Howdy

I have a 67 ford mustang with a 302. I have the MSD 6A ignition box with an MSD distributor and the MSD coil. Whenever i start my car the tach jumps all over the place while its kicking over and then once the car starts and i release the key its fine. Also when i turn the car off the tach will sometimes stay where the RPMs were when i turned the car off. Its an old tach an autometer. It has done this for a while. I dont know what the problem can be. My car has been rewired with the Painless wiring harness and everything else works fine. Actually everything is brighter in terms of lights..its nice. Anyone else have this problem? Thanks

 

Does the tach work? And it it connected to the coil or the tach output on the MSD box? Either way if the tach works then I wouldn't worry, it's prolly just how the tach picks up signal. The MSD units create a series of sparks at lower rpm, so it may be that the since the tach is older it can't account for it, when you crank it's jumping all over because it's getting widely carrying spark rates as the engine cranks.

Some timing lights can have the same issue when used with CD type ignition boxes.

 

makes perfect sense thanks. ya i have it hooked to the msd box like it said which explains the jumping now that you pointed it out. Thanks again

 

That's also why MSD advises to not use adjustable timing lights, and to set timing based on total timing with the engine over 3k rpm, to avoid any abnormalities caused by the multiple sparks.