electronic tach problems (over-reving??)

[fantasticmachine]

so we have an electronic tach on our gs1100 budget drag racer, it was originally a mechanical tach bike. the tach is a sunpro automotive tach, and the signal wire is connected to one of our green dyna coils. it has a switch on the back for 4-6-8... of course its on the "4" and i know its only sampling two cylinders since its only on one coil... but i dont think i could connected it to both coils because then it may not run at all (although to be honest i havent tried) at first i was convinced it was halving the rpms (saying 2000 at 4000 etc) but after a while i realized it was a waste spark setup and perhaps thats not the case (because each plug fires twice per revolution, someone else called it a dual fire setup but i dont think thats the right terminology)... well my problem is it falls on its face at 5500-6000 (according to the tach) and ive been trying to tune the high speed mixture to stop that. but perhaps im actually hitting 11000-12000 which is seriously overrevving it and causeing the valves to float? i havent ran it since i realized that, but im not sure how to test it... it just doesnt sound like 10000+ to me when its goin but i could be wrong and i dont want to risk my engine... does anyone have any suggestions?

[salty_monk]

You're probably supplying two signals per RPM rather than the 4 signals the tach requires. I had to set my Koso gauges as if they were a 2 cylinder to get it right from memory.

In that case your displayed RPM will be half what it should be. One way you could investigate is to change the setting to 8 cyl & see if it goes down by half again.

How about connecting up the mechanical one again to test against it in the shop?

Usually tachs are either set to go off of 1 plug (inductive setup) so 1 signal per rev or off the coil & as most cars have only 1 coil then it takes 4, 6 or 8 signals per rev.

Motorcycle specific ones can have a variety of different settings for 2 cylinder, 2 cycle etc etc