The pulse (signal) from the HEI may not be strong enough to drive the tacho is all. The tacho may probably work for quite some time then stop. Some people have been lucky and not had to do anything to their tach to get it to work but if you're planning on keeping it for a while it would be worthwhile to get the tach modded to suit the High Energy Ignition (HEI) output. Occasionally you'll get one with a weak module which wont run the engine without the tach wire disconnected (rare, but have seen it a couple of times over the years). Usually the engine only just runs in that case....






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