Use an analog tachometer?

Greybeard

Greybeard

New Member
I've noticed a few aftermarket analog mini tachs available for motorcycles.
They range from 8k to 10k units, are in the 2-2.5" size, and designed for V twins.
All of our two strokes, and probably many of the 4 strokes we use fire the plug every revolution. On my Honda one's a waste spark. That's how often a large twin fires as well.
The tachs require a "ground" signal to operate, and we don't have that, but we do have enough "signal" to operate some electronics without killing our ignitions or the mini digital tachs wouldn't work.

So here is the question. Can we not build a circuit with a transistor, where the transistor switch is going to ground. I don't know what the signal voltage from the CDI would be, but I'm sure a few resistors could drop it to where it wouldn't pop the transistor. Then, a 12 volt supply would need to be established to operate the tach. ??
 
All 4 strokes fire every other revolution. There are 4 phases to the combustion process, and its intake, ignite, exhaust, compress, but maybe not necessarily in that order. two strokes complete two phases with each stroke of the piston, which is why they fire every revolution. four strokes complete only one phase of the combustion process with each stroke of the piston, which is why they fire every other revolution. There is no such thing as a wasted spark, as every time the ignition fires, it is happening immediately after the fuel has been compressed. I think I understand the order in a two stroke, and im going to lay it out. if I am wrong, someone will correct me. on the upstroke, the fuel is being compressed as exhaust is being pushed out. (compress, exhaust or push/blow. on the down stroke, the compressed mix is being ignited and fresh mix is being pulled in right after ignition. (ignite, intake or bang/suck). on a 4 stroke, I am not sure how the order goes.
 
The 4stroke Honda 50 I have, fires every revolution. If it had a distributor running off the cam, it would fire every other rev, but it fires every time the flywheel magnet comes by the coil. When I say fires, I'm talking about the plug, not the engine. It occurs as the exhaust valve is closing and that spark is called a waste spark as it does nothing. The however part is, it gives a signall to the cdi as often as a distributor fired 4stroke twin, as does a 2stroke single.
 
I found the above tach, but thought it might look a tad large on a bicycle. Like the inductive pickup, and a 12 volt operating supply isn't difficult. If I were to start from scratch to build one, I'd use inductive pickup because it'll fit so many different engines and uses.
Found an Autometer Tach in a 2 1/2" face, designed for Jr. Dragsters. Picks up it's signal from the kill switch wire, but it's listed as Briggs only. Found someone who installed one on his Honda Ruckus, so it may work with our motors as well. http://www.jegs.com/i/Auto-Meter/105/6652/10002/-1?parentProductId=745408
 
Greybeard said:
The 4stroke Honda 50 I have, fires every revolution. If it had a distributor running off the cam, it would fire every other rev, but it fires every time the flywheel magnet comes by the coil. When I say fires, I'm talking about the plug, not the engine. It occurs as the exhaust valve is closing and that spark is called a waste spark as it does nothing. The however part is, it gives a signall to the cdi as often as a distributor fired 4stroke twin, as does a 2stroke single.
Click to expand...

At first I thought you were wrong, but after thinking about it, its actually logical that a magneto powered 4 stroke would fire every revolution, as the magnet does spin with every revolution, and there isnt anything to stop the spark plug from firing when its not needed.
 
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