Digital Tacho

Hi George,

Is it a mag or coil ignition? Plug caps suppressed?

Its possible the sensor (suggest it maybe some Hall effect device) could be picking up noise and translating that into psudo RPM.

HT is a difficult area and the pulse / wave forms can be erratic & noisy; for £20 squid, I doubt if this unit has professional digital filtering to clean up the signal:

Have a squiz here for some simple theory and good graphs:

https://www.picoauto.com/library/automotive-guided-tests/moto-primary-voltage/

https://www.princeton.edu/ssp/tiger_cub/library/ignition_waveforms.pdf

There was also some good theory and waveform analysis (and data) here:

http://www.brightsparkmagnetos.com/faqs/FAQs%20about%20magnetos%20generally/Are%20suppressors%20and%20resistor%20sparking%20plugs%20OK%20to%20use%20with%20magnetos.htm

High voltage engineering is not my area, but someone else may have an answer.

Good luck and see how you go

Rgds

Steve

Hi George,

I bought one last year from Hong Kong for £4.67 on ebay and was very surprised how quickly it arrived. There was no wiring to do except for winding one lead around the HT lead, so maybe it's the same unit where you just select the correct buttons?

I found that in practice and on the road, I couldn't read it properly as the digits change so quickly that it was almost impossible to apply a rev limit and it's so unlike a tach with a pointer. Anyway, after riding through a tropical rainstorm of biblical proportions for about 30 mins (south east UK) the tach went haywire and was consigned the bin.

Hi George

On a Honda classic racer, I have a Scitsu Tach. These are not cheap, but common on classic racers. Had some initial problems finding correct length of of pickup wire taped along spark plug wires. When I changed from half to full fairing, the tacho went beserk. Reason was that the pickup lead was too close to one ignition coil. Moving the lead some 4" away solved the problem. Had a problem with tacho on one race, borrowed one Chinese cheap one, worked perfect. As Steve hinted a resistor cap might solve the problem.

Good luck

Mike