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I have an 87 pickup with a 22r in it (carb). I just installed an aftermarket tach. I plugged the signal wire into the open connector next to the coil. The tach reads fine at idle or if I rev in neutral but as soon as I put the truck in gear and put a load on the engine the tach starts jumping around. It is so bad I can barley tell what RPM the engine is running at. I would say it jumps +-500 RPM but only under heavy load, in neutral it is completely smooth and if I only give it a little gas while in gear the tach is smooth also. Anyone have any idea why this might be happening?
 

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I just installed a new long block and I am using the tach to help me with break-in, but I guess it isn't really necessary.

Thanks for the reply, I'll check my ground.

You do mean the ground for the tach and not my engine-chassis grounds right?
 

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I am having the same problem in my 67 Chevelle, it worked fine when I ran the stock distributor, but when I switched to a stock GM HEI distributor, it went haywire. I am thinking the ground could need to be moved to where there isnt any eletronic interferance.
 

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Bad contacts inside the dizzy, bad plug wire or wires as already stated, bad spark plugs. I think the ground is ok because you said it only did it under load.
 

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Just for reference - I've got the same issue. I started with a stock coil/igniter setup, I notice that I was having troubles with my inductive timing light - it wouldn't fire consistently.

I replaced the cap/wires/rotor and went to a slightly hotter coil.
Now the tach "wanders" when the key is in the ON position. It's *really* annoying..

I'd like to keep that coil, the spark is a lot better, especially for a turbo application.. Grounds are good.. I might be willing to even reground the tach, if it comes to that - any input?
 
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