Need opinions merc sportjet 120 hardly moves in water

[desertratz]

i have a 1996 boat with a mercury sportjet 120 jetdrive. I bought the boat used non running so have no history on it. new stator and rebuilt carbs and it runs now but has hardly any thrust. in reverse it will hardly move and forward it moves a little more but no where near where it should be. ran it at the lake this weekend and i was chugging along trying to build speed and as its getting higher in the revs it felt like it just let loose like it popped out of gear or something. ive read that it may be cavitating but im stumped so far. i took the whole jet drive apart to inspect it and it is somewhat worn but still doesn seem enough that it would run this way any help is appreciated.

 

[QBhoy]

Hi. Believe it or not...that looks like the clearance between the prop and wear ring is well beyond tolerance or acceptable. I know it doesn’t look that bad but In reality any gap there at all over a mm (even at that) is bad and will cause cavitation.
Failing that check your pump seal and all faces on the plate and intake.
I’d almost bet that all else being well and pump bearings being good too...a new wear ring and prop willsee you good.

 

[desertratz]

thanks for the reply, i did the jb weld trick and resurfaced the wear ring with it and replaced one of the foam/rubber seals on the rear of the ride plate it lookd like it had blown off from a backfire. i will hopefully get it in the water soon and see what it does. runs good on land with a waterhose and plenty of air flowing out the jet lol

 

[Scott Danforth]

What is the gap. Should be less than .01". That looks like .050 or more

 

[QBhoy]

What is the gap. Should be less than .01". That looks like .050 or more

Spot on Scott. Can’t say enough how little it takes to throw the whole thing to ****. The clearance needs to be tiny. If you are new to the game...very easy to think it looks not too bad. I was blown away at how little it takes to totally reduce the propulsion. A mm or two is enough to drastically effect it.
I’d be getting a new wear ring and preferably an insert type for ease next time.

 

[desertratz]

it was probably well over tolerance, i built up the wear ring with jb weld but i think its still beyond tolerance didnt realize the tolerance was .01 that is almost nothing. pretty crazy ill probably take it out again and build it up again before it sees the water i have a couple of weeks anyways