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1994 mercury force 90 hp tear down engine

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blueweasel

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My 90 hp force quit running. Shop said the power head is bad. no compression on two cylinders and low on third. I will tear down engine to see what is wrong. How do I separate the block from the lower unit?
 
You might want to just remove the cylinder head and exhaust covers at first. You should be able to see whats wrong at that point...
And then decide if you want to do a full tear down after that....
 
That will accomplish nothing since there are no removable cylinder heads.

Buy a manual from this site--please! But don't bother trying to rebuild the power head--it would cost you a fortune. Look around for another power head (75 to 90 hp--they're interchangable).

Jeff
 
Thanks for all your replies! I have removed the head to find not much wrong, I think. It appears that something went throught the center cylinder port and was caught between the lowest exhaust port and the piston causing a small dent in the piston. No scratch in the cylinder. Going by the coloring on the piston it appears that this is older since all the coloring is the same in this area. This piston does appear to move much more side to side than the other two pistons. There is a small pitted area in the lower cylinder about .300" from the top and about an inch in length, all seems to be above the ring travel. I plan to remove theintake side in order to expose the crankshaft. Are there any points to mark for timing purposes during reassembly other than tdc? At this time I have not gotten the lower cowl to come loose, I think I have removed all the bolts and screws but its still tight. Am I missing something. This doesn't seem to difficult, I have some experience and spent several years in the machining industry. I have never worked on a 2 stroker yet. I just want to get the grandkids out fishing again. Again, thanks for all your input, its good to know that there is a little help out there.
Blueweasel.
 
From memory, the lower cowl will not come loose on that engine, as it is attached to the steering tube. To remove the powerhead you need to remove about 8(give or take) 9/16" bolts that hold the adapter plate and powerhead assy to the exhaust housing. You also need to disconnect the rubber mount that is under the carb.
You must lift the powerhead, adapter plate and exhaust housing all as one unit...
 
I did a rebuild on a 90 Force a couple years back. The block actually has quite a bit of "meat" to it. The machine shop bored all three cylinders to 30 over and said there was still lot's left (you can get 45 over pistons/rings for this model).

Force takes alot of crap (some deserved), but the block itself is actually fairly well built - it's the crappy carbs (old Merc Tillotson's that Merc stopped using in the mid 70's) and that cluster $%&# ignition, aka, Prestolite.

My advice to you is get the machine shop to source your parts, forget about using a Merc dealer. Wiseco makes pistons/rings and most shops can get Torrington bearings (or even Timkins), which is all you will get from Merc (paying double the price for the Quicksilver logo).

And yes, the above comment is correct. Simply unbolt the powerhead and pry then lift it off.

Don't forget to correct the likely source of the failure in the first place - the piston or cylinder is the result - the culprit is likely the carb(s). Rebuild them just on general principle.

The total (restore) cost for the one I did was just under a grand. That included, 3 new holes bored (that was $150 at the shop plus 25 bucks to put a helicoil in the number 2 sparkplug hole - when the pison went and the rod broke (or vice versa) it really launched and drove the plug right out of the hole - kinda cool :)) , 3 new pistons, rings and connecting rods. New bearings - crank - upper lower and main, and rod bearings both ends, powerhead gasket kit (for that you should use OEM), 3 carb kits (again OEM on these - Tillotson parts are getting scarce - the kits are close to the only source now), new trigger (coincidental? - but appears to have died the same time as the number two piston ate it's rings, broke the rod and toasted the cylinder) and waterpump kit.

Like I said, the machine shop sourced all the "guts" (pistons, rings, bearings, rods) - got all 3 for what Merc wanted for a single piston/rod. Carb kits/powerhead gaskets from Merc. Waterpump kit from Sierra (Merc stopped making impellers for these - grab a couple extra's in the aftermarket now or you will be out of luck when the last of the stock runs out).
 
Some great advice there Graham...
I will add, that your machine shop is key here... For a succesful rebuild, make sure they know outboards well...
If you use wiseco's, they need extra clearance, If any cylinders are not bored, make sure 'out of round' is checked.
Also. special tools are required to press pistons and rods on and off... So, make sure your machine shop can do this...
 
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