Tach works under hard acceleration, but reads high otherwise -- what gives?

[Rich911E]

I have a weird problem with my tachometer. It bounces around and reads very high (like twice the real RPM) under part throttle. If I mash the gas, it reads just fine. I have an Mallory Hy-fyre MSD style ignition box with a point type distributor. I tried wiring in the MSD tach adapter but that didn't work either.

Any ideas?

Alternator, voltage regulator???

Thanks in advance.

Rich

[Rich911E]

bump -- anybody?

[B D]

The "Mallory Hy-fyre MSD style ignition box" is a multiple spark unit correct? Does it have a wire or plug in for a tach?

[doozer]

could it be only reading incorrect when the ignition is not advanced / retarded?

Does the Hy-fyre produce 20deg of spark at low rpm and reduce to single spark at high rpm?

could be you connected to the coil output as a tach trigger rather than the dedicated tach output on the ignition box ( if it has one )

Hope this helps...
Todd

[jluetjen]

I'd double-check with the folks at MSD. It sounds like you have something wired wrong because your tach is reading the multiple sparks rather then the RPM. I had some questions when I was installing my MSD and they were very helpful and knowledgeable.

- John

[Rich911E]

Thanks for the suggestions.

The Mallory box is a multiple spark unit and has a line for the tach and that is where it hooked up. I am not sure about the advance characteristics. I didn't get a lot of help from Mallory with my last request by e-mail. Maybe I'll talk to MSD -- I am sure they know the unit as well.

Thanks -