 180 degrees out, or they were right for the first application and reused on this engine which is different.

To be correct, with No.1 at TDC on its compression stroke, the drive slot on the gear should be aligned 8 o'clock to 2 o'clock, with the slot below the centre-line of the gear. It's relatively easy to correct it when the rotor is 180 degrees out. Remove the cover plate, screw in a long 5/16" bolt i.e. one from an SU air-cleaner, lift out the gear until it stops turning, rotate it 180 degrees, then lower it back in. Of course if it's the distributor that is wrong, it will probably be wrong again if you swap the distributor.
PaulH Solihull

Ha Paul, thanks once again, thought I'd seen this before.

The engine is runs well, it is no trouble at all just been rebuilt wrong I think.

Ta all,

Iwan
Iwan Jones

This thread was discussed between 08/03/2013 and 09/03/2013

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