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MG Midget and Sprite Technical - Powerspark Ignition / Tacho query

Hi All,
I fitted the Powerspark electronic ignition in a new dizzy body with a decent rotor arm and coil all supplied by Powerspark and quite frankly, it works a treat.
Question is that when driving , the tacho starts off OK but seems to drop off after a few miles.
There was an article in the MGOC mag about this happening with accuspark units and it gave a re wiring fix.
Powerspark are not aware of this happening with their units but the symptoms are so similar it does look the same.
Does any body have experience of either this or of using the MGOC re wiring fix?
My car is may 1972 Mk III midget 1275 with the RVI tacho. There is NO external loop on the back of the tacho unit
Thanks all
Cheers
colin
colin frowen

I have had a similar problem which raised its head again yesterday. My friend had a GPS in the satnav and checked my speedo for me. It was spot on. At the time we checked it I was getting 60mph at 3000rpm (Ford type 9 in 5th), but after about half an hour after a stop, to get the same speed the rev counter had to be at 3500rpm. What's going on. Is it, as had been suggested the ignition system? Mine is Petronix Igniter.

Bernie.
b higginson

An RVI tacho indicates inductive loop so has this one been converted in some way? In any event the RVI's were designed for points systems and often do not work well or accurately when warm with electronic systems. I've heard of people limiting the spike with voltage stabilisers, but it may be easier to just fit an RVC tacho.
F Pollock

pertronix / Aldon with RVI. My tachometer would sometimes do its own thing. I traced the problem to a poor connection with an on line fuse.
Alan
Alan Anstead

Ive got Pertronix with an RVI and when the engine's warmed up the tacho occasionally goes bananas swinging about wildly or under reading. I thought I must have a duff connection somewhere but I'm buggered if I can find it. Its alsmost as if I'm getting voltage/current spikes from the coil. Would some form of voltage stabiliser do the trick. Strangely as I said it only seems to happen when the engine's hot. Turning the indicator on can also seem to set it off although this may be just coincidence and my paranoid imagination!
Matt1275Bucks

I fitted the pertronix unit and flamethrower coil several years ago (ditched the points) and the RVI tacho worked perfectly.
I've now fitted a Simon BBC unit, ignition improved but tacho not registering, so I am going to change it for an RVC and see if that works.
I also thought the RVI was current sensing?
aj robinson

This thread was discussed between 22/10/2012 and 30/10/2012

MG Midget and Sprite Technical index

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