tach jumps around
 

Can someone please help? 1985 rx7 gs and the tach jumps around when at idle. Once you press the gas the tach responds appropriately at the appropriate level. example. at 1100 rpm idle the tach will jump up to 3000 and jump between 3000 and 5000 will no increase in engine speed.

Any ideas how I could fix it?

It's just annoying...

The tach runs off of a wire on the coil closest to the front bumper, clean both terminals on both coils to be safe and see if this fixes your problem.

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Also check alternator and battery

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Interesting ideas, I think it will help