Hi, Anonymous you may have squeaked your top end this is commonly known as "HEAT SEIZE" the piston heats up expands and becomes extremely tight in the cylinder bore and usually occurs mainly on 2 cycle engines right after a fresh top end rebuild, but it can also happen to brand new motorcycles, these are rare instances due to better quality control and education. Overheating failures that cause pistons to seize are many and each one should be carefully diagnosed to prevent future repeat offenders and the usual suspects are:
1. Loose fasteners causing intake or base gaskets to leak or crankshaft seal failure caused by age, overheating, or walking.
2. Improper assembly of reed valve gaskets.
3. Improper oil/fuel premix ratio.
4. Broken, pinched, or cracked fuel line.
5. Carburetor fuel passages clogged or restricted from stale fuel varnish, ethanol sludge, dirt, rust, or fuel level in float bowl too low.
6. Main jet size too small.
7. Air filter improperly installed, missing or not sealing.
8. Faulty or improperly adjusted oil pump and or cable.
9. Failure to bleed oil pump after running oil tank dry, rebuild of pump or replacement
10. Among the causes of lack of cylinder, lubrication is clogging of lubricator by dirt or waste, obstruction in oil pipes/lines, leaky check valves, leaky pump packing, broken oil pipe/line, oil too cold to feed.
11. Wrong type of oil used in premix or oil tank
12. Improper spark plug heat range (too hot) and or gap.
13. Ignition timing retarded, faulty CDI.
14. Excessive engine load, brakes dragging, clutch slipping, transmission lubricant level too low or viscosity to high.
15. Engine coolant level too low, too old, or improper mixture ratio.
16. Engine coolant pump failure, damage, or passages restricted.
17. Radiator clogged, fins damaged, or faulty cap.
18. Failure to follow proper break in procedures.
For more information about your issue, please visit the websites below. Good luck and have a nice day.
DOWNLOAD Aprilia Sr 125
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