Wiring Tacho to a twin

[SIII]

As some of you know, I have moved on from the gixxer and now on a Triumph America.
The Speedmaster, which is almost identical bike comes with a tacho, whereas the America does not. There is a nice space for one on the dash panel and a suitable one has been sourced from Ebay. (Triumph one costs £250ish !!!!!)


So my question - the tacho has a single feed wire, if I connect this to one of the coils, It will only read 1/2 revs, how do I connect a pair together?


If I connect them with a simple split wire, they will then fire the coils both together, as you are connecting them together. I have an idea of putting diodes in line, but unsure of the best way to do it.


So, oh mighty org, anybody better qualified than me in electronics please tell me best way to go about this ?????

[johnnyrod]

So when you say it reads 1/2, you mean you've tried it or you think it will?

[DarrenSV650S]

Is it an aftermarket one you got? It should have an adjustable adjustmentiser to adjust the adjustable adjustment, maybe

[SIII]

Not tried it yet (waiting on delivery from china), but lead to believe it will only read half speed, as only firing every 2 revolution, Because it has two coils and not just the one for both cylinders.


Tis just a budget Chinese tacho - no adjustable adjustment of the adjustmentiser. not much choice, because it gotta fit in the hole in the dash panel.

[ophic]

just stick a "2" over the "1", a "4" over the "2", etc

[johnnyrod]

Newer bikes with one per cylinder or built into the plug caps will still fire a cylinder once per two revs though

[ophic]

Originally Posted by johnnyrod

Newer bikes with one per cylinder or built into the plug caps will still fire a cylinder once per two revs though

This. Unless it's designed for a 2-stroke it should be fine.

I'm led to believe some 4-stroke engines fire the spark plug every revolution anyway. This could lead to the rev counter reading double if it's expecting a standard 4-stroke input.

[SIII]

Tacho needs a pulse every rev to give correct reading. Some systems do fire every rev and just 'waste' the spark, but believe not so on the Triumph parallel twins.


I believe I need to combine the two pulses to get a correct reading, and will hopefully confirm this once in hand.


So back to original question - how do I combine the signals without cross feeding to the other cylinder?

[ophic]

Originally Posted by SIII

Tacho needs a pulse every rev to give correct reading. Some systems do fire every rev and just 'waste' the spark, but believe not so on the Triumph parallel twins.


I believe I need to combine the two pulses to get a correct reading, and will hopefully confirm this once in hand.


So back to original question - how do I combine the signals without cross feeding to the other cylinder?

Found these arguments here: http://www.triumphrat.net/air-cooled...-question.html

Didn't read far enough to find out who was right or wrong tho. Upshot being, there might be a wasted spark so you don't need to do anything.

If there is no wasted spark and the tach you have has a "ht sensor lead" as some do, just wrap it around both plug leads, somewhere near the coils. It'll then get one spark per revolution. Cable tying them in place works well.

Failing that a couple of diodes off each coil, feeding back to the pickup wire will do the same job. This stops both coils firing together. Just make sure they're rated for whatever voltage it's peaking at.
C.