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MG MGB Technical - Dwell measuring

Hi,

I have just got hold of a digital multimeter with a dwell function on it. Will I be able to measure dwell on an mg with it and if so how?
IA Jones

Never had a digital meter with a dwell function on it. Mine is an analogue old meter.I would think you can use it on an MG and the best advice on how to use it would be in the manual for that meter.


AdrianS

Don't see why not. Most have 12v and earth croc-clips which you connect appropriately, and a third wire which goes to the points or other trigger connection on the coil.

I take it you still use points, electronic ignition systems can give very different results from the original points specification, and as I've never heard of one being adjustable for dwell there's no point looking at it anyway unless you think there may be a fault with the system somewhere.

You can also measure dwell on the bench with a more basic analogue multi-meter with an ohms setting, as described here http://www.mgb-stuff.org.uk/ignitiontext.htm#dwell
paulh4

I have both digital and analog, but I have never seen one with 3 wires on, every type of dwell meter I have ever used only has 2 wires on, as Paul has stated you must observe polarity, which means, POSITIVE EARTH vehicles, red lead from meter to earth, and black lead to coil + or CB however its marked, and NEGATIVE EARTH vehicles, black to earth and red lead to coil - or CB however tis marked, depending on which distributor you have in your B the dwell angle is 25D 60 degrees + or - 3 degrees, and if you have a 45d its 51 degrees + or - 5 degrees, if you just want to check your dwell just connect up the wires and run the engine,if you want to adjust, take off the distributor cap and rotor arm slacken the screw put a screwdriver in the adjuster slot and crank the engine over watching the meter and adjusting when the engine is cranking, when you get the correct reading stop cranking tighten the screw and recheck, you will normally also have a tachometer as well which is connected up the same way as the dwell meter, which means you can switch from one to the other when the engine is running, also if you switch to the volts scale you can set your static ignition timing, to do this you will need the ignition on, also pull the HT lead out the coil so the engine cant start as you are turning it over by hand, these are great bits of kit well worth spending a bit of time learning all the things you can do with them, and how to use them,
Andy Tilney

You can't really go by the coil markings as originally +ve earth cars may have been converted, in which case the coil connections may or may not have been reversed. Likewise a 'modern' coil marked + and - may not be connected that way round, on any car be it positive earth or negative earth. That's why I say the coil connection should be to which ever wire is connected to the distributor when you have points.

Fiddling with points gap on the car is a right pain, especially with an RHD, which is why I suggest doing it on the bench. Especially when changing the points, and really if the points have been in some time and they have drifted out of tolerance, you are probably better of changing them anyway. Subject to getting a decent pair, that is.
paulh4

I would have assumed that anyone looking at coil markings would also notice if the black/white wire was on the correct terminal, and if it wasn't would swop them over, but I think another clue would be the dwell meter not working,
Andy Tilney

That's why I mentioned the wire ...

paulh4

Hi,

Thanks for all the information.

Paul, I was really interested in your bench system. Almost went out to whip the dizzie out, then I looked at your web pages. Well, that stopped me!! Not a ten minute job then :). Will have to spend a wet day, with no list of chores, setting up a rig.

Will do the in engine method for now!

Thanks all

Iwan
IA Jones

The full rig is really only needed if you are planning on measuring the curve. For gap/dwell you only need the Allen key lightly clamped to the drive dog.
paulh4

This thread was discussed between 30/09/2018 and 02/10/2018

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