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Discussion Starter · #1 ·
I recently installed one of the Nova MMB 48mm tachs on my bike and have noticed some very strange behavior. What is happenning is that it is reading double what it should.

If I am doing about 2500 RPM it is showing 5000 RPM.

The wiring was very straight forward in that there is a single lead to the negative side of the coil.

Does anybody have any idea as to what is causing this and how I can fix it?

Thanks,
DLF
 

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I'm not familiar with that make/model but I looked at a web page describing the amount of impulses per revolution and it states a number of possibilities. My guess is that there might be a setting on the unit for the amount of cylinders or pulses per revolution. Here's where I got my info : http://www.mmb-instruments.de/VehicleInstruments/Technical/technical.html
 

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Discussion Starter · #3 ·
Yes, I saw the same article. It talks about ratios for the translation of impulses to RPM. On the back of the unit that I have it says that the ratio is 1:2 My guess is that it is taking each impulse and multipling by 2 to cause the double reading.

Unfortunately it is a sealed unit and I can't find anyway to adjust it. I sent a note to the manufacturer but so far no response.
 

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You running it on an old dual fire bike? Or an aftermarket ignition set to dual fire?
 

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Discussion Starter · #5 ·
It is a new Merch 120 with a new Compu-fire ignition module that is wired for dual fire.

With the Compu-fire module you can wire for either single or dual fire and I have gone dual fire.
 

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That might be the reason. If you had a siamesed coil with two signal inputs and you switched to single fire it should work. They run a bit smoother on single fire anyway.
 
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