Tachometer Problems

Gentlemen,
   recently I installed a Holley Sniper EFI including HyperSpark ignition, coil, and distributor which are all basically MSD equipment.
After instillation the tachometer didn't work. Tach is wired G & 8 to ground. 1 to brown tach wire coming off MSD ignition and last post to switched +12V. I called tech support and they said for Ford I needed a tach/fuel adapter #8920. I bought and installed the adapter and the tach now works but RPM reads double the actual amount. Does anyone know of a resister or shunt I can install inline to split the signal going to the tach? Gary I know you once created your own ICVR.

I don't think a simple resistor or shunt can fix that problem. The issue apparently is that the ignition system is presenting two pulses per cylinder firing to the tach. Or what looks to the tach like two pulses. Perhaps it is actually firing the plugs twice?

I'd call the manufacturer again and ask how to solve that problem. If they don't know, then all I can think of is taking a look at the waveform being presented to the tach and seeing if there are spikes that can be filtered out.

Or, use a 555 timer to give one pulse per cylinder firing regardless of how many pulses the ignition gives. This spreadsheet tells me that if we make the pulse width one millisecond wide and then prevent the timer from firing again for 1 millisecond that the circuit would work to clean up the input spikes. But, the question becomes how far apart the two pulses are. If they are further apart than 2 ms then you'd still get the two pulses through.

So, I'd start with the manufacturer 'cause to design a circuit is going to require a 'scope trace to know if it is even going to work. And/or go online and Google to see if other are having that problem and, if so, how they solved it.

Gary Lewis wrote

I don't think a simple resistor or shunt can fix that problem.  The issue apparently is that the ignition system is presenting two pulses per cylinder firing to the tach.  Or what looks to the tach like two pulses.  Perhaps it is actually firing the plugs twice?
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Or, more? I found this:

MULTIPLE SPARK Under low RPM cranking conditions, the HI-6 generates up to 12 sparks. This assures quick starting even under the most adverse conditions. At idle and cruise, the number of sparks fired is adjusted to maintain a total spark duration of approximately 20 degrees (crankshaft), assuring smooth idle, improved throttle response, and eliminating the lean surge characteristic of some late model emission controlled vehicles. Above 3,000 RPM, the HI-6 generates a single powerful spark with many times the spark gap current of most competitive systems.

Then my circuit won’t work. That is far too many to filter out with that circuit.

They make a part to solve this problem and it doesn't solve the problem?  

I'd be calling them back and getting answers...