Odd tach behavior after Pertronix install

[JasonB]

I'm hoping someone here has run into this before. Earlier this year I converted my old points/resistance wire/30 year old coil to a new Pretronix Ignitor 2 and a Pertronix Flamethrower coil. The upgrade is doing everything I had hoped it would. Before the upgrade, the tach would show about 4100 at WOT, which is almost perfect for my engine. GPS speed is not know for sure (bad memory), but I believe it was 45, but the speedo would show about 42.<br /><br />Fast forward a bit. With the pertronix, WOT now shows as 3200-3400. Idle is right where it should be. Max speed still registers on the speedo at 42 and the boat GPS speed is 47.***. I am still turning the exact same 19p prop and the same load. I ran the numbers through a prop calculator and to run the GPS speed witha 19p prop, I would have to be turning 4100-4200 rpm with 10% slip. With this and judging by the sound of the engine, I believe the calculator to be about right. I have also noticed that before the upgrade, at 30mph, I would be turning ~3000rpm. Now 30mph shows ~2800. It seems the tach gets more inaccurate as the engine revs.<br /><br />The ignitor is hooked up accurately as far as I can tell. I removed the resistance wire from the coil and ran a key switched verified 12v power lead to the positive off the coil and attached the ignitor to the pos of teh coil. The negative of the ignitor is connected to the neg of the coil. <br /><br />I am building one of Pertronix's filters like they use for the OMC shifters to see if it helps to hook the tach through it (Pertronix suggestion). Anyone ever see this before? It's like the tach is out of calibration (stock 1976 Mercruiser tach).

 

[loadnet]

Re: Odd tach behavior after Pertronix install

that IS weird?<br /><br />Will the engine rev to 4200 to 4600RPM out of the water?<br /><br />It be understandable to be doing 42 speedo at 3200 to 3400 - after the Pertronix install - if the engine's WOT was reaching 4200 to 4600rpm on a test drive, but because it is NOT pulling the even CLOSE to that at WOT I think I would have to start leaning towards timing advance maybe, or shortage of fuel..or something goofy like that, causing this dilema..<br /><br />If WOT out of the water is hitting 42 to 4600rpm,<br />I'd hook up an auxilary additional tach and go for another spin to see if the 2 tachs are matched RPM. or close to match.<br /><br />If it does match, or within 150 RPM I'd head for timing advance or shortage of fuel.<br /><br />Pull a fuel pressure or fuel volume test just to satisfy your brain that it's getting all it needs on load..<br /><br />Have you installed new plugs since putting in the Pertronix?<br /><br />Just my 2 cents.

 

[JasonB]

Re: Odd tach behavior after Pertronix install

I haven't changed anything else except installing teh pertronix and coil. She's still making the power to maintain the same or better speed on the GPS. I'll be pulling the carb in a couple weeks for a rebuild anyway, but I'm pretty sure the problem is with the tach since the top speed via GPS actually went up 1-2mph after the pertronix.<br /><br />I meant to try revving it out of gear today, but forgot. I may have to buy a second tach to get this figured out.