83 911SC runs poorly when tach connected

[Jim Dunham]

Crazy as it sounds, we installed a 1983 911 engine in a 1983 911SC roller we bought and the car runs poorly when the tach is connected. Disconnect the tach and it runs just fine. To make things worse, it's not always consistent when the tach is hooked up - sometimes it's OK but 1/2 the time it runs poorly when you try to accelerate (fine at idle). disconnect the tach and it's fine. Really stumped on what this could be - looking for any help on root cause. Have not been able to find a decent 1983 911 wiring diagram to sort out what the tach leads might be connected to. Thanks.

Jim

[proporsche]

Hi Jim
..wait a little Timmy2 will be here to help you..he is an expert..For me i wonder what is your connection back in the engine compartment with the back purple wire near the coil and fuse box..
ivan

[Jim Dunham]

So, tracing the SC wiring diagram it looks like the white w/red striped tach wire heads to the RPM limiter switch so wondering
if the RPM limiter switch is acting up (shutting fuel off) and the signal from the tach is "disrupting" it. Next we will try to "tape off"
the white w/ red wire and see what happens. Have to wait for the tach to come back from the rebuilder (it was found to be just fine).