I just installed a 351C in my 73 Mustang. I haven't had time to tinker with it yet, but the RPM gauge on the speedometer isn't working. Can anyone point me in the right direction?

 

Stock tach?
Stock ignition system?

 

Factory tach goes to the posive side of the coil points type ign. aftermarket tach goes to the neg side if a points type dist
and
if a electronic ign, it probably won't work at all.

 

With a MSD, you will probably need a tach adapter. If you still have points, then there are two options depending on your type of tach. There are two different types of tachs...a current triggered or voltage triggered. Voltage trigger tachs connect to the negative terminal of the coil. Current trigger tachs connect to the positive terminal of the tach.In both cases, whenyou install electronic ignition like MSD, you cannot connect the tach to the coil because of the multiple sparks, so you either connect the tach to the MSD itself or a converter if that does not work. On my '69, the tach is current triggered, so it went orginally to the positive side of the coil, but now goes through a MSD tach adapter.