Re: Piston is Melted in #2 cyl on 85HP
First off, that is detonation damage. You were lucky that it did not progress further and completely destroy the piston and score the cylinder. It usually happens when you go to full or near full throttle and it happens so fast that you can not react. It usually shatters the rings, scores the liner, and melts the piston so badly that there is zero compression.
That cylinder is running LEAN. Find out why. Disassemble and clean the center carb and be certain that the low speed needle is no leaner that 3/4 to 7/8 turn out from lightly seated. Less than 3/4 is too lean and will almost always melt pistons. Also be certain that the float is not set too low. Double check to see that no crap is stuck in the inlet needle. Finally, put a cheap 3 buck plastic auto filter between the fuel pump and carbs.
If the cylinder is indeed not scored--a very lucky break-- you can replace only the center piston. AND, you need not disassemble the engine to do it. There is no oversize to account for wear. If the cylinder is not scored and does not need to be bored, buy a standard one.
Take off all three carbs, the manifold and reed assembly, and rotate the flywheel until the big end of the rod is closest to the opening.
Using a 12 point 1/4 inch socket with a 6 inch extension, remove the cap screws. Remove the rod cap and as many of the rollers as you can. Push the piston and rod out. Some rollers may stick inside the big end of the rod so be careful not to let them drop into the engine pan. Some will fall inside the engine but they can not get stuck or lost as long as you keep them away from the exhaust ports. A bent piece of wire will fish them out nicely.
ACCOUNT FOR ALL 16 ROLLERS
You will need to press out the wrist pin and there is a special tool necessary to press it back into the new piston unless you buy a WISECO piston. They come with rings, wristpins and retainer circlips but you must buy a bearing kit for an extra 13 bucks.
Pay attention to the orientation of the rod when you remove it from the old piston and install it into the new piston. While it will work either way, the factory recommends one way only.
At 150 PSI, forget about the other two cylinders. You can not get any better. Lubricate the new piston, rings , and rod assembly and push it into the bore. Line up the big end of the rod with the crankpin. Using greaseto keep them from dropping, replace the cages and rollers one by one. Work slowly because the cages will tend to ride over each other as you rotate them to install the rollers. Count the rollers as you install them. It pays to be paranoid here. Once all rollers are in place, replace the rod cap and gently just snug the bolts--finger tight only, no wrench. Now, check the ground portions of the rod to be sure they are completely aligned and a pencil point or scribe passes smoothly over the break. Once they are aligned properly--be very fussy here--good enough is not good enough. Perfect is the only option. Once they are aligned, torque the cap bolts to 180 INCH POUNDS. If you do not have a torque wrench, borrow one.
Replace the manifold--you will need new gaskets--replace the carbs and linkages.
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