Re: Impeller Failure. 68 Merc 200 (20hp)
Re: Impeller Failure. 68 Merc 200 (20hp)
The motor was severely overheated. This 'fried' the piston and/or rings in the top cylinder, which takes the worst of it since heat rises. The only fix is to pull the powerhead, split the crankcase halves, pull the entire rotating assembly out of the block, and replace the damaged parts.
The powerhead is bolted to the exhaust adapter plate with studs threaded into the powerhead.
Remove the 'horse collar' cover at the bottom of the lower engine cowling and you'll see a series of nuts to either side and one each per side at the very front.
Remove these nuts, undo all hoses/wires/linkages that would prevent the powerhead from being removed, and the powerhead pulls right off the exhaust tower.
You'll need a flywheel puller to remove the flywheel, an automotive harmonic balancer puller will work for this. Along with the correct-threaded puller bolts. Get Grade 8 or better bolts or they'll just break or pull out of the holes. If I recall, on a 20 there's a decal on top of the flywheel that tells you which bolts to pull. If you don't have that, pull (2) opposite bolts, if there are any with lock washers those are the ones to pull.
Once you have the flywheel off, strip the ign parts off of it; pull the carb; unto all the bolts which clamp the perimeter of the crankcase halves together; undo any bolts/fittings that are secured by locking tabs in the center or side of the manifold (front) crankcase cover. These are a main bearing shell securing bolt and a reed block securing bolt.
Undo the upper and lower main bearing cap securing bolts. 4 on top and 3 on the bottom cap.
Note, any bolt that won't move with normal force is stuck, try a combination of heat from a propane or MAPP torch, and penetrating oil. Use caution with the oil since of course it's flammable. Usually some heat will help break the bolt loose, once it's started to move you can get penetrating oil in there to do the rest.
The rotating assy (crankshaft, bearings, pistons, rod, reed block) pulls out of the block.
Once things are apart you can assess the damage. I expect you'll find 'smeared' aluminum where the piston has melted across the ring grooves. And likely some aluminum transfer to the cylinder bore.
If you're fortunate, the bore wasn't too hurt by the overheating. Carefully applied muriatic acid will dissolve the aluminum transferred to the cylinder bore. Your 20hp has the same pistons as a 44 c.i. Merc (think 50 hp 4-cyl), so those should be commonly available. You could probably find an inexpensive, very servicable piston on eBay, a new set of rings, some new bearings, gaskets and seals, and maybe you're back on the water.
Just a thought, HTH.............ed
If you take that cover I mentioned before could I see into the cylinder to see how bad the damage is? Just to get an idea of what is going on before I rip everything apart?