I would have to ask first what kind of plug wires and what type of tach? If the
wires are solid core they make loads of RF noise that is hard to get rid of. My
preference for most any car is a set of wires with around 700 ohms per foot.
They are the best compromise between noise and power to the plug. I have run
turbo 4 cylinder 2 liter engine just under 800 hp with them so I suspect they
will not hurt the ignition output that much over solid core. All the MSD units
if you have one is designed for this type of wire as well and are labeled not
for use with solid core wire. If you are using a simple set of points and a coil
with the tach hooked up to the negative side of the coil, many new tachs an some
old ones can not stand the flyback voltage when the coil fires. GM had a bunch
of trouble with this and many GM cars from the 80's had a tach filter. If any of
the tach wiring is around a set of plug wires or the coil wire you are in
trouble as well.
You might start with going to a GM dealer an asking the parts guys about the
tach filter used with GM cars with the HEI ignition. My old fiero has one though
the filter is no longer made. I might be able to scare one up for you though if
you would like to try one. When it is bad the tach bounces all over the place
while the engine is running just fine. If you think it is a shielding problem I
have lots of shielded wire in stock for ignition pickups so if you need 5 or 10
feet I will send you some.
Dave Dahlgren
Richard Fox wrote:
>
> List; I have noted that Jerry (the driver) says the tach needle flops all
> around at speed. At one time with a different motor I taped over the tach so I
> wouldn't have to listen to things I knew didn't happen.
> But I got to wondering, with two coils very close to each other and the points
> firing within 5 degrees of each other, could the field in one coil collapsing
> affect the other coil right next to it? Should the coils be separated by a few
> inches? Anybody have a thought on this?
> Rich Fox
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