Powerhead swap 90HP to 120HP

[kandil]

I have a 1994 90HP Force that has low compression on piston #1 it has some damage on the corner of the piston and the cylinder wall has some glazing on all the pistons and I figured that to remove the powerhead and hone all cylinders and get all the parts will end up coasting too much!!+ the time so I found a 1994 force 120HP complete powerhead with 150 psi on all pistons and very Little hours for under $800.So my questions are 1-is the swap possible? 2-is it going to fit? and 3- is the gear ratio the same for this outboards? 4-or should I rebuild my powerhead?
thanks for the advice in advance

 

[Frank Acampora]

Re: Powerhead swap 90HP to 120HP

The powerhead mounting pattern is exactly the same so the 120 will mount on your leg. Since they are both '94 engines, the drive shaft splines should be the same.

Usually, prior to 1991 the 90 had a lower gear ratio than the 120 --BUT--this will actually work in your favor if it is still the case in 94. It will actually improve performance of the 120.

The coils, stator, and CD boxes from your 90 are usable BUT you must have a four cylinder trigger, another coil, and a four cylinder mounting plate. Also, carbs are different so I hope you are getting the carbs with the four cylinder 120.

Of course the hood will be too low and a four cylinder hood must be purchased--or you could cut out the top of your three cylinder one. LOL.

 

[kandil]

Re: Powerhead swap 90HP to 120HP

Hi frank Acampora Thank you very much for this good news. the 120 comes complete with every thing on it!!! I only need the gasket to install and I am getting the hood that was on it as well

 

[kandil]

Re: Powerhead swap 90HP to 120HP

I got the new head in and installed it that went smooth and added a pee hole I like that it shoots a stream about 3 feet on idle and that was hard to remove the nylon plug I had to drill it out and re thread it. over all it seems to run good beside the carb are over flowing and they drip gas!! time to rebuild or clean!! and the other problem is I can not shut it off!! when I turn the key to stop it it stays running I wonder if this is a bad ignition switch?or a bad relay? I have it on a 1994 sea pro CC180 and I don't have the weiring diagram for it

 

[CharlieB]

Re: Powerhead swap 90HP to 120HP

You have the kill wire disconnected, or plugged into the wrong terminal.

If I remember correctly, the WHITE wire from the main harness is the ignition kill wire, it must be connected to the ????? wire from each ignition box.

Late model may have used the same black W/yellow trace that Merc uses.

Hopefully Frank will see this and correct me if I am wrong.

 

[pnwboat]

Re: Powerhead swap 90HP to 120HP

I did as power head swap a while back and ran into the same problem. Turned out to be a jumper on the terminal strip on the port side of the motor that was supposed to be connected to ground. The jumper connects the very bottom terminal to ground. It does this by connecting between the bottom terminal and the lower screw that holds the terminal strip in place. What happend was I loosened all the screws on the terminal strip when doing the swap, and that jumper fell off. I didn't see exactly where it went and it didn't make sense to me to jumper a terminal to ground, so I didn't do it. Fired the motor up, but couldn't shut it off. Had to choke it to stall it out. Played around with it and looked at the manual to finally figure it out.

 

[kandil]

Re: Powerhead swap 90HP to 120HP

I checked all the wiring and it looks fine there is one wire near the harness plug that is not connected to any thing it is brown and white and by the panel there are 3 wiers that are not connicted to any thing one has a plug on it I thnk it is prbel and the other 2 are a brown wire and a black one that are not connected to any thing I think it was like this befor!!!or do you think this 2 wires should be connected together?all the grounds are connected to the motor on both side I think it is the ignition SW I ordered another one and I will see if this fix it how ever today I cleaned the carbs and it stoped the leakand it started and ideled fine and when I shifted in forward gear and went back to reverse it engaged the starter!!!! I wonder if I did some thing wrong?what could it be? it too late now I will work on it tomworw thanks for your advice but I will take more

 

[kandil]

Re: Powerhead swap 90HP to 120HP

Some photos off the panel you can see the brown and black wiers at the top right of the panel