Have some questions about a 125m topend rebuild

[saturnthegiant]

My '84 125m had an issue with smoking when i first got it, and over time it has progressively started losing power. I rode with a friend during some pretty high winds the other day and barely had power to go at times, i had to power down to third gear sometimes when we were riding directly into the wind. Also it has what i think is gas or oil coming out the exhaust. I just wondered if it sounded like i needed new rings? And what is involved with replacing them? Do i have to measure the cylinder bore to know what size rings i need? And is it a must to get the cylinder bored or can i just hone it? I'm a newbie to these engines and trying to learn as i go, some advice would be appreciated. Thanks.

[saturnthegiant]

Also what things can i do to tell if its rings or valves?

[peco250sx]

smoking means you need rings but for valves you need to do a compression test or leakdown test. or if its already apart, lap the valves with lapping compound or a dry erase marker

[oscarmayer]

you should probebly get a new piston first about .40 over bore to be certain will clean up. once you have the new pisotn, you can take it and the jug to a good shop and have it porte dand honed to the new piston size and specs.
Also order a new set of valves ( your intake probebly has a lip around it) and get a shop to do a basic valve job on it. cost is probebly $45 on the vale job and $35 shipped for the valves.

you will need a top end rebuild kit and a bottom end rebuild kit and all oil seals (kit sold seperate from gaskets) you shoudl probebly repalce the clutches, check front sprocket for wear if worn, replace it and the chain, repalce the 2 oil seals dealing with the sprocket and any other oil seals you see in the manual. This will ensure you end up with another 20+ yr running motor.

[saturnthegiant]

I would like to do all that eventually, but could i just replce the rings and gaskets for now and see if it helps? And from what i've seen the rings come in two different sizes, .25 and .5, how do i know which to get?