tach related misfires

gdsmithmd · 05-14-2009, 10:48 AM

i have a 73 914-4 stroked to 2.2 that has been a great performer and generally reliable. Recently I started having the oft reported oscillating tach problem. i have used the crane/allison ignition in the past until it failed then switched to the pertronix ignitor and until recently the tach worked fine. if I hook up a dwell/tach meter to the coil it reads fine but direct connection of the car's tach doesn't. the weird thing is about where I start getting wild needle behavior (4K) I get misfiring. If I disconnect the sensor wire (BLK/PRPL) the misfire disappears. It seems as though I'm getting a short through the tach triggering the coil. I took the tach to San Diego Speedo tach and they couldn't find anything wrong. Any ideas?

Greg Smith, PCA SDR

SGB · SGB's Garage

How sound is the spade connector connection? Sometimes its the simple shi...

gdsmithmd · 05-27-2009, 08:23 AM

I take it you mean on the tach. It seems fine and I don't see how it might be grounding out, and if so why only at higher RsPM? I've been living without the tach (I can't really over-rev this engine unless i miss a shift). Thinking about either contacting pertronix or replacing it with points/condenser just to see what happens.

SGB · SGB's Garage

Actually, I meant the connections to the coil- either the wires to the connectors or the connector sliding/ contact area. The tac connects directly to the coil. I think it is the + side, but I haven't checked.

gdsmithmd · 06-03-2009, 11:28 AM

No it's to the negative, the positive's always hot. the point is if it were loose it would make a more open circuit not a short, which is what appears to be happening, because the misfire stops when I disconnect the tach.