Mercury 80 won't crank with plugs installed

Twin Mercury 80 HP 4STK (2007) not turned over in 2 months. Hot climate and stored under shade. Port engine fires up immediately. Starboard engine does not crank. Starter engages but cannot turn flywheel.

Attempted to manually turn flywheel but very stiff. Removed all 4 spark plugs and manually turned flywheel. Lots of gasoline vented from lower two cylinders. Initially flywheel was stiff, but then was able to make several smooth complete rotations. Additional gasoline vented from lower cylinders as the flywheel was manually turned. I do not know if this is normal but seemed excessive.

Cleaned up gasoline and made safe. Re-installed and torqued spark plugs and pencil coils. Tried starting but issue not resolved. Tried to manually turn flywheel, but was again stiff and would not move past a certain point.

Since flywheel turns smoothly with the spark plugs removed, the issue seems to not be a jammed impeller or seized piston rings. Any thoughts or guesses before getting the workshop involved? Thanks in advance!

Bent/Twisted Connecting Rod, if the Motor Hydrolocked when cranking

jimmbo said:

Bent/Twisted Connecting Rod, if the Motor Hydrolocked when cranking

Click to expand...

Thank you very much for the quick reply.

Motor last started in shed with muffs in Jan/Feb and run idle for 10 mins to keep things operational. Boat has not been in the sea since.

Assuming issue is hydrolock (and potentially bent rod), why would removing spark plugs from cylinders allow flywheel to be manually turned? I can also see the pistons rising and falling through the spark plugs holes. A bent rod or hydrolocked engine would not allow flywheel rotation, if understand this correctly.

Sorry for the questions, I am just trying to understand the mechanics involved.

Removing the Plugs, allowed the Liquid to drain, removing the cause of the Hydrolock. A very slightly bent/twisted Rod, can cause Binding/Tight spots, but still allow rotation, even complete rotation of the Crankshaft.

jimmbo said:

Removing the Plugs, allowed the Liquid to drain, removing the cause of the Hydrolock. A very slightly bent/twisted Rod, can cause Binding/Tight spots, but still allow rotation, even complete rotation of the Crankshaft.

Click to expand...

Thank you. I will call the Mercury service centre.

Bottom line is....there should not be liquid in the cylinders like that...you have a problem but I can't say what it might be. Better to get it looked at by someone that knows.

Updating in case it helps someone else.

Worked with Mercury techs today to diagnose the issue. With the plugs removed and engine oil drained (there was some gasoline in the oil), the motor cranked easily enough but a lot of fuel was spurting out of cylinder 4. We swapped an injector to rule out a faulty injector, and the issue (for now) seems to be a misbehaving fuel pump. The techs have taken it back to their workshop for diagnostics. I will post an update.

Stuck on FULL CHOKE ?

*Final Update*

In case it helps anyone else, the issue was caused by a ruptured hose in the fuel pump. See attached photos. The techs were able to replace the hose. The fuel pump was reinstalled, and the motor is now working fine.

Thank you to everyone for their input and suggestions.

The ruptured hose which was replaced.

Glad to hear you found the problem

Thanks for posting the solution to your problem......

Gee, my guess was ruptured diaphragm in fuel pump. That allows gasoline to enter crankcase in 2 cycle engines.

Glad it was resolved.

4 stroker can be filled through a bad float needle if it is carb engine or through a bad injector and an open intake valve.
Have hade this kind of issue, but it was a B&S engine on a lawn mover.
Especially if it has an over head internal fuel tank (no fuel pump).
Crank case was filled with about 1 liter (I am in Sweden Europe) fuel !!!