Bad tach causing a miss on an hp500 Carb engine
 

I have a sporadic miss around 2500 rpm on my hp500 carb engine. When the engine misses I also notice that the voltage guage draws down with the miss I remember reading somewhere that a bad tachometer could be the culprit causing this miss. Does anyone have any experience with this issue in which this was the cause?

i read that too ...

It happened to me. It took me a year to diagnose it because it was so intermittent. Finally engine acted up long enough to figure it out. Cut the tach wire and engine ran fine. Replaced tach, and I was back in business.

Mainly curious if there is a diagnosis that woukd rule out the tach, pick up coil, or coil...

Look for a grey wire on negative side of the coil, that should be the tach signal. Disconnect it and run the engine without the tach and see if the problem is still there.

If i disconnect the gray wire that will leave one pole on the coil completely unconnected.... i don't even think it will l run that way?

You can disconnect the gray wire, it just uses the coils pulse to signal the tach. It doesnt power the coil. Or, if easier, disconnect it from the tach behind the dash.

Not saying it the cause of your issue. But I've had it happen where tach was bad and caused ignition woes

Yep, buddy or mine (BadDog) has an HP500 clone in his boat, and had that very same issue. To this day, he is still running without a tach. Says he knows by sound and speed about where his RPM's are. :)

More info needed engine serial # why because do you have the overspeed assembly in your ignition set up ? Later HP 500 carbs apps did not have it. Older Hp 500 had them as they can be a problem child.

next ignition sensor under the distributor cap and rotor can be your problem. While you are in there, I would replace the cap and rotor if not already new and how old is the ignition coil - if 5 years or older I would change that to. They are maintenance items anyways

also make sure all connections are tight and clean form coil to tach and also all grounds especially back side engine grounds to.

The engines are 1999 model electronics/wiring. I do have the knock sensor setup. I pulled the cap yesterday and ran some fine sandpaper over the ignition sensor. After restarting it after this it seemed to idlena hair higher and smoother but this may have been a flook. The miss was still there. I'll check some more things out here when it gets light out and go from there.

Swapped coils and now it runs as smooth as a babies ass. I've never had a coil partially fail. It's always been all or nothing. Coil had a "98" on it which I can only assume was a year of manufacture, so I guess it shouldn't be a surprise that it was failing.

Thanks for the update. Posted what I did in post # 9 as I really thought your problem was ignition component related. Like I said cap, rotor, ignition sensor and coil basically have become maintenance items these days.

It really sucks but thats what its come down to I guess.... Either way, a coil is alot cheaper than alot of things!