View Full Version : tachos

jon_curtis - n/a
20-Nov-07, 06:21 PM
just wana get this clear in my head, might sound like stupid question, but dont want to buy the wrong tacho!

rotax 447, with tacho on 1 ht lead, (one spark every two revolutions??) thinking about it now, is it once every once rev?

briggs engine 12hp single, im farely confident is one spark per one revolution?

anyone have any imput regards ht lead triggered tachos? are they pants? how do they compair to a rotax (microlight) tacho regards accuracy?


profqwerty - n/a
21-Nov-07, 09:52 AM
I just bought a digital ht lead one. It's clip on really simple to use. 2 strokes are 1 spark/ 1 rev, 4stroke and 1spark/ 2 revs (on each lead).

so the 447 would be 1spark/rev on either lead. the briggs would probably be 4stroke(?) single cylinder, so would be 1 spark/2revs.

the one i have has a button to cycle through different engine types (number of cylinders, 2/4stroke).

however, it is set off by ignition leads nearby, so it would have to be connected to the pto ht lead, so it's away from the other one, otherwise you get double rpm reading (2 sparks/rev).

My one seems prefectly adequate, more than accurate enough! updates every 0.5secs.

but tbh, nothing beats a good old fashioned moving needle (that's just my own opinion).

Iceman - n/a
23-Nov-07, 08:36 AM
The number of sparks per rev varies with ignition systems, for Rotax electronic setups, there are two sparks per rev (per cylinder) but on points ignition its just the one.

Alot of tacho's are adjustable for number of cylinders etc etc and its fairly easy to tell when your rev counter is reading double or half (for example) so selecting the right setting without knowing anything about your igntion system is easy to do.

If you are getting erroneous readings due to intereference from adjacent HT leads, wrap the signal wire around both leads and adjust the setting to suit.