What causes this on cylinder walls?

[bored&stroked]

Last season my superjet got really hard to start and felt slightly down on power. Over the winter I replaced the vforce reeds [they looked ok] and made sure the 2 season old oem mikuni carb diaphrams and what nots were still good and rubbery. All back together, I decide to do a compression test. I get 115 from the front cylinder and 160 from the rear. Not good with 31.7cc domes. I pull it apart and the head gasket looks fine no signs of being bad. Pistons+rings look great. Only thing I can find eyeballing is these spots on the cylinder walls. What causes this? Can this really cause 100psi loss of compression?

 

[JC-SuperJet]

Looks like Rust to me. Leaking Cylinder Head O-Rings?

 

[bored&stroked]

oem 760 head gasket with shaved stock head. Gasket looked great...

 

[DylanS]

Wouldn’t cause that much loss, a steel wool and oil will rub that surface rust right out. Any stuck rings? Could’ve been a sneaky head gasket leak.

 

[Half flip95]

Moisture sitting in the cylinder over winter

 

[WFO Speedracer]

Rust , have the bore honed and checked for straightness and piston to wall clearance , like has already been said wouldn't cause a compression loss , keep looking for stuff like stuck rings etc.

 

[subnsxr]

Gotta crank it over weekly especially if you have a washer and dryer in the room and don't laugh, A famous crank builder told me that.

 

[WFO Speedracer]

Yep dryers create massive amounts of humidity

 

[mikidymac]

Fogging oil after every ride.

 

[fastgtfairlane]

Fogging oil after every ride.

I do this after every ride and also run 34 to 1 oil mix. My current 735 is pushing 195psi and still running strong with 5 seasons on it.. it's fully ported with all the matching parts.

 

[mikidymac]

I do this after every ride and also run 34 to 1 oil mix. My current 735 is pushing 195psi and still running strong with 5 seasons on it.. it's fully ported with all the matching parts.

 

[DylanS]

Fogging oil after every ride.

 

[bored&stroked]

Really didn't think I'd need fogging oil with it sitting outside in the Arizona desert and -20% humidity lol

 

[kevbo]

My solution for preventing rust is cheap TCW3 oil and blowing it out until the head gets too hot to hold onto. Then I leave the hood cracked so all water in the hull can dry up. I have a couple of stand ups with 20 seasons and their cranks and cyl walls are spotless.

 

[mikidymac]

Really didn't think I'd need fogging oil with it sitting outside in the Arizona desert and -20% humidity lol

Yes, you do as water still sits in the waterbox no matter what you do and with a port open will get rust into the cylinders just like you have.

 

[Quinc]

Take it one further and leave the sparkplugs out of the ski as well.

 

[mikidymac]

Take it one further and leave the sparkplugs out of the ski as well.

Remember these from back in the day?