Diputado

Folks,

I posted something about this about a year ago, but have NOT yet got this solved. My '65 289 has a Duraspark II ignition, and I've got an Autometer Model 2895 tach hooked up. Ever since installing the tach over a year ago, it has never given correct readings. It seems to read about 1/2 the actual rpm. For example, at an idle of what I KNOW is around 900-1,000, it will read about 400-500. I'm running a 3.50 rearend with a wide-ratio Toploader, and the tach reads exactly 1,000 rpm at 38 mph in high gear....no way! However, as the rpm increases, the "error" seems to diminish, as I have shifted at over 4,000 rpm (according to the tach) many times, and if it was reading only 1/2 actual rpm that would be impossible. I have called Autometer tech support on this to make sure I have the wiring correct, and they even swapped out the tach for another of the same model, but with the same results. ANY IDEAS??????

kenash

Hi,
Don't they have DIP switches? Are you sure they are set correctly?

Gun Jam

My initial thought was wrong cylinder selection....but if that was the case the error would be off exactly the same amount at any rpm it shouldn't become less.

Does anyone near you have a timing light with a built in tac or a plug wire run tac? It would be interesting to see how they compared.

Diputado

Yeah, that's the first thing I checked. But even if it was wrong cylinder switches, with an 8-cylinder it would actually read MORE rpm....given your other selections would be for a 6 or 4-cylinder. The timing light suggestion is good...but this engine has no timing pointer. I actually asked the PO who built it how he timed it without one, and he just said "by ear". To "thicken the plot", I've also noticed that the windshield wipers work fairly normal (although slightly slow) with the ignition in the "on" position and engine off, but while it's running they just creep very slowly across the glass. I've checked the ground cable from battery to engine block, and it looks fine and snug. I also installed a ground strap (copper factory orig) from one of the heads to the firewall (it didn't have one), but still no change.

Chromeshadow

Do you have a DMM? Can you measure at the negative side of the coil with the key in the run position?
The original ignition had a current limiting resister in series with the coil, most analog tach require this to remain to give the correct reading. Sounds like someone replaced it with a higher value. You could have a problem with the switching on the Duraspark. Some analog tachs don't measure speed of the ignition pulse, they measure the length of the ignition pulse(dwell)and covert it to an RPM value. If the Duraspark has a sloppy switching pulse it might affect the reading. You could tell by putting a scope on the negative side of the coil and look at the pulse.