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Pertonicxs affect the tach?

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AUSMHLY

AUSMHLY

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Does replacing the points in a positive BJ8 with the positive Pertonixs affect the accuracy of the tachometer?

Yesterday I drove a negative Healey with points and tachometer was always near the 4000-4500 when shifting.

My car with Pertonixs was around 2500-3000 at the same speed.

If I pushed my car to 4000-5000 before shifting, (2rd gear for example) it sounds to fast, compared to the negative Healey.

My tachometer was rebuilt before switching to Pertonix. So I'm assuming it's reading accurate?

My car idles about 750 and runs great.

I was surprised to see the difference in the tachometer when shifting at the same speed between the two cars.

Thoughts?
 
with my 64 BJ8, pos ground, with a pertronix ignition, I have found the tach fairly reliable at lower rpms. Above 3000-3500, however, and I question its accuracy. It jumps around at higher rpms and even falls back to zero around 4000+ rpms. In order to get the tach to work at all with the pertronix, I had take a second turn in the loop at the back of the tach with the ignition/coil wire. MoMa indicates they can rework the tach to work better with a pertronix.
My opinion, for what its worth, is the Smith's tach isn't particularly good anyway, so why bother? But then I drive mine fairly conservatively.
 
Roger, going home yesterday I was paying close attention to the tach, looking for any sign that it's indicating incorrect rpm. (it was my BJ8 Roger was comparing against).

Like I told you, my tach has not been rebuilt, at least by me, and very likely does need recalibration. She's cruising at 70 mph at an indicated 3500 rpm in overdrive. 60 is about 3000 rpm in overdrive. When I shift my car at 4,000 rpm it feels and sounds like your car does at 3,200 rpm. When you shift your car at higher revs (4k and up), it sounds to me as though you are over revving and headed for a world of hurt. Hope this helps you sort that out. I'm not running a Pertronix dizzy.
Randy
 
Re: Pertonix affect the tach?

The short answer is yes, a Pertronix can screw up the tach.

If your tach is marked RVI on the front it should have a Nylon block on the back. The white wire from the ignition switch to the coil is wrapped through this block which acts as an inductive pickup for the tach.

As Skip says, sometimes adding an additional wrap of wire will help. You may have to use smaller wire to get an additional loop through the Nylon block. Skip is also correct that this method doesn't always work very well. Actually the original tach was not very accurate in the first place.

Doug Lawson has some helpful ideas on rebuilding your tach to work with electronic ignition.
https://www.britishcarforum.com/lore/article.php?id=063

If this looks to be more work than you want, you might be able to find a shop that would rebuild your tach to work with electronic ignition.

Here is an article on the tach reading high due to a defective capacitor.
https://hometown.aol.com/bgahc/01_jwha_tachreadshigh.html
I know you have the opposite problem, but I have cured a couple of cases where the tach read low by replacing this capacitor. Possibly worth a try?
D
 
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