Technical Tachometer

[AndrewHarvey]

Having collected a large amount of period gauges for my project I have a problem.
I have a four cylinder tachometer that looks the part - I'm aware that it will require a internal resistor change - does anyone have any information to aid this work please?

 

[MATTSDAD]

Hi Andrew,
it's very rarely I look at the forum as Matthew is on a house project at the moment and I've just noticed your post. It's something I looked into for Matthew.
You cannot make a 4 cyl tacho into a 2 cyl tacho by altering a resistor. Most older Tachos are current impulse types which sense the coil firing by passing a wire to the coil negative or CB terminal through a pickup coil.These are not suitable for modern electronic ignition systems. Later tachos are Voltage Sensing and are wired in parallel with the ignition coil. For a 4 cyl engine turning over at 1000 rpm there are 2 sparks per revolution therefore it is delivering 2000 impulses per minute to the tacho. A twin cylinder Fiat 500 produces one spark per rev therefore turning over at 1000 rpm will only deliver 1000 impulses so a 4 cyl tacho will read half the true rev speed and at tick over this reading will probably be inaccurate. What is required is an impulse doubler circuit. SPEEDY CABLES may do one.
Alternatively you could try one of these on Ebay
http://www.ebay.co.uk/itm/52mm-White-Waterproof-Petrol-Rev-Tacho-gauge-ideal-Kit-Car-or-Marine-/170996648094?pt=LH_DefaultDomain_3&hash=item27d031c89e.
This claims to work on 2 cyl and at that price is worth a gamble.
A pricier option but more retro is the SUNPRO FZ88R but I can't find a UK stockist and at this price you would definitely need to check with SUNPRO it would work with your ignition set up.
Hope this helps
Richard

 

[AndrewHarvey]

Richard
many thanks for your response...its great to share our expertise.
I have found a UK company to carryout the modification for a conversion thank you.
My gauge is about to return and I will report my outcome.

Many thanks once more

Andrew

 

[AndrewHarvey]

received my adjusted rev counter back today -great service!!
new pod ready to be fitted