Michael,
Depending on what software and hardware version you have, the fix can be as simple as a jumper setting or as difficult as a PC board hardware modification.
Assuming you have a fairly standard DPU P/N 4010066 or 4010067 with software ver 1.00x or 2.0 and a Rev H v1995 digital PC board, then all you need to do is remove the bottom cover from the DPU, orient the DPU lengthwise with the larger blue capacitors to your left and look for a set of 2 jumpers in the middle of the PC board separated by a small black chip. The bottom most jumper (as oriented) controls the pulse count, it is likely set to A for mag ignition, if so move it to position B and you should get the correct RPM reading.
If my description does not match what you are seeing, then you probably have an older version that will require a hardware modification.
All the best,
Reggie
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