Tachometer Bouncing Causing Engine Surging

[Kola16]

Hello all. I need some schooling on how the RPM signal is generated. I just replaced the carburetor with a Holley Sniper EFI system on my Mercruiser 260 setup (old carbed-style GM 350). My tachometer and the Holley Sniper unit will jump up and down in RPMs when that is not what the engine is actually doing. From 800 RPMs to 4000 to 400 to 2500 and all over the place.

I have a Pertronix Ignitor in my distributor that runs to the positive and negative side of my coil. When I had the carb on, it did not matter what the tachometer was reading since the fuel delivery was regulated by the air flow through the carb. But now the negative side of the coil is where the Holley Sniper system gets the RPM signal to inject fuel into the system. Because the tachometer bounces periodically, the Holley Sniper thinks it needs to inject fuel when the tachometer goes up, even though the throttle is at idle.

The Holley Sniper has its own display with an RPM reading and it bounces around concurrent with the tach on my dash. When the tachometer is not bouncing around, it runs great.

The tachometer only just started jumping around at the end of last summer when I had the carb on it. It seems to stop bouncing around after it has been running for a while. I tried unplugging the tachometer on my dash to see if that helped and it did nothing. I put new plugs and wires in it and nothing changed.

So how does the RPM signal work? Every time a spark goes off in each cylinder it counts or something? Could it be missing in a cylinder?

How is the system grounded? Can I run a ground from the negative of the coil to my common ground or is that a bad idea?

Could the Pertronix Ignitor be sending a bad signal? I am not sure how the Pertronix Ignitor works...

I'll get some pictures and video up here by the end of today. Please help! Thanks!

 

[alldodge]

Have some conflicting statements, or at least how I'm taking it
The Tach bounces but the motor RPM doesn't actually change
The Tach bounces but the motor RPM does change from the sniper fuel supply

Fist the Tach gets the signal from the Neg side of the coil, which is when the Petronix switches and grounds that side of the coil which cases the coil to spark the plug

So I would say its the Pet since both are getting the same signal

 

[Rick Stephens]

Is there any tuning available for the Sniper in a marine deployment? Just wondering how well that works. Automotive fuel flows are massively different from marine.

 

[Kola16]

Have some conflicting statements, or at least how I'm taking it
The Tach bounces but the motor RPM doesn't actually change
The Tach bounces but the motor RPM does change from the sniper fuel supply

Fist the Tach gets the signal from the Neg side of the coil, which is when the Petronix switches and grounds that side of the coil which cases the coil to spark the plug

So I would say its the Pet since both are getting the same signal

I see the confusion I created. When the carb was on it, the tachometer would bounce around but the actual engine RPMs did not change. The tach would just momentarily read wrong.
With the sniper on it, the tach will bounce and the Sniper will inject fuel thinking it needs to for the increased RPMs, which does change the RPMs a little bit, BUT the tach will momentarily say something like 4000 RPMs, when really the engine is only at maybe 1500 RPMs. So my problem is a bad tachometer reading at the negative post of the coil. And I'm guessing it is the Pertronix Ignitor thats causing it? I just hate throwing parts at stuff haha.

 

[Kola16]

Is there any tuning available for the Sniper in a marine deployment? Just wondering how well that works. Automotive fuel flows are massively different from marine.

It says it is self-tuning so we will see if I get the darned thing running properly. I can manually change the AFR per the RPMs too. It has Coast Guard compliance so I'm hoping it works.

When the tach isn't on drugs it sound great on the hose.

 

[alldodge]

With Tach not reading correctly, disconnect it. The Tach may be what's causing the issue and not the Pet

 

[Bondo]

The Holley Sniper has its own display with an RPM reading and

Ayuh,.... Unhook the dash tach from the coil, 'n try it,.....

 

[Kola16]

I tried unplugging the tachometer on my dash to see if that helped and it did nothing.

I mentioned I tried that in my long-winded post, but good suggestions.
I found the problem though! The positive side of the coil was only getting around 9.5 volts! The Pertronix wants 12 volts. The positive side of the coil is where the Pertronix gets its power from. I do not think there is a ballast resistor inline, I think it is just the resistance of the old wiring in my boat that drops the voltage. That is why the tachometer would work as time went on since the voltage would go up with the alternator charging the batteries. Anyway, I ran a jumper from the battery to the positive side of the coil, and it ran great! Thanks for the suggestions guys! I guess I should rewire the rest of my boat too

 

[alldodge]

The boat had points at one time so it has a resistor wire.

Find the purple wire at the 10 pin connector or purple wire at the alternator excite and splice in new wire to coil