Do a compression test and verify the compression is low (though it sounds like it is).
Assuming it is low:
The first thing to check are the valve clearances to make sure they are not too tight and holding a valve open.
Other than that, it's a stripdown - cylinder head off and check for burnt valves or seats, if they are OK it needs either new rings or a rebore & new piston.
If you only change the rings, make sure you get a set with a stepped top ring - a standard one will break on the "wear ridge" at the top of the cylinder.
Also hone the cylinder bore to take the glaze off it and allow the new rings to bed in properly.
Get manual and look at the 'removed head' section. Static compression test with a air tank, pressure gauge and bleeddown lock valve will give a big hint as to rings wear. do one test 'dry', then repeat after putting in a spoon full of oil, turn over twice to coat rings. 20 pounds higher compression with a 2 minute bleed down instead of 30 seconds implies unbroken but worn rings. Sometime 'service' ring sets are available for stock bore pistons with .010 or less cylinder bore wear, other wise looking at a 'stock' set that will help some but won't get back to specs with .005 taper and wear. Over .010 wear usually means larger taper and the cure is the .020 oversize piston, ring sets. Sometimes a 'sizable' .010 or .020 ring set can be adjusted with grinder to have proper ring gap of say .006 plus/minus- some of the .010 service rings had the soft ends just for this reason. a stepped top ring is put in smaller side up
A lack of compression indicates worn valves and/or piston , rings and cylinder .
While it could be as little as a valve spring or a ring ,,, on a 35 year old bike I would say a complete rebuild including a cam is in order .
Not sure which you are asking about- a 125 two-stroke or a 250, 2 or 4 stroke.
If a 2 stroke (either size) it probably needs a new piston/rings and maybe a bore to go along with that.
If a 250 4 stroke it may be valves and or a new piston/rings.
Time to open it up to find out either way.
Hi this indicates the bore is very worn or a broken piston ring. the usual cure is a rebore and a new piston with new piston rings. these days however a new barrel and piston is the way they do it now.
