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How can I measure my engines RPM?

Joined Dec 2003
169 Posts | 0+
Rockford, Il.
To any member,
How can I measure my engines RPM? Is there a multi meter or small hand held device I can use. Would like to be more exact when tuning. Appreciate any help.
Thanks,
Mackberg :wink:
 
Many multimeter companies make meters aimed a tuning petrol engines. They generally have most of the basic functions (voltage, high current, resistance, continuity etc) of other multimeters as well as a couple of extras good for petrol engines. Duty cycle, dwell angle & tachometer are usually available. If you want to fix electrical problems a multimeter is a must.

The tacho signal on older meters is a bit useless on CDI engines as it requires connection clips onto the primary side of the coil - same place you would be measuring dwell - but this is only works for points or transistor assist ignition. The coil signal on CDI is differnt.

Most later ones use an inductive pickup for the tacho signal. The pickup is a big clamp that clips around the spark lead but does not accually connect to it electrically. It senses the spike in electric field around the lead when the plug fires & sends this back to the meter. This is how the meter that Dale listed will work. You want one of these as there is no physical connection necessary to the bike to make them work.

Also, if buying a spark timing light, the easiest ones to use have the same type of inducive pickups.

Just be aware that many meters will have a selection switch that is marked "2 stroke" or "4 stroke". This can be misleading. What it is really selecting is an internal calibration value that corresponds to 1 spark per crankshaft rev (2 stroke) and 1 spark per 2 crankshaft revs (4 stroke).

All engines that have the spark timing sensor on the crankshaft (like all 'bergs) spark once every crank rev so the meter must be set to "2 stroke", even if the engine is acutally a 4 stroke. 4 stroke engines that have the spark timing sensor on the camshaft (like most old Honda singles) spark once every cam rev or two crank revs so the meter must be set to "4 stroke".

Some can be confusing by having the CDI charge coils on the crank, but have the CDI spark timing coil on the camshaft.

Some cheaper meters don't have a selection swich at all so for some engines you have to do a divide or multiply by two to get the right reading.

The meter I use is a Fluke 178 Automotive multimeter. With this one you have to buy the inductive pickup as a seperate item. Fluke stuff tends to be expensive & I would only recommend it if you use it for a living.

I have heard of small battery powered tachos built especially for bikes. They are suitable for mounting permanently up on the bars and use an indutctive clip on the plug lead. I suspect the one Dale recommends from Sendec is like this. Sorrry I don't know any more details abaout them.
 

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