1999 90hp Mercury 2 stroke- Dropping RPMs

[99tracker90]

We have had an intermittent issue with the engine running fine and then dropping rpms in the higher range.
Friday changed the 2nd and 3rd CDM packs. Saturday we took it out and it ran great, cruising at 4000 to 4500 rpm down the intercoastal, no more surging or loss of power. Cruised around for an hour or so. We put the tube in and had two people on it, then switched a person out who weighed about a 80 more pounds. A few minutes into the second tubing event, the surging/ drop in rpms came back. We pulled tube in and headed back. Engine was still acting up so we shut the engine off and inspected the prop to see if anything noticable was wrong. All checked out. Start it up again and it runs fine.
Any ideas on what is going on here? Not sure what else to look at. This is super frustrating!! Help...

Items replaced thus far:
2 batteries with battery switch, all fuel lines from tank to engine around the fuel filter including primer bulb replaced, fuel filter, 3 spark plugs, 3 CDM modules, starter solenoid, fuel diaphragm, impeller.

Tracker party deck 21"- Boat has been out every week or every other week all summer. Fuel is not old.

 

[Shawnee]

Re: 1999 90hp Mercury 2 stroke- Dropping RPMs

Not a mechanic and new to boats but have worked on a few engines of other types. Could it be the vent in the fuel tank? ie cannot draw air? Try running it with cap ajar or when problem occurs, loosen gas cap to see if you hear a "hiss" as air enters fuel tank, then see if it makes a difference? Another thought is check spark when problem is there, put a timing light on plug wires and see if you are getting a pulse? Only other thought is something floating around fuel tank and occasionally, partially plugs fuel line pickup? Is fuel mixture right? Is oil pump putting out the requisite amount of oil for the RPM? Try running a can of premix but you have to redirect of disconnect oil pump as it is certainly moving some oil or I think you would have cooked engine by now. Sounds like you have already explored my other ideas.

Maybe some of the others with more marine experience on here will chime in.
Interesting but sounds frustrating...good luck.
Shawn

 

[CharlieB]

Re: 1999 90hp Mercury 2 stroke- Dropping RPMs

We have had an intermittent issue with the engine running fine and then dropping rpms in the higher range.
..........................then switched a person out who weighed about a 80 more pounds. A few minutes into the second tubing event, the surging/ drop in rpms came back. .............

How old is the fuel pump diaphragm?

Alcohol in the fuel causes a 'stiffening' of the diaphragm, reducing pump output. The increased load of the heavier tuber causes the motor to use more fuel, requiring maximum fuel pump output.

The service manual may list a volume for testing the fuel pump, but a fuel pump rebuild kit is cheap enough and may solve your problem.

 

[99tracker90]

Re: 1999 90hp Mercury 2 stroke- Dropping RPMs

The fuel diaphragm was just replaced about a month ago. Everything I replaced has been over this past summer.

 

[CharlieB]

Re: 1999 90hp Mercury 2 stroke- Dropping RPMs

Additional load also requires maximum output from the ignition, it may be possible that the stator is heating and beginning to fail.

PROVE IT, use and inductive tach or inductive timing light, sample RPM (or timing light flash) on each cyl when the condition is occurring. If the inductive tach reading on on or two cyls reads lower than others it is a pretty sure sign that cyl is misfiring. Same if the timing light starts failing to flash.

When this occurs, switch the CDM between the cyl to see if the failure stays on that cyl or moves with the CDM.