Upgraded to HEI and now the tach jumps around

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mad accountant

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I upgraded my ignition to HEI over the weekend, and now my tach jumps through a range of 800-1000 RPM. Tach is an old Sunpro, connected to the negative post on the new coil as it was with the electronic ignition I replaced. Tach was rock steady with old ignition. What other information does anyone need to help diagnose and/or what should I look for?
 
I know it shows on some of the Ford Duraspark conversions a diode between the negative side of the coil and the HEI module. I think that's for spikes to the tach.
 
What was the original idle speed? What happens at higher RPM?

Just guessing because lack of information. It could be too much gap in reluctor, or wiring from pickup, where the path is too long, to close to plug wires, or no twisted pair. A look with a scope on the (-) terminal would provide certainty.
 
KitCarlson said:
What was the original idle speed? What happens at higher RPM?

Just guessing because lack of information. It could be too much gap in reluctor, or wiring from pickup, where the path is too long, to close to plug wires, or no twisted pair. A look with a scope on the (-) terminal would provide certainty.
Click to expand...

Original idle was 750. The tach acts the same at any RPM. Wire I'm using is the same as with the electronic ignition I replaced but routing has changed a bit so I'll check that out and maybe re-reroute it. What do you mean by twisted pair? Could the same reluctor gap make a difference between 2 different ignition system?
 
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In the route you want to keep as short as possible and the wires together and away from other wiring, especially plug wires. The twisted pair helps insure that they are together. The concept is if there is noise, and the wires are close, like a lamp cord with twist, the noise is canceled, positive one way negative the other, the sum is zero.

You could also have the pickup polarity reversed, causing significant running issues. But you have not told us what happens at higher RPMs. Likely if the tach is jumping the ignition is not correct, and may be doing extra sparks and missing others. It would feel rough in running, a miss.
 
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KitCarlson said:
View attachment 1714963702 In the route you want to keep as short as possible and the wires together and away from other wiring, especially plug wires. The twisted pair helps insure that they are together. The concept is if there is noise, and the wires are close, like a lamp cord with twist, the noise is canceled, positive one way negative the other, the sum is zero.

You could also have the pickup polarity reversed, causing significant running issues. But you have not told us what happens at higher RPMs. Likely if the tach is jumping the ignition is not correct, and may be doing extra sparks and missing others. It would feel rough in running, a miss.
Click to expand...

The tach acts exactly the same at all RPM, engine runs great.
 
I am not sure what exactly the same means. Same as before HEI, or always waver of 200 RPM.

Tach basically counts ignition pulses, more pulses per time, greater the reading. A waver means noise is making more pulses, but only for some cylinders. That suggests some plug wires could be too close. At some point in higher RPM, the additional false triggering, will waste time in coil charge resulting in weak spark.
 
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