67fish383S
Well-Known Member
So I am trying to run down an issue with my '71 Duster 340 4-speed. The problem is that the aftermarket original-looking rallye tach jumps quite a bit at high RPMs. It seems accurate at idle and low speeds, but whenever I get on it, it's all over the place. It will usually settle down at the higher RPMs when I am decelerating. I am going through the wiring trying to determine if the tach is bad and needs replacing, or if perhaps there is a simpler fix.
The ignition system on the car was inherited from a previous owner. It has an MSD 6AL box and a Blaster 2 coil. The distributor is vacuum advance with a magnetic pickup. The tach is fed from a dedicated tach output from the MSD box. I'm not at all familiar with the MSD system, but I downloaded a manual and it talks about some tachs needing an adapter. The first thing I wanted to try was moving the tach feed from the box to the (-) coil terminal to see if that improved anything.
BUT, the first thing i found when running down the existing wiring is that I am 99% sure the coil is connected backwards. There are two wires running from the MSD box to the coil. The black wire is connected to the (+) coil and the orange to the (-). Logic and the manual say this is backward, but the car seems to run fine. A little Googling on the subject yielded results all over the map. Some sites say it is the end of the world, others say it's no big deal. I tend to believe the second thing, but I also don't want to fix something that doesn't seem to be broken.
So this seemed like the perfect question for the brain trust here on FABO. I want to just reverse the wires and see what happens. Does my tach bounce go away?
Or does the car immediately burst into flames?
Any thoughts?
Thanks!
The ignition system on the car was inherited from a previous owner. It has an MSD 6AL box and a Blaster 2 coil. The distributor is vacuum advance with a magnetic pickup. The tach is fed from a dedicated tach output from the MSD box. I'm not at all familiar with the MSD system, but I downloaded a manual and it talks about some tachs needing an adapter. The first thing I wanted to try was moving the tach feed from the box to the (-) coil terminal to see if that improved anything.
BUT, the first thing i found when running down the existing wiring is that I am 99% sure the coil is connected backwards. There are two wires running from the MSD box to the coil. The black wire is connected to the (+) coil and the orange to the (-). Logic and the manual say this is backward, but the car seems to run fine. A little Googling on the subject yielded results all over the map. Some sites say it is the end of the world, others say it's no big deal. I tend to believe the second thing, but I also don't want to fix something that doesn't seem to be broken.
So this seemed like the perfect question for the brain trust here on FABO. I want to just reverse the wires and see what happens. Does my tach bounce go away?
Or does the car immediately burst into flames?
Any thoughts?
Thanks!
