MY SUPERJET 701 ENGINE SEIZED...MY QUESTION IS WHY?

dlrainer0217

dlrainer0217

Location
Pennsylvania
Hey everybody, I recently purchased a 2009 Yamaha superjet 701 with some work done to it. Her name is blue magic and she runs like a raped ape! Unfortunately I had her out yesterday and I ran it wide open throttle for a good stretch (1/2 mile -1 mile) and then while riding at partial throttle the engine got tight and locked up. Instead of trying to restart it, I let her cool down and once cooled, then engine was no longer seized.

So I sprayed some WD-40 in the cylinders and fired her back up and she seems to run ok again. However, I did a compression check today and my rear cylinder only has 110 PSI, when they both used to have 165 PSI.

Here is a list of some of the motor work that was done to the ski by the previous owner:

stock top end
ADA girdled head kit
factory b pipe
pod filters
Unknown manufactured jet kit


I've always run 50:1 fuel mix with premium gas. This is my third time taking it out, only in fresh water.



So today I decided to tear down the top end and find out why I have low compression, and here is what I found (see pics). Rear cylinder bottom ring is seized into the ring land, and both pistons have two seizure marks each.

What are your guys thoughts as to what may have caused this?? I'm thinking a lean condition, but I still haven't taken the carbs apart to see which jets are in them. I'll be doing that today. One thing that I did notice about the carburetors is that all 4 fuel screws were set at only 3/4 of a turn out. Does anybody know what they should be set at?

Thanks for your help!


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Blue

Blue

Judging your cheapness
Location
St Cloud Florida
That sucks. We need to know what size jets your running to tell you where the adjustment screws need to be set but typical is 3/4 to 1 turn on bottom and 1&1/2 to 1&1/4 on top screw. You really need to run 40:1 oil mix and use a good quality oil. And a picture of the top of the pistons and the cylinder domes will help determine if it was too lean or just too little oil.
 
Jakobi86

Jakobi86

Did you hold it in one spot? Always vary the throttle position and don't let it stay in one place. Move it, blip it, brap it, whatever you want to call it, just keep it moving. My guess is you just came off a long pull and right afterwards gave the motor a dose of part throttle cruising, and in my opinion, doesn't have enough fuel to cool it down fast enough and you had a seize


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dlrainer0217

dlrainer0217

Location
Pennsylvania
Jakobi86 said:
Did you hold it in one spot? Always vary the throttle position and don't let it stay in one place. Move it, blip it, brap it, whatever you want to call it, just keep it moving. My guess is you just came off a long pull and right afterwards gave the motor a dose of part throttle cruising, and in my opinion, doesn't have enough fuel to cool it down fast enough and you had a seize


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Click to expand...


haha...that is exactly what took place before the seizure, so it sounds like you are pretty on point!
 
dlrainer0217

dlrainer0217

Location
Pennsylvania
Do you guys all run 40:1? I'll take more pictures now.

Do you guys think I could get stock sized pistons and just hone the cylinders? Or should I go 81.25mm and get them bored?

I really appreciate the help and I'll go check my jet sizes too...
 
Blue

Blue

Judging your cheapness
Location
St Cloud Florida
Yes thats lean. Your pistons are almost completely black. Open the low screw to 1 turn and set the high at 1 &1/2 when you put it back together. Your jets should be at least a 75 pilot and a 135 main with a bpipe and you should have a lower popoff around 40 psi or so with prok filters. If you can hone out the scoring its possible but ur best to have a machinist measure the bore and confirm the exact clearance. Most likely it will need a bore as its 6 years old already.
 
CRJ

CRJ

Hibernating
Location
Toronto
50:1 is fine to run. anyone who runs less is just being cautious, 40:1 will run fine, its just more protection, and you have to consider you are now getting less fuel per drop or ml. it was very lean, pistons shouldn't be all black. Ski was most likely setup for freestyle, not meant to be held wide open, and you got a WOT lean seize. I'm sure it was scoring while you were WOT, but most motors will continue to overpower the locking effect for some time till they lock solid, in your case till you got off the throttle.

Blue mentioned adjustment screws and where they should be set, but frankly the turns out don't have preset adjustment, its relevant to your jetting and riding style, so its impossible to say "they should be X turns out". However 2.5 turns out on a screw is one jet size up from whats in there, so when at 3/4 turn out you would be around 0.3 above your current jet, which isn't enough fuel clearly. Go up by 10 on your main jet, leave the screw at 3/4, tune from there. always start rich, and work towards lean. Too much fuel won't lock a motor.

time to bore it, port it and have some fun!
 
dlrainer0217

dlrainer0217

Location
Pennsylvania
So I pulled the carbs apart, they look nice and clean. They're running a 135 main jet and a 75 pilot jet. I'm still in between getting 81.25 pistons or just getting stock pistons and honing the cylinders. There's not really any deep grooves and the cylinders measure 31.189 throughout.
 
Dartman

Dartman

Location
usa
clean out the cylinder wall with some acid, give a light hone with a precision rigid hone then re-measure the bore
you always could use wiseco pistons also, they like looser bore clearance
 
Roseand

Roseand

The Weaponizer
Site Supporter
Location
Wisconsin
Hmmf, looks familiar.. Just stuck a piston myself. With muriatic acid it cleaned up the cylinder and there was no scoring whatsoever. Maybe you got close to that lucky too..Never know.
 
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