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MG MGB Technical - Tach/Coil/Electronic Ignition

I have a 1980 MGB with a 3.9 V8; electronic ignitions without a control box.
I had the original tachometer refurbished and calibrated to a V8.
The tach reads 3000 rpm when it should be reading 1000 rpm.
The company that refurbished the tach was Nisonger. I had them recheck the tach and was assured that it was dead on.
They gave me the following reads:
1500 rpms @ 100 pulses per second
3000 rpms @ 200 pulses per second
4500 rpms @ 300 pulses per second
6000 rpms @ 400 pulses per second
The tach is connected to the negative side of the coil (distributor side)
The car runs great.
With this information, I’m obviously getting incorrect pulses from the coil to the tach. What would be the logical next step to trouble shoot?

Richard Cohen
ricmgb

You are trying to work in two measuring systems, seconds and minutes. Converting the 1500 rpms@100 pulses per second to only minutes yields:

1,500 revolutions per minute at 6,000 pulses per minute.

Each revolution of your crankshaft should cause four cylinders to fire (four stroke engine requires two revolutions per cylinder to fire that cylinder). Thus:

6,000 pulses (cylinder firings) per minute at a rate of 4 firings per revolution would indicate that the engine is revolving 1,500 times during that minute.

If the tachometer has been accurately adjusted, as per the information you have provided, it should be reading accurately. Thus, the problem lies in the fact that the tachometer is not accurately reading the firing impulses on your electronic ignition system, a problems sometimes encountered with certain electronic ignition systems.

What electronic ignition system are you using and what had Nisonger to say about how well the supplied tach would work with such a system?

Les
Les Bengtson

First I liked to thank you for your help.

I'm a novice with this, but I love my B and driving it is a blast.

I can't find mechanics to get it right, so I'm working on it on my own.

With the info Les provided, my next questions would be:
As I understand it, if I had the tach recalibrated to a 4 cylinder engine, it would read half of what it is reading now, ie 1500 instead of 3000. Then could an internal adjustment be made to adjust for the additional 500 rpm's.
If this is not possible how would I identify the current electronic ignition now in the car and begin trouble shooting. Or should I replace the electronic ignition with one that is my suited to the tach?
THANKS
Rich
ricmgb

Rich - post a pic of the dizzi/amplifyer - or tell us where you found your engine. I suspect the answer may be a "New" tacho - I would be loathe to upset a system that is running well.
Roger
R Walker

Is this picture helpful?


ricmgb

One more


ricmgb

I am sure that someone will recognize and second guess which amplifyer coil arrangement you have. From that angle it doesn't look like UK kit. I am assuming that the box on the side is the amp and that there isn't another mounted on the leeward side ?

R Walker

If Nissonger are correct then you are getting three pulses for every one you should have. If the tach is being fired from the coil pulses as MGBs were originally, then that implies each cylinder is having its spark plug fired three times instead of one. That seems unlikely, unless you are getting three rapid sparks with the first at the correct time. Spinning the engine on the starter will all plugs removed may show this on a timing light. If the tach is being fired from a 'tach' terminal on a box of electronics then the box is probably faulty.
PaulH Solihull

Hi,

As Paul intimated, perhaps you have a multi spark ignition. These were developed to go with capacitor discharge system, so that, because of the very short pulse duration, there is a better chance of the fuel being ignited.

Herb
Herb Adler

I don't see a control box, and in his first post Richard says "electronic ignitions without a control box." So that would have to make it something like a pertronix (is there any other under-the-cap conversion unit?)

My guess is that the tach is [wrongmiscalibratedmalfunctioning].
Rob Edwards

Is there a box under the coil?
Need pics of distributor innards.

I've seen some very odd stuff with parts of different systems wired together.

FRM
FR Millmore

Hard to tell. I stared at it a while but couldn't be sure, though I'm leaning toward "no".

I agree -- pics of the innards would help!'

Also, I just noticed that my "pipe" (vertical bar) character got gobbled by the forum software in my post above, so read that as:
[wrong OR miscalibrated OR malfunctioning].

(Computer geek speak ;-)
Rob Edwards

Thanks for everyone's help. I'm out of house for the weekend, I'll try to get a couple of detailed pictures on Sunday.
ricmgb

This thread was discussed between 02/11/2011 and 05/11/2011

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