'93 5.7L Tach reading double after DUI-HEI install

New to me boat (1993 Cobalt 220) 5.7L carbed Merc w/Alpha 1 ran great for about 4-5 trips before a progressively worse miss/backfire issue. Was also very cold natured at start up. I suspected ign module but started with a comp test and blew water out of #7. Net net had blown head gasket and a wet corroded ign module on the back of the distributor.

Heads off to reveal .040 pistons and a pretty fresh looking bottom end. Had heads surfaced, valve job, new 4bbl Marine Edlebrock carb and Davis Unified Marine HEI. Figured at this point I would cover all my bases with fresh fuel, and spark.

Engine started on first rotation after assembly. Never ever did that before . Timing was 8* BTDC at 600rpm on the timing light. Then I noticed the dash tach showed 1200 rpm. Quick check on the back of the Faria gauge showed it was set to 8 cylinders. This gauge read correctly before the changes. Engine starts great, idles just fine but tach is off. Timing light with rpm read correctly on #1 wire.

This Dist set up only has two wires- a Batt wire the I connected to the old + Purple coil wire. A Tach wire that I connected to the shift interrupt and the old - Grey coil wire.

What would cause the tach reading to double?

Re: '93 5.7L Tach reading double after DUI-HEI install

Possible its picking up on the field collapsing and being energized. Did you connect the tach to the tach lead, or is it connected to the -(neg) of the coil? If neg of coil, your tach is picking up the field of the HEI, connect to tach lead and it should be ok.

Re: '93 5.7L Tach reading double after DUI-HEI install

No luck changing the tach settings to 4cyl or otherwise. It read about 500 rpm too low. 1500 actual read 1000 on dash.

As for the wires- there are only two wires on the dist a red BATT wire that is hooked to the purple + wire from the old coil. the other is TACH and that is hooked up to the grey wire that leads to the tach. According to the wiring diagrams I have out of Mercruiser service manual the grey wire goes to the tach on the dash as the signal wire. Looking at the back of the tack there is the same grey wire hooked to the SIG input. The coil is integrated in the HEI set up so there is no Neg coil hook up other than the Tach wire.

The only other thing I have in this hook up is the white/green shift interrupt wire. It is also hooked to the TACH wire as instructed by Performance Distributor rep I spoke with on the phone. Searching other forums before buying this dist I see that was also done by other. I sent PD an email late today and will call them tomorrow to see if they have an answer.