measuring rpm, tachometer

How can I measure rpm on a Honda Express? I have an old car tachometer (measures 4, 6, and 8 cyl - I figure that I may have to do some conversions), but I don't know where to hook up the positive lead. Any ideas? Thanks.

Re: measuring rpm, tachometer

InfectedBootSector /

You should have 3 cables... one for the negative wire, one for the posi, and one signal wire...

Your tack may only have 2 wires ( In this case, the body has to be hooked to Ground)

Anyway.... hook the signal wire up to the coil....

I don't think that tach will work on your bike though...

Re: measuring rpm, tachometer

A car tach is going to be 12Volts DC... you ped is probably 6 volts... maybe even AC (I was surprised to learn recently)

soooo.... it might not work

You can get a ... Briggs and Stratton "Tiny Tach" for $16.90 from M&H Mower in Ohio... but I cannot tell you if it is 6V or 12V (or even if it might work either way).

I guess they are small... like a wristwatch LCD.

Let us know if you try it and it works.

Re: measuring rpm, tachometer

Fred wrote:


> A car tach is going to be 12Volts DC... you ped is probably 6

> volts... maybe even AC (I was surprised to learn recently)



Most peds use ac for the lights and horn because it is cheap. Most of them have two lighting coils in the magneto, one for the running lights and one for the stop lamp and horn. I am not sure what the peds with turn signals use for turn signal power, but I would expect an extra dedicated coil.

The coils are designed to reach saturation at fairly low rpm, this means that they are then putting out their maximum power and further increases in rpm do not change the output voltage. Each coil is matched to its load, consequently, if you lose a bulb, the remaining bulbs on that circuit get an increase in voltage and are likely to burn out shortly.

It is important to replace bulbs with the same wattage as original. A little higher will not hurt anything although it is likely to dim the lights a bit. Using lower power bulbs will reduce their lifetime.

The lighting coils are allways either 6 or 12 volts so standard auto bulbs can be used as the bulbs do not care whether you are using dc or ac to power them.

The ignition coil is powered by a separate coil in the magneto and I do not know how to tell if this is a 6 or 12 volt coil. As far as I know, when Motobecane went to a 12 volt lighting system, they did not change the ignition coil as it has nothing to do with the lighting circuit.

The ignition energizing coil is positioned in the magneto so that the energy from the coil is at its peak when the points open. The relationship between the magnets in the flywheel and the ignition energizer coil cause the coil output to increase to its peak, just as the points open. This means that the ignition system is actually running on dc. It just happens to only be there whem it is needed instead of all the time as in a battery powered system. How you relate this to a 6 or 12 volt coil beats me.

If I had to try, I would use a 12 volt coil first and if I did not have a fat enough spark, try a 6 volt. I am not sure, but I don't think coils used with magnetos, like lawn mower and ped coils are even marked with a voltage rating as they are never subjected to a continuous voltage like they would be in a battery powered system when the engine is stopped with the points closed.

As for tachometers, most good ones which connect to a power source other than the tach input (usually the points terminal of the coil) have internal circuitry which senses the voltage change when the points open, and converts that to a calibrated energy pulse. The closer together these pulses become, the more the meter deflects.

In theory, for testing, you can connect an auto tachometer to a 12 volt battery (you may have to ground the battery negative to the ped) and connect the tach to the coil just like in a car. The actual rpm would be the 4 cylinder reading * 2 as that range would expect the points to open twice each revolution.

If the tach uses an inductive pick up on a spark plug wire, the actual rpm is the meter reading / 2 because the ped is a 2 stroke and is producing a pulse every revolution. I would guess that the Briggs unit would read correctly because I think it runs the points from the crankshaft and sparks on every revolution.

Sorry to ramble on, I got carried away.


Re: measuring rpm, tachometer

hey ron! you're back! how was TX?

Re: measuring rpm, tachometer

I'm guessing, but your tach is probably independent of the basic operating voltage of the ped. It is just looking for a pulse from the low side of your ignition coil. Hook one lead to ground and the other to the low-side coil wire (it comes from the points). Try the polarity both ways. If you get a response, select the scale for the best range. To calibrate, find ratio of motor turns to wheel turns. Do this by marking your flywheel and your back wheel. Hold your clutch in and turn (take your sparkplug out first) your motor over counting the turns for one wheel revolution. Measure the perimeter or your tire and with these bits of data you can calculate the ratio of RPM to MPH. It is an exact ratio when your clutch is not slipping. Mount your tach on the bike and take a ride. Compare the speedometer to the tach and you have it.

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