Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

ib18

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Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi, third piston has low compression, motor cannot turn over, so tearing it down. Got stuck twice (1) could not get screws off - had to buy a deep wall wrench #10mm socket
(2) had to buy a flywheel puller.

mercury 90hp tear down part 3 - diy outboard motor rebuild - YouTube

give me your comments if you see something i did wrong. thanks.

I will load more videos as I continue to tear it down and rebuild this outboard motor
 
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ib18

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

I am stuck on the last screw in removing my Mercury 90hp powerhead. I read from the manual that I need to remove 8 long screws, but I also read it from the internet there is 2 more screws in the front that people often missed. Please help.

My mercury 90hp outboard information - 1987 Mercury 90 hp 2 cycles 3 cylinder outboard (Serial No: 0B194831 Model No: 1090727)

mercury 90hp tear down part 6 - diy - YouTube
 

racerone

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

The crankcase screws should not be taken out before the powerhead is off the exhaust housing.----There is a possibility that the driveshaft is stuck in the crankshaft.-Try removing the lower unit.--It has to come off anyway to replace the waterpump impeller.----Do you have a factory manual ????
 

ib18

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

The crankcase screws should not be taken out before the powerhead is off the exhaust housing.----There is a possibility that the driveshaft is stuck in the crankshaft.-Try removing the lower unit.--It has to come off anyway to replace the waterpump impeller.----Do you have a factory manual ????

Thanks, yes, i have a manual, I know I didn't disconnect the gear link arm on the port side, so I am going to do that next.

I will proceed to remove the lower unit, but will have to manually lift the (power tilt/trim) motor by finding the manual release valve and turn it to lift the motor so I can remove the lower unit. Hopefully crankshaft is not stuck with the driveshaft.
 

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ib18

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

Do i need to remove my lower unit in order to remove my powerhead? Please help. I am having trouble raising the motor. The power trim is not connected to power, so cannot tilt the motor up.
 

ib18

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

I found a wiring diagram to directly connect my 12v battery to the trim motor solenoid, so will try connecting the blue wire from the solenoid with the red wire from the battery and pray that the trim motor will lift up my motor
 

ib18

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

Success! was able to raise the motor this am with power trim, and took the powerhead off the motor from the boat by myself (it was so much easier) after removing my lower unit. Split open the crank case and piston rings were big time loose, so will replace them all. Anything else?

https://www.youtube.com/watch?v=u2efJvJs5pE - trim motor wires direct connect to 12v battery

Do i need to take it to machine shop to hone and bore the 3 cylinders? please help.
 

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racerone

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

Post a few pictures of the pistons.----Post pictures of cylinder bores too.
 

ib18

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

thanks, just posted them
 

racerone

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

Pictures of the damaged piston by any chance ?----#3 piston does not look very good.-----What about the bearings on the #3 connecting rod ??----Take a good look at those too.
 
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TD_Maker

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

ib18
I enjoyed your series of videos on youtube. I used to have the 70hp version of your motor, so it was a trip down memory lane for me. I was amazed you actually got the exhaust plate bolts out without breaking any of them off. Down here in Florida (with the salt water) it almost a given you will break off multiple bolts when removing. For what its worth, be sure you take a tap and clean out the bolt holes, and clean up the threads before reassembly.

Racerone is right, the bottom piston looks suspect. As I stated in the youtube video, as long as you got it broke down this far, and if the budget permits, I would go ahead and have it bored and sleeved, and replace the piston and bearings on the bottom. It would be a real shame to pull the powerhead and do the work over again trying to cut corners now.

Good luck and keep us posted.
 

ib18

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

I am going to remove these rusted screws from the motor adapter, should i buy new screws or can i soaked them in something to reuse them?
 

ib18

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

IMG_2253.jpg
I am going to remove these rusted screws from the motor adapter, should i buy new screws or can i soaked them in something to reuse them?
 

ib18

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

IMG_2254.jpg

This shows my manual relief valve for the trim motor
 

TD_Maker

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

As long as the heads of the bolts are good, you should be able to clean up the threads (with a die) and use them again. Be sure to chase the threads with a tap and clean out the holes as well.
 

TD_Maker

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

That is not the manual relief valve for the trim motor. That is the filler screw for fluid. The relief valve is on the opposite side.
 

ib18

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

Thanks for correcting me. The screw on the opposite side is a plastic screw and broken as shown in the pictureIMG_1444.jpg
 

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TD_Maker

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

Your issue is going to be that manual relief valve. Parts are very hard to find. I am curious why you are messing with it anyway? I noticed (in your videos) that the trim and tilt unit seemed to work fine. Is it leaking down, or does the motor hold its position when tilted up? If the tilt position is holding, then leave it alone. The older Mercury units are very difficult to get serviced...the "o" rings and so forth are obsolete.
 

oldman570

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Re: Rebuilding Mercury 90 hp 2 cycles 3 cyclinders (1987) - 100, 100, 75 psi

Bump 1 on TD_Maker remarks. If the till/trim is working fine, do not mess with the unit. You might open up a can of worms that eats money quickly. The one cylinder dose have some buildup on the wall and can be cleaned up and honed if the ware is within limits. As others have stated, now is the time to do the job correctly and repair/replace everything that looks bad or you think needs attention. It will only save hard earned money in the long run. I never put my dirty shorts back on after taking a shower. JMO
Oldman570
 
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