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MG MGB Technical - Tachometer and Ignitor

Hello,
I have installed a Ignitor electronic ignition in my distributor. The engine runs fine but the tachometer (electric) reads now about 25% high! Can this be corrected? I have checked the actual rpm with help of two other instruments.
Kjell Magnusson

Kjell,

Apparently there is some new data emerging regarding this phenomenon. My understanding is that it takes some time before the problem begins to manifest itself. Prehaps someone from this BBS can shed more light on the subject as I am no expert.

Cheers - Dennis - Sacramento
Dennis Rainey

Kjell,
I had the same thing happen on my '66 when I switched to an Allison ignition. I drilled a small hole in the back of my tach oposite the adjustment pot and adjusted the tach to match my Snap On tach at 3000 rpm. It's now accurate up to 4500 where it starts reading low; 300 rpm shy at an actual 5500. This seems to be a common occurence.
David
David

David,
Thanks, I did not know there is an adjustment pot, I will try to find it.
Kjell
Kjell

What is an "adjustment pot"....I've been back there and had tach apart for painting the can white but can't remember it too well and am not familiar with the term. Planning on putting Ignitor in mine in next few weeks. Damned inconvenient.

JTB
J.T. Bamford

Kjell and JT,
There is a very small potentiometer on the back side of the tach circuit board that, when turned with a small screwdriver, varies the resistance of the internal tach circuit. At least on my '66's tach.
David
David

I suspect that this may only be a problem with the earlier induction pickup type of tachos. My 73 B has had a pertronix Ignitor module fitted for two years with no apparent impact on the tacho reading. I believe that my car has the voltage sensing type of tacho. I am not sure what year the tacho type changed - some knowledgeable contributor should be able to say.

Good luck.

Barry
73B
B.J. Quartermaine

great feedback, thanks. Here is info on various ignition/tach systems.

http://www.mgb-stuff.org.uk/s_ign.htm#ignition1
J.T. Bamford

also a possible fix for the problem on the same site...Paul Hunts.

"If your inductive tach (64 to 72) doesn't work with your shiny new electronic ignition system try the Crane tip (page 16) of changing wire going through the pickup from two passes to one pass and recalibrating. Will need to dismantle the tach to get at the pick-up on later versions. " Go to Site, go to Spanner, then electrical then tach

That link to Crane doesn't appear to work but I found this:

http://www.cranecams.com/pdf/90008920.pdf

Don't know if Crane stuff would work with Ignitor, though.
J.T. Bamford

I got a whole lot of info when I was getting my tach rebuilt. I was going to put in a Mallory Unilite distributer and was told that the Early tach's would not work with the electronic ignition system and would have to have the internals replaced. It seems that there were at least 3 different internals in B tachs and not all would work with all electronic ignition systems. Go the someone in your area who does tach rebuilds and you should get the correct info on your tach. Ed
E kulick

Two basic types of tach internals - current sensing up to 72 and voltage sensing from 73 on. With the earlier current sensing type there were at least two if not three arrangements for the pickup.
Paul Hunt

Thanks everybody for all the information. I have dismantled the tach, found the potentiometer and adjusted the rpm. It seems now that the indication is pretty accurate up to at least 4000 rpm. Have not tested any higher rpm yet.
Kjell
Kjell

This thread was discussed between 19/05/2004 and 25/05/2004

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