I have a 2005 Merc. 90hp two stroke on my pontoon. Never had any issues with it at all until last week. When I went to full throttle it starting surging...I could watch the rpms surge back and forth from 3500 to 5500. Figured I have issues on one cyl. Changed plugs and took for a test run. Ran like a top, better than ever. Went back out the next day..same thing...back to the surging at Higher RPMs. I pulled plug wires off of the top two and they jump spark all over the place. Pulled the bottom one off and not jumping fire from a distance like the top two but jumps when I but the boot back on. Put a new coil on and same thing. Also my surging issue seems to be at higher rpms and not really lower ones. Could this be a coil issue?
Thanks
Ken

more likely a power pack problem.

Switch around the coils with a new one etc..and no difference. What I did notice after running it was that if you feel the rubber boots on the plugs the top two were hot, bottom one was not. I think the bottom cyl. is where my problem is but changing the coil out did nothing. So that would have to take me back to the power pack right? Also there are two larger yellow wires coming out of the pack and both of them look burned or have gotten very hot at the clear connectors. Still making contact but Im wondering whats going on there?

Go to www.bassboatcentral.com and post your problem on the Merc board ..You will have to post your serial number to get an answer.. The burnt yellow wires sounds like the voltage regulator... Mercs don't have a power pack.. they have a switch box...Good luck ...hope you get it fixed...

Sometimes you can reset the ignition control module by disconnecting it while motor is running, then plug back in and re-crank. That motor also uses CDM modules instead of coils and you cannot test correctly by pulling plug wires. Needs to be plugged to a lap top. I agree the yellow wires sound like the voltage regulator connections.