Former Yamaha tech looking for help on 1998 GTX

[newguy111]

Just got a boat thrown in my lap. Not too excited to work on it so thought I might see if there are some common issues I should look for out of the blocks. Engine lost the rear cylinder. Indications show a lean condition with complete erosion of the exhaust side crown and welded in rings. The power valve on that side was freezing up as well and required force to remove.

Are there any other issues with these boats I should know about? I am still not sure if there is a carb issue or anything other than that exhaust valve as of yet.

 

[seadoosnipe]

Rave...

The RAVE (rotax automatic variable exhaust) valve would not cause any conditions that would affect the lean or rich running condition of the engine. The problem losing the PTO cylinder is either going to be a lean condition or possibly, losing a rear main bearing. That model being the 951cc engine is a powerhouse. It's the largest 2 stroke Rotax made for the Seadoos. I have the 97 GTX and love it.

If you are pretty sure the problem was caused by a lean condition, you might look into the carb to see if it's clean or could have caused a problem. THe first thing I'd do, with that suspicion is to close the low speed adjuster screw. Count the turns in. If it's less than 1 turn out, then I'll agree that you lost the cylinder from a lean condition. I'd also turn in the MAG carb LS adjustment screw to see what location it's in too.

You might be able to go with a top end rebuild then have a ski worth $2800 bucks or more, depending on the asthetics.......:cheers:

 

[newguy111]

Thanks for the reply. I did check the LS settings and I have 1.25 turns on the PTO side, 1.5 turns on the mag side. The dead cylinder has a large amount of gray matter in it. I do not see any major signs of detonation. I will clean the plug up a bit and do some more inspection.

The piston ring lands on the exhaust side are completely eroded and I suspect a large amount of blow by contributing to squirt heating. I cannot get at the entire engine just yet to see what the bore looks like. I was expecting piston crown damage but looks pretty good.


What is an SBT motor worth these days? Are these worth a rebuild if the crank is good or is a short block the way to go?

 

[seadoosnipe]

Motor replacemnet?

I don't know if I'd just replace it. I don't think you yourself will actually know that until you pull that puppy out. You sound very well experienced in these motors so it may be worth pulling and doing a full inspection, then decide from there.

If your crank looks good, deflection good, bearings in good shape, you might decide to just go with the upper end. If not, the replacement for those 951's are running right at $1200 bucks with a good core.......:cheers:

 

[seadoosnipe]

High speed?

I was just in another thread and a friend posted that this model 951 is suppose to run the high speed screw on the PTO cylinder open. I think he said 1/4 turn. With the amount of manuals I have gone through, all are closed. So, this is new to me. But, the guy who posted is someone I have known for a long time so I trust he knows what he's talking about. So, check that high speed bypass on your PTO carb. If it's closed, this would def be a reason for that cylinder to run lean.......:cheers:

 

[seadoosnipe]

Shop manuals....

I have just gone through the shop manuals for the 951 and I can't find any of them that run the high speed bypass open. So, according to specs, this adjustment should be at "0".....:cheers: