Tach – Single-fire vs Dual-fire

If you are adding a tachometer to your bike, you need to know if your ignition is 𝙨𝙞𝙣𝙜𝙡𝙚-𝙛𝙞𝙧𝙚 𝙤𝙧 𝙙𝙪𝙖𝙡-𝙛𝙞𝙧𝙚.

Rebel 300/500 is single fire. Modern bikes using an electronic ignition control system are single fire system. Single fire means the spark plugs ignite once ever 4 strokes of the engine.
The older Rebel 250/450 are dual-fire. Dual fire (wasted spark) means the spark plugs ignite once during the combustion stroke and once during the exhaust stroke. (Twice per every 4 strokes)

Dual fire (wasted spark) is the most simple way to run the mechanical ignition system used on older engines like early Harleys before electronic ignition control was being used.

A dual fire tach would read 1/2 RPM on a single fire system.

2013 Fury – single fire
VTX1300/1800 – single fire
2006 Kawasaki Vulcan 1600 – dual fire

1500/1600 Vulcans use a dual fire or ‘wasted spark’ ignition system that provides a pulse from the coil every revolution of the engine.
Vulcan 900 and 2000 engines are ‘single fire’.
Triumph, written Dec 2012: “360º Bonnies T100 & Thruxtons are dual fire wasted spark systems with a single, double-ended, ignition coil.
Scramblers, America & Speedmaster are single fire as they have 270º cranks, with twin single-ended coils.”

Most carbureted HD engines are dual fire.
All Twin Cam & fuel injected Evolution engines are single fire. All carbureted Evolution engines are dual fire.
KZ 440 360 crank dual fire
Yam XS400 twin with 180 crank – single fire?

SINGLE CYLINDER bikes – mostly single fire? By gearing the cam for half speed it can fire only on compression stroke – single fire.
There is no reason for a properly designed single cylinder motorcycle with a single spark plug to have either a “dual coil” or a “dual fire coil.” Back in prehistoric times when engines had points, it was cheaper (and simpler) for the factory to fire the spark plug as the piston approached TDC. One cam rotation equals one spark vs two engine rotations for one cam rotation. A 50+ year old Norton single has the points geared so that the plug only fires on the compression stroke. (Those Brits did not want to waste anything!) It may well be cheaper to fire the plug on both strokes with today’s electronic ignitions, but not better.

Distributors turn at 1/2 crank speed so they only fire on the correct cycle, crank triggered systems use the computer to “turn off” the spark on the exhaust stroke. All direct injection systems are already designed to recognize what stroke the engine is on so it never allows the ignition to fire twice as many times as required.

If you used to ride old British bikes (or American bikes as well, with magneto or point systems, the ignition system was driven off the cam and ran 1/2 engine speed, hence no “extra” spark. Only in crank shaft magneto systems did you have one spark per revolution. Like my Husky TE510, an extra spark on exhaust stroke.

Some bikes, notebly old Honda twins, had spark coils with a spark plug driven off both ends of the secondary coil and the major energy went to the cylinder with compressed charge and the other cylinder looked like a dead short and got trivial energy.

Some say “dual fire (wasted spark) is a Harley thing”, because other manufacturers found a workaround.
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People get this mixed up all the time. The number of spark plugs or ignition coils used has nothing to do with single fire or dual fire. Our bikes use two spark plugs powered by a single ignition coil, per cylinder. But this does not indicate if it’s single or dual fire.
Single fire means the spark plugs ignite only once, which happens roughly during the beginning of the combustion stroke. (Once per every 4 strokes)
Dual fire means the spark plugs ignite once during the combustion stroke and once during the exhaust stroke. (Twice per every 4 strokes)
Dual fire systems don’t provide any more power so there is no advantage, it was just the most simple way to run the mechanical ignition system used on older engines like early Harleys before electronic ignition control was being used. If your bike is using an electronic ignition control system, as all modern bikes seem to, then it’s safe to say it’s a single fire system. Some electronically controlled systems in automobiles fire during the exhaust stroke to help burn up any remaining unused fuel but this is merely for pollution control purposes. Since motorcycles don’t even have catalytic converters I don’t see why they would bother using dual fire for this purpose.

I know for a fact that all Shadow Spirit/ACE 750 models from 2001 to 2007 (non-shaft) are dual spark plug, single fire systems. I don’t see why the C2 model would be any different.”

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The wasted spark system (DUAL FIRE) that he is talking about is actually very common.
It’s used because it’s VERY inexpensive to use.
On a multi-cylinder engine, you can use one coil to fire two cylinders.
The way it works is that one coil will fire both cylinders at once,
one will be at TDC compression and the other will be a TDC exhaust.
Our Shadows do NOT have a wasted spark system – it is SINGLE FIRE
Many people mistake it for a wasted spark system because it’s got two coils.
However, in a wasted spark system, as I said, one coil will fire two separate cylinders.
If you look at our setup, both plug wires from one coil go to the same cylinder.
Both plugs on one cylinder fire at the same time and the other cylinder is stagnant.
The Shadows do not have a wasted spark

I know for a fact that all Shadow Spirit/ACE 750 models from 2001 to 2007 (non-shaft) are dual spark plug, single fire systems. I don’t see why the C2 model would be any different.”

that came from here: http://www.shadowriders.net/forums/showthread.php?t=18868

but then there is still NRALuke above with the 2008 Honda Shadow Aero VT750 haunting me with the BC Dual-Fire Tach that worked without an adapter.

Greg, All i can state is that in the add where i bought the tach it stated it was for Honda shadow aero 750 2 cyl 4 stroke 1000rpm=16.6HZ and stated in add it was a dual fire although thinking about it, logic tells me that a dual fire tach would read 1/2 RPM on a single fire system. Like i told you in Pmail, my hookup instructions are the same as the one you bought on ebay but the tach you bought states for Harley, don’t know if there is a difference there or not.

If i’m not mistaken the Shadow fires on both compression and exhaust strokes. Some call it a wasted spark system.
Thats what I have understood..two coils each connected to two plugs..first coil connects to and fires one cylinder at the top of the compression stroke and the other cylinder at the exhaust stroke, it fires both plugs at the same time, the other coil does the same for the other two plugs..I think the idea is to burn any residue gas left behind after ignition.

So each coil fires two plugs at the same time but not in the same cylinder..

Here is a thread from this forum that at the end says they are dual spark.
http://www.hondashadow.net/forum/72…1252-do-i-have-dual-fire-ignition-system.html

“People get this mixed up all the time. The number of spark plugs or ignition coils used has nothing to do with single fire or dual fire. Our bikes use two spark plugs powered by a single ignition coil, per cylinder. But this does not indicate if it’s single or dual fire.
Single fire means the spark plugs ignite only once, which happens roughly during the beginning of the combustion stroke. (Once per every 4 strokes)
Dual fire means the spark plugs ignite once during the combustion stroke and once during the exhaust stroke. (Twice per every 4 strokes)

I know for a fact that all Shadow Spirit/ACE 750 models from 2001 to 2007 (non-shaft) are dual spark plug, single fire systems. I don’t see why the C2 model would be any different.”

I agree with the definition of single fire and dual fire.
I don’t agree that the Spirit is a single fire system.
The ignition pickup is off the crankshaft so the ignition module only knows when the piston is at the top of it’s stroke, not whether it’s at compression or exhaust.
The single fire system needs a cam shaft sensor which according to the VT750DC manual the Spirit does not have.
The cam shaft sensor is used to determine whether it’s compression or exhaust stroke.
That said, the 2012 model might be single fire since it’s fuel injection.
From what I understand fuel injection systems have cam shaft sensors.

I think that the only Shadows that are really single fire would be the ’85-’86 VT1100s which have two crank pulse generators PLUS a camshaft pulse gen which is used to tell compression stroke from exhaust stroke – the manual even states the cam pulser is to kill the spark on exhaust stroke. All of the other models until they switched to digital ignition are most likely “dual-fire” = spark plugs fire on every “up-stroke” whether compression or exhaust.
BTW, a wiring diagram will not show the difference…

My two pennies,,,There is no advantage to power or anything that has to do with performance on a dual fire system, meaning the spark fires at the end of the COMPRESSION from about 12 Degree’s BTDC at idle to I believe 32 degree’s max advance BTDC at high rpm varies somewhat model to model. THEN IT FIRES AGAIN on the exhaust stroke. That is only an attempt to burn unused fuel for cleaner EPA reasons.

If your tach you bought was for a “dual” fire ignition, fires on compression and exhaust, and it is reading half the rpms it should then you have a single fire ignition.