Your Scitsu racing tachometer can tell you many things about the tune of your engine. Learning to understand the tachometer will help you understand your machine better and get better results from your engine.

Q) I have fitted a Scitsu tachometer to my motorcycle which is fitted with a magneto. The tachometer reading is fluctuating and inaccurate, what can I do?

A) There are several reasons why the reading on the Scitsu tachometer can be inaccurate.

1) The magneto could be in poor condition. For example if the slip ring is worn, the pickup will bounce over the brass contact and create a dirty spark or multiple sparks. The Scitsu tachometer is extremely sensitive and will read all sparks produced in the HT lead.

2) The voltage out put of the magneto may be too high for the Scitsu tachometer. The Yellow and Black (Magneto type) Scitsu tacho's are supplied with a variable resistor in the pickup lead which will need to be "tuned in" so that the correct pulse strength is received. (see image below this text)
This is because magnetos vary quite considerably in their output voltage and the Scitsu relies on a set voltage band width. Once tuned in it will remain stable and accurate.
Note: The adjustable resistor has an effective range of 18 - 20 turns. If either end of the range is reached, the adjusting screw will continue to turn against a clutch. It is therefore possible to "loose your way". If this happens, you are advised to cut the wire between the resistor and the tachometer and fit bullet connectors. You can then disconnect the resistor and measure the resistance and re set the resistor to its factory position of 50 - 100k ohms.

3) Your magneto may produce a "false or dirty" spark or it may produce a spark with two or more peaks. The Scitsu tachometer may read these peaks and interpret them as true sparks. To overcome this problem, it may be necessary to fit a capacitor between the tachometer and the pick up lead. You should try a 5, 10, or 100pf capacitor, depending upon how large the peaks are.

4) "Ringing" can occur. This is a situation where an unwanted rebound spark is counted by the Scitsu tachometer. Imagine a water pipe with high pressure water flowing through it. If you suddenly shut the flow, a reverberation will occur in the pipe. Electricity behaves in a similar way. If the voltage necessary to fire the spark is too high, when the spark finally jumps the gap a rebound spark can occur in the HT lead.
The voltage can rise too high if for example: The plug gap is too wide, the compression ratio is too high or the fuel/air mixture is too rich. If the problem cannot be solved by adjusting one of these contributing factors, it may be necessary to fit a diode between the tachometer and the pick up lead. You should try a 1N5408 diode.