I just finished installing new 500efi motors in my boat and took it out for the first time today and my port tach is jumping around is there a place that a wire is plugged in for this that could be loose causing this? Finding all the little bugs from the change over.

i had to replace my tach becasue it got some water in it and was doing the jumpping thing. at WOT i was bouncing around 7k haha

swap tacks,if its still their check alternator volts..suspect alternator feedback to tach and might be on its way out..

I was just thinking as they worked when I pulled the 496's that it might be a wire issue on the 500efi's didn't know if there was a tach wire on the distributor that could be causing it.

Could very well be the tach although they are only 2yrs old so I'll try switching them and see if it does it on the opposite side or not.

Check the ground.

Ground on gauge?

didn't see efi i replaced a alternator last week on a 365 mag,,tach was jumping sporadic idle (high & low)..when a alternator goes bad it will throw ac voltage,that's what was causing my issue's on a customers boat..

still doing this now on both motors

Check out my thread. Search for tach 500 efi. I would give you the link but I'm on my phone. Post #12 is what did the trick. Sounds crazy but it works. Call me of you have any questions. 864-420-4932

Here you go. Back home with a computer.

http://www.offshoreonly.com/forums/g...s-500-efi.html