Mercury 120 Sport Jet in Sugarsand Mirage

[Belleville Lake]

We put boat in first time this year, ran a few minutes, and stopped. Boat had been winterized. We pulled plugs, water in cylinders. Drained water, replaced exhaust manifild gaskets and head gaskets. Ran for about 20 minutes, even full throttle, then 'bang'. Engine stopped and froze. What should we check first?

 

[H20Rat]

Re: Mercury 120 Sport Jet in Sugarsand Mirage

We put boat in first time this year, ran a few minutes, and stopped. Boat had been winterized. We pulled plugs, water in cylinders. Drained water, replaced exhaust manifild gaskets and head gaskets. Ran for about 20 minutes, even full throttle, then 'bang'. Engine stopped and froze. What should we check first?

my guess is your wallet...


first of all, is the engine locked up solid? If it isn't, do a compression test.

 

[Belleville Lake]

Re: Mercury 120 Sport Jet in Sugarsand Mirage

starter won't turn it over..but a big wrench on the flywheel bolt will turn it over, slowly with a lot of grinding. Guess we pull the motor next.

 

[kend301]

Re: Mercury 120 Sport Jet in Sugarsand Mirage

when you ran the boat for 20 min was it in the water or on the trailer , if it was out of the water did you have a hose hooked up to it ? If it was out of the water with no hose hooked up you blew the drive plate off

 

[Belleville Lake]

Re: Mercury 120 Sport Jet in Sugarsand Mirage

We were in the water, ran for 20 minutes, cruised around and ran full throttle. Engine sounded good, no hesitations, no shaking. Until the big bang. Now it will not turn over, even with spark plugs out. But with a big wrench on the flywheel nut it will turn, but very tight, and grinding sound. Next step? Pull engine, pull head, look inside. What should we look for?

 

[kend301]

Re: Mercury 120 Sport Jet in Sugarsand Mirage

Well the first thing I would do is prior to pulling the motor , which sounds like you are going to have to would be to look in the spark plug holes to see if all pistons are moving . If you can't get a good angle to look , try sticking a long pencil in the hole so it sticks out and measure the travel of each piston . Diffrent movement or no movement is a broken rod or crank. Pull the motor and first thing turn it over by hand , If you still have a tight motor , time to take it apart , If it seems to be ok out of the boat , look at the jet drive unit for a broken driveshaft or pinion gear.