1998 Mercury Force 90HP Powerhead Rebuild

[bwillaub]

Lost a cylinder on my motor last week. Was told that the powerhead is not available. Is there a reliable place I can send the head to be rebuilt? Can the cylinder be bored and sleeved or all cylinders bored larger to change HP to 115? Any help will be greatly appreciated.

 

[pnwboat]

If you are unable to re-build the power head yourself ( many of the Forum members here have done this), then your best bet is to get a used power head in good condition. If you're looking for more power, then you can get a used 1996 or later 120HP 4 cylinder power head. It should bolt up to your lower unit. There are re-built power heads available, but I myself would never pay what they're asking. It would take a lot of machining and major engine modification to get a 90HP 2-stoke Force motor to produce 115HP. Nothing as easy as boring it out or changing carburetors. Probably wouldn't be as reliable either.

Be aware that the transom on your boat has to be rated to handle the weight and thrust of the 120Hp motor or you run the risk tearing the transom right off the boat and sinking. There could also be possible handling/control issues if your boat is not rated for more horsepower, plus possible legal issues if you are stopped by law enforcement and they see that your are "overpowered".

 

[bwillaub]

Thanks for the reply, pnw. What are the steps to "rebuilding" a power head? I've watched a few videos on youtube but they aren't very specific.

 

[Frank Acampora]

You must first disassemble the powerhead and remove it from the leg. Take a LOT od digital pictures at each step because you may forget something by the time you reassemble. Then take the block to a good auto machine shop and let them tell you how far the bad cylinder needs to be bored. Standard over bores are: .010, .020,, .030, and .040. I do not like to go .040 because it leaves the cylinder walls too thin. Order a standard overbore piston to match what the block is bored. If the other two cylinders are good, only the affected one need be bored. The added cubic inch (partial) is so little that the engine will never notice it.

It is not possible to bore out all cylinders and increase displacement. Even going .030 on all three yields less than one cubic inch. You can increase power slightly by porting the cylinders but if you have never done it, stay away. If you want more power bolt a 120 or a 125 block onto your leg. However, given the cost and fuss, you would be better off buying a used running 125.

almost no special tools are needed to rebuild. A harmonic puller to remove the flywheel and both a foot pound and inch pound torque wrench are necessary. Expect to spend about 5 hours disassembling and a bit more with reassembly. It is more than likely that you will twist off a couple of bolts and they will need to be drilled and re-tapped. The machine shop can do this for you but if you are real careful use some penetrating oil and heat, and are lucky, you may not break any.

If you do decide to rebuild, come back and the guys will certainly provide more detailed help.

 

[scout-j-m]

You can buy a used model specific factory service manual off of Ebay for $20-$60. You can also buy a Clymer manual which covers many years and HP models for the same price as well. I am currently rebuilding my 70hp and have the factory service manual which has been invaluable. The Clymer one is helpful as well as it has better info on the torque values but you can definitely get away without it between the two.

The machinist I took my block to charged $50 per hole for boring/honing. He also chamfered the ports, pressed out my wrist pins to get me my connecting rods back, pressed on my upper main bearing, and washed the block. My total was $165...I think he didn't charge me for the wash and installing the bearing and the extra $15 was for pressing out the wrist pins. The chamfering must have been included in the cost of the bore. I have no idea if i was given a good price, however, it was cheaper than some of the places you can ship the block to get it done. Those places were charging closer to $70/cylinder and then you have to factor in shipping.

I'm going to pass on offering too much technical advice at this time because I am not done so for all I know all of my work may have been for naught if it doesn't run afterwards lol.

 

[bwillaub]

Thanks for the replies. I really appreciate the info I am getting. I have one more question and I will be getting started. My motor hadn't given me any reason to suspect something like this was about to happen. It had started good every time I needed it and it had ran good from fishing spot to fishing spot. I always make sure water is jetting from the motor when I start it. I was just running up river from the marina and the motor just lost power and died. I attempted to restart it and it wouldn't start until I gave it throttle. It would run in neutral with throttle. I would lower throttle to engage in gear and it would die. Does the fact that the motor would run tell me that the damage to the bad cylinder isn't too bad but just bad enough to cause the motor to not run under torque?

 

[bwillaub]

Forgot to add this: The results of the compression test: Top cylinder = 120LBS Middle cylinder = 30 LBS Bottom cylinder = 120 LBS. I'm no rocket scientist, but I'm guessing the middle cylinder is the bad one. Thanks again, guys.

 

[Frank Acampora]

Ahhh! I was under the impression that you had already removed the head and looked at the cylinders. As I state numerous times, these engines have a nasty little habit of breaking the top piston ring for no particular reason. It is because the top ring is a semi-keystone design and sometimes will get caught in the exhaust port. The engine will run beautifully with no sign of trouble until this happens.

. Depending upon the damage the engine may start and run at reduced power. At 30 PSI, you probably have a little more damage than usual but you can't tell until you actually eyeball the piston and cylinder. Repair can range from simply replacing the piston and rings up to disassembling and overboring the cylinder. The amount of damage dictates the course of action.

 

[bwillaub]

Hoping for the best. Should have the disassembly finished by tomorrow. Busted my fanny when the flywheel broke loose! Harmonic balancer puller worked just fine.

 

[bwillaub]

No Title

Got into the motor a bit today. Here is what I have found thus far. The damage to the cylinder wall seems minor to me. It will still have to be bored but hopefully only +.010". What do you guys say?

 

[Frank Acampora]

It does look relatively good. You may be able to simply hone it (with a 30 dollar Sears hone) and slip in a new piston. However, take it to a machine shop and let them tell you what it needs. It is obvious that the piston has broken the top ring but if you look at the baffle you can see the pitted appearance due to detonation--in your case mild. The broken ring may have stopped it before really severe damage occurred.. At any rate, check all three carbs and clean them thoroughly. Then put a cheap in line fuel filter between the fuel pump and carbs--BEFORE you try to run it again.

BTW: Stock bore is 3.375 inches.

 

[bwillaub]

Thanks, Frank. I appreciate your input. I measured the bore and it is 3.375". I already have a fuel filter between the fuel pump and carbs. I will replace it before I try to run after the repair. Will also add oil to fuel to achieve the 25:1 ratio for the break in. I'm waiting for my repair manual to get here before I go any further. It should be here within a couple of days.

 

[bwillaub]

No Title

Got the powerhead off and disassembled this morning. Been looking for a machine shop to do the work but have been unable to locate one as of yet. Need to buy a good gasket scraper and get to work cleaning the mating surfaces. Best place to buy the gaskets and piston? Thanks, all.

 

[scout-j-m]

Since you are rebuilding you need to replace all of the gaskets anyways so I would try to find a complete gasket kit for your motor. It's a good bit cheaper if you price these compared to individual gaskets on the parts sites. Ebay has lots of them and they are around $100 which is a good deal I believe. My rebuild kit I ordered came with a gasket set. It's possible they may sell you one by itself if you call and ask. My kit came from powerheadkit.com.

 

[bwillaub]

Thanks, scout. I plan on buying the rebuild gasket set. I'm also needing an OS piston w/rings and new rings for the other two pistons.

 

[Frank Acampora]

These are not auto engines You need to think of a different paradigm.

Because these engines get so little hours running time and there is a fresh shot of oil with each rotation, rings hardly ever wear to the point of needing replacement.. Chances are rather good that you don't need rings on the two good pistons. However, if it makes you feel better, or if you see obvious scratches in the surface of them, go ahead.

 

[bwillaub]

Frank, the only reason I would feel better is to perhaps get the cylinder compression up to the 135 lbs they are supposed to run at per factory specs. The top and bottom cylinders don't have a blimish on them. The boat trimmed and ran right at 33 mph before the lost ring. Maybe I should rethink the new rings for top and bottom?

 

[scout-j-m]

I'd like to ask a question pertaining to new rings all around, even on the good cylinders. Would you not want to go ahead and get new rings everywhere so that after the rebuild the compression between all 3 cylinders is within 10% of each other? I was just thinking if he did the one bad cylinder it's compression may end up being greater than 10% of the other two which I seem to remember bwillaub saying were around 120 psi each. Just a thought.

 

[bwillaub]

It is very possible that I checked the compression wrong. It was the first time I had done it. I did the checks with all plugs removed, the kill switch down, and cranked for a few seconds. Checked the pressure guage and got 120 lbs on top and bottom, 30 lbs on center. I'm open to any suggestions as I plan on putting it back together next week if I receive the gaskets, piston, and rings.

 

[bwillaub]

Got my repair manual the other day and was looking into the factory specs on the cylinders. Compression for the 3 cylinders should be as follows: 1=135-155 psi. 2=145-160 psi. 3=145-160 psi. Why would they be different? I think I will lightly hone #1 and #3 cylinders to create a crosshatched finish. I understand that this will help the oil create the seal needed for better compression. Ideas? Anyone? Frank?