Super Jet '90 SN with 701 top end, Power loss and now knocking

N

Norcaltrd

Location
Redding, CA
Took my two new to me super jets out to the lake with my brother for the third time.
All went well the last two times, but this time we made it maybe 3 minutes from the launch and I noticed my brother signaling to me and heading to shore.
As I pulled up he said that he was riding along at about 1/2-3/4 throttle and the power suddenly dropped out.
I tried checking the impeller and intake grate to make sure he hadn't sucked up a stick or something, everything was cleared out.
I started it once and noticed it had no power, then killed it.
I removed the hood so I could hear the engine better and upon starting again hear a pretty obvious knocking noise. We found a length of rope on the shore and I towed him back and we called it a day.

Best guess as to what the problem is?
Im thinking top end rebuild which sucks since I just bought it :\
 
Quinc

Quinc

Buy a Superjet
Location
California
Check your exhaust, intake, and head for loose bolts. Might as well pull the top end apart. Only takes an hour or so and will be a good time to make sure everything was put together right and throw on some new gaskets etc.
 
N

Norcaltrd

Location
Redding, CA
By knocking I mean an abnormal engine metallicy noise, I cant be specific just that it doesnt sound like it should.
Its rough to get started and will die if the throttle isn't being played with.
When I started it at the shore and gave it gas it would hardly move and was very rough, died soon as i stopped throttle.
 
Th3 VVay

Th3 VVay

Sounds like you found the issue. Hopefully its just a hole in the piston and nothing came apart. I don't know how familiar you are with two strokes but they are incredibly easy to work on. Remove the cylinder head and see how that piston looks.
 
N

Norcaltrd

Location
Redding, CA
I have worked on Honda/Subaru/Toyota auto engines and Honda street bike/dirtbike engines.
Never a two stroke before though or a PWC engine.

Ill pull the head and get some pics.
 
N

Norcaltrd

Location
Redding, CA
Piston #2 was melted. The #1 dome has a few nicks, the #2 dome has about double the nicks but nothing major.
The case sleeve had a bit of slag stuck to the very top wall where piston had melted, I knocked it off with a light tap on a screwdriver and then scraped with a razer blade.
Now I am pulling off the piston head itself.

I cant see any large chunks in the bottom end, only fine metallic powder residue around the #2 piston head.
Should I just replace the piston, re-tune the carb, and call it good?
 
Th3 VVay

Th3 VVay

I would just do a top end rebuild. Pistons of dif manufacturers weigh slightly different and can unbalance the engine. Its only like 200 bucks for a full top end rebuild. get the cylinders honed (another 20 bucks) and reassemble the top end. Bottom end should be fine just wash it out with premix. Check your plugs once its running to verify its not running lean. If it is, asjust carb accordingly.
 
Quinc

Quinc

Buy a Superjet
Location
California
Download the manual and check the tolerances for the crank too. Don't want to put it back together to find out there where other issues.
 
Th3 VVay

Th3 VVay

Yeah that would definitively do it. Get the top end done, get rid of that head or run race gas. You'll be good to go either way.
 
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