Re: 1999 Force 120 HP cuts out at 4400 RPM
You have two distinct problems: The melting on the baffle of #2 piston is due to lean running. Judging from your compression results, the low reading is probably due to a broken top ring. The broken ring limited your engine's ability to deliver power and saved it from more severe damage from detonation. Consider yourself lucky.
These engines with semi-keystone top rings are prone to breaking the top one. It is a relatively common occurrence. The broken top ring then scores the cylinder and piston and sometimes the cylinder head as the broken piece rattles around before being blown out the exhaust. The exhaust ports may have a nick in them where the ring was caught or may not. At any rate, a small nick there will not hurt anything.
It is difficult to see the extent of the scoring . However, if the scoring is light enough that you do not catch a fingernail when dragged across it, then simply honing should do. It is a judgement call but do not let economics dictate the repairs. If a friend has a bore micrometer, Mike the bore--It should be 3.375 +.002 max. If the scoring exceeds that or if cylinder out of round exceeds that then it needs to be bored either .010, .020, or .030 depending upon severity. If you don't have a friend, take it to a machine shop. Let them tell you if it needs boring.
From the photos, again, I can not tell you if the #2 piston needs replacing but if it did flip a ring the ring groove will be damaged and then yes, it will need replacing. The damage to the baffle is not so great by itself as to require replacing the piston. However, Aesthetically, it bothers me and if it were my engine yes, I would replace it.
Pistons are cam ground to an oval shape and tapered up top to account for uneven expansion due to uneven cylinder cooling. YES, they do rattle in the bore when cold and all four should do it about the same amount--although it is a difficult call by hand unless damage to one is severe.
Because of the nature of the intake, twin carbs, and manifold design, one cylinder will always run leaner than the other in a pair. This is no problem when the carbs are set correctly. However, If one carb goes lean for whatever reason, ONE cylinder will be too lean and will enter detonation, destroying the piston and scoring the cylinder severely--sometimes beyond boring limits.
SO: When you go to reassemble the engine, remove and disassemble the carbs. Clean them well. When you put them back, be certain there is a filter between the fuel pump and carbs (I believe late model Force engines did have a fuel filter here). When you start the engine set the low speed needles to 1 1/4 turns out to start. Never adjust to less than 7/8 turns out no matter how poor the idle. Usually, these engines are happiest at somewhere between 1 and 1 1/4 turns out from lightly seated.
Rings very rarely wear enough to be replaced so on the other three cylinders: Remove the pistons and clean rings and ring grooves. LIGHTLY hone the cylinder walls. Coat the entire piston skirt and rings with TWC-3 oil or assembly lube and reinsert the pistons.
