Are plug wires new? Crossfiring, especially between 5 & 7 could also set off the knock sensor and cause a surge.

You have the Thunderbolt V ?
Oh boy, not only does it have knock sensors, but it has mean best timing. Wonder if that is causing your surging ?

BTW: To set base timing with these requires a different procedure than normal distributors.
See the V6/V8 Thunderbolt V procedure here:
http://www.perfprotech.com/blog/arti...eengine-timing

and:
http://www.boatfix.com/merc/Bullet/94/94_02.pdf

wires havent been touched this year. I have whatever ignition comes with hp500 in 1999. going back down today to check the timing. When i got the boat they didnt have it timed right, i had retimed it was only getting 24 degrees total. Thanks for the above procedure will try it later. i did ground the wire when i timed it. this new issue hasnt been happening very long week or so. thanks again will post results!

Swapped in a known good ign module same thing

Checked timing at dock have 35 degrees at 3500 rpm and held at 3500 to watch it. Of course the problem will only happen at cruise speed

Dam. Maybe try unhooking the knock sensors? Swap coils?

Trying that right now. I can't find the knock sensors because the stupid waste tank I can't lay infront of the motors. Where are they if your standing infront of the motor

Dang Jiff, you are putting more hours on your boat troubleshooting it than I have put on mine all summer! I admire your persistence. Good luck with it. Pretty sure the knock sensors would be located low on the side of the block, but I'm sure someone who knows for sure will chime in.

Maybe this will help - down low, in front of the starter.

http://ww2.justanswer.com/uploads/ol..._mercknock.jpg

Thanks unplugged knock sensor and it ran right for the first time was able to turn 5k and cruise at 3500 no surge for a few miles. Then on the way in it started again. In the process of swapping ign modules i unbolted the coil and move it around to get in there, swapping couils it the only think i have not done yet. will try a new coil and see what happens. I dont think there is anything left! I have eliminated just about everything.