weird - any body seen this

[Scott S]

Hi All -
Took the car to a show this weekend - had a blast, car ran great, nice weather - fun day. When I drove down it was early and cool. On my way home my tach started acting up as the engine got warmer (200 degrees). I am running an MSD 6a with the appropriate tach adapter and a comp-u-fire in the dist. The tach started to get a bit jumpy as the car warmed up and when I was at idle at a stop light hitting the brakes caused it to drop at least 1k on the guage - but the car did not lose any rpm's and ran great. I started it this morning and it was ok again with the car cold. Any thoughts? My MSD wiring is very neat and all the connections are tight, and the tach was perfect prior to the MSD install.
Wierd stuff...
Thanks!
Scott S

[SteveStromberg]

Run a lead from the neg. post to the engine block. It sounds like a bad ground. Steve

[John Rogers]

I had the same thing happen with my 914 race car and the MSD tech asked me if I had the wires bundled tightly together for neatness and of course I said yes. He said to loosen the wires up and try to protect them from the heat so the insulation will not break down and the problem wiil go away. I seperated the wires so they only barely touch and have had no problems since. Good luck.

[ringmorph]

I have a msd 6a and I had a similar problem on my bug when I equiped it. The bug is gone now and a 75 914 replaces it. Any way I had a autometer tach and it was very eratic near idle to 2000 rpm but steady after 3000. It turns out the tensioner in the stock (205 vacume advance) distributor was two loose and the vacume plate was moving around uncontrollably untill it hit max advance. I had to loosen the scrow and slide the metal tab down to put more tension on the metal ball that supports the vaccume advance plate. That fixed the problem. BTW it seemed to run fine with the bouncy tach as you described but obviously the tach was sensitive enough to the changing advance.