Hi, Graham you may have hydraulic lock caused by excessive liquid in the cylinder or you 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. Lack of lubrication due to low or no oil.
2. 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.
3. Wrong type of oil used in premix or oil tank.
4. Improper premix ratio you should start with 32:1 as a baseline.
5. New rebuild piston to cylinder tolerances set up too tight.
6. Failure to follow proper break in procedures with a 16:1 premix ratio.
7. Loose fasteners causing intake, base, or head gaskets to leak or blown crankshaft seal, failure caused by age, overheating or walking. 8. Holes in the intake manifold bent or warped intake flange or reed cage any stray air coming into a cylinder will cause a super lean condition.
9. Improper assembly of reed valve gaskets.
10. Insufficient warm up time for temperature conditions before wide open throttle.
11. Broken, pinched or cracked fuel line.
12. Carburetor fuel passages clogged or restricted from stale fuel varnish, ethanol sludge, dirt, rust, or fuel level in float bowl too low.
13. Main jet size too small or partially clogged
14. Air filter improperly installed, missing or not sealing.
15. Faulty or improperly adjusted oil pump and or cable.
16. Failure to bleed oil pump after a rebuild/replacement or running oil tank dry.
17. Engine coolant level too low, too old, or improper mixture ratio.
18. Radiator clogged, fins damaged, or faulty cap.
19. No liquid circulation due to failed oil or water pump.
20. Cavitation due to air in oil or coolant system.
21. Water and oil mix due to blown head gasket.
22. Timing retarded due to faulty CKP sensor or CDI.
23. Improper spark plug heat range (too hot) and or gap.
24. Excessive engine load, brakes dragging, clutch slipping, transmission lubricant level too low or viscosity to high.
For more information about your issue and valuable "FREE" downloads that you will need please click on the blue links below. Good luck and have a wonderful day.
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