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MG MGB Technical - Dwell Tachometer - Points

I have just been given a dwell tachometer, I suspect that ther are a lot of these lying around that get little use now.
Hooked it up to my 70 MGB which is running O/K, I understand the dwell and the Tacho reading that it gives (both were O/K) But this meter has a setting for points which uses the needle of the Tachometer to say weather the points are good or bad.
Can someone tell me what this meter is measuring, the car is running O/K but the meter shows that my points are bad.
Side issue - I love it when you tune your B and you can feel an improvement in performance which gives great joy & pleasure. My late model Holden costs me hundreds of dollars to be tuned at the garage (because it is beyond my expertise) and I can feel no differace in the performance when I get it back.
David Levy

David - Is the dwell/tachometer one of the gunson AutoRanger? If so, I have an instructions booklet and can make a copy of it for you. Let me know. Cheers - Dave
David DuBois

I guess it is measuring the voltage drop across the points - as they wear and the "pip" & "dimple" develop the resistance increases.

While this may not impair the engine running - it isn't optimal.
Chris at Octarine Services

Thanks for the quick response and offer of instructions. Unfortunately it is not a Gunson unit.

My intention is to check the points with the unit next time I do a points change before and after to see if the reading is indicative of the points condition.
David Levy

David ,
The suggestion by Chris , above , is spot on.

The "points" setting is another label for a range specific ohm meter.

This does measure resistance and will show on a scale on the main dial but as most people have no idea what an ohm meter is and how to use and read one the makers have opted for a more easily understood "good / bad" scale.

By the way on the "DWELL" function it should be a steady reading too. Any wavering of the needle will indicate a bent shaft or worn bearings in the distributor showing up as the time taken varies as the shaft rotates.

HTH , Cheers , Pete.
Peter Thomas

I don't know why you say dwell meters get little use, since getting mine I have never used feeler gauges to check the gap. And since doing that I have never had to readjust points between replacement intervals. Whilst a spike and pit interferes with the feeler gauges, so you take the points off the face them up, by which time you might as well fit new ones, and go through the hassle of getting the gap right anyway. My current set of points have been in over 10k miles on both cars, and the roadster is still within spec. I did have to make a small change to the V8 gap recently but it only has a tolerance of 1 degree either way unlike the 4-cylinder which has plus or minus 3 degrees. But then again the V8 has an external stud to adjust dwell with the engine running, and it is about the easiest thing to reach apart from the steering wheel, so that is neither here nor there. When I *do* change the points on the roadster I take the distributor out as it is such a pain to get to, especially on an RHD car, and have a bench tester whereby I can operate it under real-world conditions and so can use the dwell meter rather than feeler gauges.

If you *do* use feeler gauges always pull a strip of clean paper through the closed points afterwards, sparking through oil or grease from the feelers can cause a resistive film to form. Even then don't pull a torn edge through the closed points as that can leave fibres behind. Both these things can cause intermittent misfires, which is annoying when you have just tuned it all up and it didn't do it before.
Paul Hunt 2

This thread was discussed on 15/03/2006

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