787 Top End or Full Engine Rebuild Needed - How to know?

[670hoth]

I have a 97 XP. It runs good other than starting hard in the water but has 120 psi in both cylinders - low but dead even. How can I tell if I can get away with a top end rebuild (hopefully w/o removing the engine) or if I need a full rebuild. I don;t know the history on this on the machine and don't know if anything has been done to the engine before. The hull does have a fair amount of battle scars which indicate that this machine must have a lot of use on it.
What can I check w/o completely disassembling the engine? Thanks

 

[AdamXP800]

Have you tried a different compression gauge? Hard starts could be low compression but also a number of other things. Do you have grey fuel lines? Have you cleaned and rebuilt your carbs with oem parts? Cleaned or replaced fuel selector?



96 XP800!
Keep the 2 strokes alive!

 

[Coastiejoe]

If the bottom end has not been rebuilt, then doing a top end on a 20 year old engine will likely cause the bottom end to fail quickly....

 

[670hoth]

Yes, I have checked compression with two gauges and they both agreed at 120 psi give or take a psi. I have replaced the fuels lines, cleaned carbs, etc... but I did not replaced any parts in the carb or check pop-off. I do think I need to go through the carbs again with rebuild kits and make sure the pop-off is correct but I have also decided that I pretty much need to also do a top end or full rebuild.

Coastiejoe, I do agree with you reasoning that a 20 year old motor probably should have a full rebuild but I don't really know for positive if this is the original motor and/or if its been rebuild before. For that reason as well as the fact that I will probably ride the thing a half dozen times or so next summer and sell it in the fall I was hoping there were some specific tests or things I could look at w/o disassembling the complete engine to give me a good idea if the bottom end will hold up to a new top end. Maybe the only true way to know is a fall disassembly and component inspection. If this is the case then it would not make sense to not rebuild the entire thing at that point. I am just trying to avoid this if its not needed.

 

[SabrToothSqrl]

compression is spiffy, but how's the real world performance?

what's the RPM at WOT, in water? what's the MPH?

what's the acceleration?

I wouldn't do anything if it's performance is just fine.

 

[Minnetonka4me]

My logic is....jetski motors are unlike any other motor. They are subjected to water all the time and with that...and being old like Joe brought up...I think ALL motors that have no confirmed history of a full rebuild should be split and inspected.

 

[670hoth]

SabrToothSqrl, Actually, other than the hard starting it runs really good. I don't have a tach and also need to replace the speed sensor so I can't provide RPM or speed info. From my own perspective, the RPM sounds where I think it should be and the top speed is good. Acceleration is good after you get out of the 1st qtr throttle range but I think that low end throttle issue may be tied to the hard starting. It gets way to much gas when trying to start it and at low end until it clears out. I guess this could possibly be attributed to low compression but the carbs really could use carb kits and the pop-off checked so it could be related to that as well.

Minnetonka4me, your logic makes perfect sense and before I was divorced I would think just like you. Now, between the US govt and my ex-wife I get to spend quite a bit less than half the money I make. The only way to have a decent life is to scrutinize spending and don't spend on things if I don't have too. I have a lot of experience with snowmobile engines and in many cases you can do at least one top end rebuild between full rebuilds. This can even apply to some old sled engines as long as they are not of the extremely high strung type.

 

[AdamXP800]

Sounds like you might have some leaky needle and seats



96 XP800!
Keep the 2 strokes alive!

 

[670hoth]

Yes, maybe leaky needle and seats. I will say that the needles looked really good and had no ring around them. I can't say I got a good enough look at the seats to say what condition they were in.

 

[howie099]

The solution that is the best rebuilt engine and rebuilt carbs. Real money like $1200-$1300 yourself. What could get you buy is a top end and carb rebuild. When you have the high hours or just age of the seals usually the bottom goes out not that long after the new top end. I would just do some new rings and carb fix and bank some money to do the rebuilt next winter. If you are going to sell the ski I would for sure just do the rings and carbs. It makes no sense to dump that much money if you are going to sell it. What could you get for that ski? Its not worth doing an engine just to sell it off.