Hydrolock is the term used when one or more cylinders has been filled with enough petrol, that when the piston tries to up-stroke, but it can’t move high enough to continue the rotation. This is because liquid does not compress as much as air, and therefore the piston is locked from rotating. Hence, it has been hydrolocked.
If you press your starter button, but you just hear the starter engage but not turn, then there is your first symptom. You might think at first that the battery or starter motor is dead, as the headlights will go real dim when you try to start. The difference is, with a flat battery it will probably get some rotation or possibly the start relay with click on/off (unless it's really flat!), whereas with hydrolock it’s more like the starter is locked. Don't keep trying, it’s not good for anything... particularly the battery, rods, rings and starter motor brushes.
I wouldn't rock or move the bike in gear to see if the engine is actually hydrolocked, as you are putting a lot of stress on the piston rods and bearings. You could measure your battery voltage to confirm you battery is ok.
The worst thing you can do at this stage is to try and roll start the bike. You will effectively be putting the moving force of yourself and the bike onto one or two of the piston rods and bearings. They were never designed to withstand such a high force. In addition, the petrol in the cylinder is trying to get out, and will squeeze past the rings and contaminate the engine oil. Why do I know this… because roll starting my hydrolocked bike bent two of the rods, damaged two big end bearings and subsequent damaged the crank beyond repair
. It was cheaper to buy an engine replacement than fix the damage. Don't wait too long to clear the cylinders of petrol, as it’s eventually going to leak past the rings and contaminate the oil.
Clearing the Petrol
Pull the plugs out! Hit the kill switch (you don't want any sparks), and give a ‘quick’ crank with the starter. Wear protective googles/clothing when you do this, because the petrol will squirt out everywhere, and can hit you directly in the face :O!
After you’ve replaced the plugs and cleared the problem that caused the hydrolocking, the engine may be difficult to start, backfire and rev high for a few seconds.
Causes of Hydrolock
The carbs provide a small reservoir of petrol for each cylinder, which supplies petrol without the pressure from the fuel in the tank. A float for each carb will rise when the reservoir is full and close off the petrol supply with a valve. If this valve or float are not operating or sealing correctly, then petrol will drain though into the cylinders.
If the bike has been dropped on its side, the floats may not be in a position to hold the valve shut. This may allow for petrol to flow through into the cylinders.
Connecting the petrol line up to the air intake tubes instead of the petrol inlet tube will also flood the cylinders.
The easiest way to prevent hydrolock is to turn your fuel tap off when you are not riding, or particularly if the bike has been dropped on its side.
I have the habit of turning the key to/from ignition with the other hand turning the fuel tap… so I don’t forget.
Turning off the fuel at the tap reduces the pressure of a tank full of petrol applied to the float valves. Only the petrol in the tube to the carbs will be able to leak past, however that is under some level of vacuum because it can't run out of the tube without sucking in more petrol. Therefore, turning the tap off will prevent the float bowls from filling up any more.