90 efi Mercury 4 stroke over revving

[Ken727]

I have a 90 hp on my 16 ft bow rider, it ran fine until 2 weeks ago, now it overrevs . I have replaced the prop hub and no effect. I have auto trim and tried every position and no help. I have a 21 inch pitch prop on the boat and I use to run at 4800 rpm at 45mph to 50 MPH but now, if I let it, it will go to 6500 rpn and will not accelerate over 45 MPH. I cannot find anything slipping and have not changed anything. I have no idea where to look now.

 

[cyclops222]

You may have junk wrapped around the Lower unit and the prop.
Also hit and slightly bent a prop blade or blades.

 

[Ken727]

I have had prop off and inspected and it was OK, I have looked at lower unit and found nothing on or around it.

 

[cyclops222]

Has the tachometer gone bad ? Need to check that out.

 

[Texasmark]

Has the tachometer gone bad ? Need to check that out.

The only problem I have with that possibility is that you have 2 instruments that would have gone bad simultaneously....tach and speedometer.

I had a 90 2 stroke triple, 2002 model and I was playing with the timing one day I accidentally set it too fast and the engine ran away....did a hard peg on the 6k scale tach and turning the ignition switch off had no effect. Had to disconnect the fuel line to get it to shut down,

It ran like that for a couple of minutes while I recovered from the shock and started looking at ways to shut it down. I lucked out in that apparently it didn't tear anything up as it still ran my little 16' tin bass boat right at 50 with a 24P Ballistic SS prop.