Merc 120XR sport jet boat overheating

[QBoater519]

Hi eveyone,

I have a 95 Euroline jet boat with a Merc 120XR jet engine. I have clean the carbs, pump and filter. Added new fuel after removing all old fuel was sitting for 8 years.

Checked compression in all 4 cylinder around 120psi. New spark plug. Added a new thermostat since it was only $20 bucks. Chained the engine oils and bottom propeller oil and also the oil reservoir too.

Boat fires right up and run normal for 2-4 minutes. I connected a hose since im running it in my driveway for water cooling.

After it warms up 2-4 minutes it starts to rev up by itself without me touching the throttle and starts to overheat since a solid beep alarm comes on.

I then turn it off and let it cool for 30 minutes.
But it keeps doing the same thing over again.
Is there a water pump that might be bad?

Maybe something stuck in the waterline or water vent casuing it to over heat?

Any advise on troubleshooting this overheating.

Thank you for any help.

 

[Cobra Jet Steering LLC]

Remove the thermostat put it in a pan of water and slowly heat it see if it opens or remains shut if it remains shut you have the cause of the issue. how to replace it

it is not really Mercury it is actually a Force outboard power head converted to a jet drive .

 

[QBoater519]

Remove the thermostat put it in a pan of water and slowly heat it see if it opens or remains shut if it remains shut you have the cause of the issue. how to replace it

it is not really Mercury it is actually a Force outboard power head converted to a jet drive .

Thanks for the suggestion.

I replaced thermostat already but still overheating.

Could it be a heat senosor?
Clogged vent?
Bad water pump?

 

[Cobra Jet Steering LLC]

if still it runs hot it is usually low water pressure in the flush hose

 

[QBoater519]

I will start to look into the flushing hose and flushing housing.

Thanks for suggestion.

 

[QBoater519]

I think i have a leak somewhere in the gasket.
Since i found water in #4 cylinder and get an overheating solid beep alarm after it runs for a while. So the exhaust manifold must have a leak??

No water in any other cylinders, all cylinder had 120psi during compression test.