Inexpensive Digital Tune-up Tach?

Brent <Brent.Evered.RemoveThis@gnb.ca> wrote:
 > Does anyone know of a source for a reasonably priced digital tach for
 > motorcycle tune-up purposes? Something that will be useful for air mixture
 > screw adjustment?

Buy a cheap digital VOM with a frequency measurement capability.
You can measure the AC output of the alternator to determine the rpms.
The frequency of the AC output will be a small integer multiple of the
rotational frequency of the engine. The exact multiple depends on how
many flux reversals per rotation there are. If the bike already has a
tach, you should be able to figure out the AC volts freq/revs per second
ratio easily.

If you have trouble figuring out what that ratio is, you can try wrapping
a few turns of wire around a spark plug lead and hooking that into the
line input of a PC's sound card. Look at the resulting WAV file with a
sound editor program like CoolEdit (any sound editor that is any good will
tell you time intervals, or you can count samples knowing the sampling
rate) and you can tell how many times the plug is firing per second.
Remember that most bike ignition systems fire the plugs once per rotation
rather than every other rotation.

Or you can just buy a cheap tachometer. But a DVOM with frequency
measurement capability will be useful for many other things.

--
Mark '01 SV650S '81 CM400T '99 EX250-F13<!-- ~MESSAGE_AFTER~ -->
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"Mark Olson" <olsonm.TakeThisOut@tiny.invalid> wrote in message > Buy a cheap digital
VOM with a frequency measurement capability.
 > You can measure the AC output of the alternator to determine the rpms.
 > The frequency of the AC output will be a small integer multiple of the
 > rotational frequency of the engine. The exact multiple depends on how
 > many flux reversals per rotation there are. If the bike already has a
 > tach, you should be able to figure out the AC volts freq/revs per second
 > ratio easily.
 >
 > If you have trouble figuring out what that ratio is, you can try wrapping
 > a few turns of wire around a spark plug lead and hooking that into the
 > line input of a PC's sound card. Look at the resulting WAV file with a
 > sound editor program like CoolEdit (any sound editor that is any good will
 > tell you time intervals, or you can count samples knowing the sampling
 > rate) and you can tell how many times the plug is firing per second.
 > Remember that most bike ignition systems fire the plugs once per rotation
 > rather than every other rotation.
 >
 > Or you can just buy a cheap tachometer. But a DVOM with frequency
 > measurement capability will be useful for many other things.

Mark:
Thanks. When I get home, I'll check to see if my VOM does frequency
measurements. If it does, I'll be back for a LOT of clarification. (I'm an
electridiot of the highest order)
Brent<!-- ~MESSAGE_AFTER~ -->
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"marksh" <me RemoveThis @here.net> wrote in message
news:bli289$50h$1@mozo.cc.purdue.edu...
 > You can buy an automotive multimeter with a tach setting at almost any
 > autoparts store for around $30. Just be sure you know how your engine
 > fires the cylinders. Some 4 cylinder bikes fire on both the power and
 > exhaust strokes, so the reading on the tach will be double what the
 > engine is actually running.

Mark (the other mark):
I'll have to have a look around in the parts stores, I guess. I've got a
couple of inexpensive multimeters now, but there's plenty of room in the
garage for another one!
Thanks
brent<!-- ~MESSAGE_AFTER~ -->
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