Mercury Sport jet 90 starting problem

[costintabla]

Hello everyone,

This is my first post in Iboats and like the majority is because i have a problem.

I have bought a 94 Northeast jet boat with a Mercury sportjet 90.

I have installed a new starter,plugs,battery and rebuilt the carbs at a Authorized Mercury dealer.

And the motor has been rebuilt and has about 10 hours on it.

The boat starts every single time with the water hose attached but when i take it to the ramp and put it in

it does not. If i keep crank it will fire just one of the cylinders, but thats it.

After i take it out of the water and wait about 30 minutes it starts right up.

Could it be the exhaust manifold,or the gasket letting water in ??

i believe the water hose connection has a slightly different path trough the motor.

i did notice that the the water that comes out looks like its been mixed with oil.(its milky gray)

and that about sums it up

Its a great looking boat but its been driving me crazy..


Any help will be greatly appreciated.......help

 

[Crater]

Re: Mercury Sport jet 90 starting problem

In theory a blown gasket could allow the exchange of water and exhaust gases back and fourth. And because water may be getting in the crank case (where there is deffient oil) you will see the milky substance in the exhaust. First check your compression and do a leak-down test. This may rule out broken/sticky rings/piston. Comp should be around 120psi on all cyl. a 10psi diff between any 2 is acceptable, mostly check to see if the're all about even. If that's ok then either remove the head (pretty simple) or the exhaust manifold and check the exhaust ports (especially the bridge) for cracks. I guess water can seep into cyl through cracks also. It will be easier to remove the head, but you can get a better inspection though the exhaust so you'll have to decide. The aluminum can be cracked but not completly through the steel sleeve if viewed though the cyl. But then again you can get a good look at the pistons and combustion chambers if the head is removed. Either way, if the head gasket is leaking you'll get water and if the exhaust gasket is blows or leaking around one of the numerous bolts you'll get water. So blow your wad and buy a head gasket and a exhaust gasket. But a compression test (especially leak down) first will tell you alot about the condition of the rings and the head gasket.

 

[CharlieB]

Re: Mercury Sport jet 90 starting problem

Was this 'rebuild' done since you bought the boat?

How do you know it has about 10 hrs?

This very first thought when a motor starts fine on the hose yet fails to start once launched is that the difference in exhaust backpressure requires a slightly fatter idle mixture.

Try backing out each idle mixture screw 1/4 turn to see if that aids starting. If so, final adjustment must again be made while on the water to BEST idle PLUS about 1/8 turn fatter (CCW) to provide the additional fuel necessary for acceleration.

Note that if you are still running extra oil for break-in, that final carb idle mixture adjustment should be repeated once you return to a 'normal' fuel oil mixture.

 

[costintabla]

Re: Mercury Sport jet 90 starting problem

Was this 'rebuild' done since you bought the boat?

How do you know it has about 10 hrs?

This very first thought when a motor starts fine on the hose yet fails to start once launched is that the difference in exhaust backpressure requires a slightly fatter idle mixture.

Try backing out each idle mixture screw 1/4 turn to see if that aids starting. If so, final adjustment must again be made while on the water to BEST idle PLUS about 1/8 turn fatter (CCW) to provide the additional fuel necessary for acceleration.

Note that if you are still running extra oil for break-in, that final carb idle mixture adjustment should be repeated once you return to a 'normal' fuel oil mixture.

Well the guy i bought it from said he rebuilt the motor and the Mercury mechanic confirmed that it looked like it was rebuilt.

The boat never never fails to start when connected to the water hose. Ill try backing out the mixture screws this weekend and see what happens.

We finished the break in mixture. its running regular 50 to 1 ...



Its funny because after i had the mercury dealer rebuild the carbs,they said that they tested the boat on the water and it ran fine......(makes me wonder if they ever launched it???)

The very next day i took it to the ramp and nothing it would not start.....

 

[costintabla]

Re: Mercury Sport jet 90 starting problem

Ill get the leak down test done this weekend and see what happens.

i am wondering how come it starts on the water hose and not when on the water???

Do you know where i could get the gaskets??

I am fed up i sold my walkaround to buy this and i didnt even get to enjoy it at all this summer.........



Thanks