The 70T runs again!! (also tach is flakey)
 

Well, I got the engine back in my 70T this weekend and got to start it up. (Intro thread http://forums.pelicanparts.com/porsche-911-technical-forum/272451-introduction-southeast-pelicans.html?highlight=intro+southeast ) The rebuild has been finished for awhile, but the college money and time budget wouldn't allow for installation until now. Everything seems to be running well and breaking in. Thanks go out to Wayne, Pelican, and the board for helping me get this project together. I do have a problem with the tach though. It reads far too high. At idle, it reads aobut 4k, and blipping the throttle swings the needle to 7-8k! I know it's not actually revving this high because I have a clip on shop tach that I checked it with, but I'm at a loss as to why the tach would behave like this. This car does have the ballast unit to reduce the positive coil post voltage from the CDI to the tach input level. Could a failure of the ballast unit cause this?

I forgot to mention this earlier. The PO replaced the stock ignition transformer with a bosch blue coil. Don't know if this would have any effect but I figured I'd mention it.

Ok, I did some asking around, and some people here at work have seen similar issues with Kettering ignitions when the dwell was misadjusted, causing point bounce and other issues. This car has the CDI unit though, but I guess the points could be causing the problem. Any thoughts?

+1 points

Just an update. I figured out what was causing my high tach readings. I replaced the Bosch Blue Coil with the proper factory ignition transformer and voila!, the tach reads correctly. I'm thinking that the inductance of the blue coil is wrong for the system and was causing some ringing, but regardless it's fixed.

That is true, the engine will run, but not well with a blue coil.