merc 75 hp 3 cyl top end surge

My 97 merc 3 cyl 75hp has been in the shop twice already trying to figure out why it has a surge while wide open. The carbs have been done new plugs , wires, all factory settings checked. It idols fine goes through the mid range fine but when at the top it sounds like its fouling but will clean out and run like a sewing machine for a few seconds and then back to fighting itself dealer having trouble wondering if anyone has an idea, I think maybe electrical a coil pack breaking down under load? The boat used to go around 31/32 now 27/ 28 and just doesnt sound clean AROUND 4750 RPM INSTEAD OF 4900/5000 YOU CAN HEAR IT WANTS TO GO UP AND RUN THEN IT JUST FALLS BACK ,AN UP AND DOWN SURGE THAT,S MY BEST DISCRIPTION ANYWAY. IDEAS MUCH APPR!!!

Re: merc 75 hp 3 cyl top end surge

Did that shop actually water test this unit?If so they should have been able to at least tell if you have a fuel or ignition problem.How are those compression numbers?How is ignition output on a spark checking device?Vaccume into and pressure out of that fuel pump is what?

Re: merc 75 hp 3 cyl top end surge

If your motor uses the same fuel pump as a 94/95 there is a TSB out advising that the fuel pump be rebuilt using the new check valves that come with the kit. Your dealer should be able to check this. Seems ethanol gas causes the edges if the rubber check valves to curl up. The replacements are all plastic.

Re: merc 75 hp 3 cyl top end surge

yes they tank tested it but told me it did not sound off under full power but on the water it sounds alot different the compression was good in all cyl, dont know the press off the fuel pump but will have them check it. Thanks

Re: merc 75 hp 3 cyl top end surge

the motor is a 97 and as far as I know they have not tested fuel pump pressure that sounds like a good place to start. Thanks