electric tach which fires every second revolution

[paul whittaker]

I am told that certain Virago tach's received their pulse from one of two coils. If so this would work fine on a limey twin. Anyone have definite info on which breeds and models use this system, and may be available from the wreckers?

[mike burke]

Hi Paul, I've heard similar (even works for twins like the cb350).

[mixstup]

Hi Paul

I have run FZ750,Honda V4 ,Early GSXR750 Tachs on the XS650 ,I have electronic ignition and the tach runs from the coils...make sure you rubber mount everything!

Mick

[racerchris46]

You might try a V35 magna tach. thats what i ran on my cb350 twin with seperate pickup for each cylinder.

[paul whittaker]

thanks Mick and Chris? I will look through the shop manuals for wiring diagrams. One of the problems with a salvage yard which has trays of instruments is that they are often not labeled. sounds promising anyway. Paul.

[racerchris46]

07-28-2010 53 small.JPG (30.43 KiB) Viewed 361 times

07-28-2010 56 small.JPG (24.76 KiB) Viewed 361 times

V35 tach picture off my 350

if I remember right the V45 tach will NOT work on a twin

black wire is power, green is ground, yellow to coil

good hunting

[Dave King]

Hi Paul,
I have a Magna tach that I tried on my old Norton special, it didn't work for me so I bit the bullet and bought a Scitsu.
I'll bring the Magna tach with me to Mosport, you can have it, free, gratis. If you can use it, good. If not file it under "G"
Dave.

[paul whittaker]

Hi Dave, Chris et al, I looked up a couple of Suzuki manuals and found wiring diagrams for certain models which show the pulse take off from one of two coils. The transition seems to have been directly after the switch from cable operated tachs. As you may have noticed this is some thing which a number of other racers are interested in. See you at Mosport albeit with a slower rig which requires a great deal of maintainence. thanks, Paul.

[mike burke]

... See you at Mosport albeit with a slower rig which requires a great deal of maintainence. thanks, Paul.

Hell, don't let Marie hear you talking like that...

MIke

[Brian FZR400]

I'm not aware of any *vintage* motorcycle tachometers that take a pulse every second revolution.

The Virago may have two coils, but it is still a waste-spark system that fires each coil once per revolution. There is no cam position sensor on that bike and there is no distributor driven at half engine speed.

I'm not knowledgeable about the old breaker-points ignitions, but the ones I've seen others tinkering with are on the end of the crankshaft driven at crank speed with no distributor, so those are waste-spark systems (one spark per revolution) also. I don't know how closely the changeover between cable driven tachometers and ignition-coil-driven tachometers coincided with the changeover to electronic ignition. If there is a bike out there in which the ignition stuff is driven by the camshaft (half engine speed) or by a separate gear driven distributor like you see on older cars, AND the tach was electronic rather than cable-driven, those are possible candidates.

Having said that ... I've been hearing that certain Yamaha R1 tachometers - after that bike changed over to coil-on-plug ignition - have the tach driven by an ignition-fire signal, once every two revolutions. I will have to ask the guy fiddling with adapting an R1 tach to an older bike, what year of R1 that the tach came from (although it may be pointless for a "vintage" application ). Nowadays, tachs are mostly driven by a dedicated communication signal from the ECU, rather than from an ignition-fire signal.

[paul whittaker]

Hi Brian, you are right in most cases, I had associated separate coils with revolutions without taking into account where the sender is mounted. Cam shaft mounted senders should = 2 revolutions and so if they are out there the tach's should do the trick. 1981 Virago does not. Nor would XJ650/750 which take the pulse from one coil. Maybe a versatile mechanic working with a variety of machines has come across something to fit the bill? Paul.

[Malcolm Lake]

I have two examples of a diode system that converts a single fire set of coils to a dual fire so you can use any cheap old tach. You connect one side to each of the coils and the other attaches to the tach trigger lead. Cost me about $20 each and would part with them for the same.

I'm not sure about the rig being slow.... sounds like Paul propaganda to me. Just have to get it to last is all.

Cheers!

Mal

[paul whittaker]

HI Malcolm, bring one along to Mosport its worth a try, of course if I start running on one cylinder there will be recriminations........... Paul.

[Janyz]

Hi Paul,

The issue at Quebec was not a diode but a 30 year old coil wire that decided to die/break on us!

That not withstanding, it does add another layer of complexity that may cause a malfunction.

We'll bring the diode sets to Mosport - it's up to you if you want to try them - don't pay unless you keep them.

BTW - we also have your replacement diaphragms...so no worries - Marie can't spring a surprise pregnancy on you.

Jan

[paul whittaker]

the simple two wires passing through diodes and into one adapter was the one thing that seemed to work well over the weekend. thanks, Paul.