Cylinder hydrolock is a condition that occurs when fluid enters the combustion chamber and prevents the piston from reaching TDC because it cannot compress the amount of fluid between the piston and head. The result of such a condition can be a bent rod.

Hydrolock can occur when excessive oil and/or fuel enters the crankcase and is then pumped up through the scavenge port to the cylinder as the piston moves toward TDC.

Defective sealing of oil line check valves, where the bike has set for an extended period, can result in hydrolock. Defective needle/seat valves in carbs can cause excessive fuel to be dumped into cylinders, especially on modded engines where the carbs are slanted down toward the intake port. Defective/open petcocks can also be a contributing factor.

If there is reason to believe that this condition exists the excess oil/fuel must be pumped from the crankcase prior to starting the engine. There are a few of ways to do this:

- Run a tube through the open intake port to the bottom of the crankcase. Attach the tube to a pump or turkey baster and suck out all fluid.

- Pull the sparkplugs and push the bike or coast downhill until all fluids are eliminated. Put a rag over the sparkplug holes prior to doing this.

- On engines equipped with crankcase check valves at the lower front of engine, they can be removed to allow access to crankcase.

- Slowly kicking the engine over with ignition off to "feel" if a hydrolock condition exists may not always detect the condition. This has been proven where bent rods have resulted after such a test.