Runaway 951

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Water Crafts
2000 seadoo rx
#1
I have a 2000 rx carbbed when I bought it had low compression (I didn’t check it got screwed) so I put a set of jugs and pistons in it now after getting it all back together the engine wants to run away on me like revvs straight to 6 grand in the water compression before doing pistons was like 80 psi but it ran fine it just didn’t go above like 45 mph kinda stuck idk what to do at this point

Things checked
Throttle linkage working properly
No obvious disconnected vacuum hoses
 
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PwcRyan

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Water Crafts
96 Seadoo XP 787 and 1988 650sx modified
#2
I don't know much about the ski, but is the throttle cable and throttle linkage functioning properly? Also, does this ski have vacuum lines? Maybe you have a vacuum leak somewhere? It's got to be getting uncalled for air/fuel from somewhere.
 
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Water Crafts
2000 seadoo rx
#3
Throttle linkage is working properly not really sure how to go about testing for a vacuum leak on one of these
 

mikidymac

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#4
If the cable has slack and the idle screw is backed out then you have an air leak. It is most likely between the carbs and the intake so check that.

Not everyone does it but you really should pressure test any engine you have apart for air leaks.
 

mikidymac

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#6
The only way is to block off the carbs and exhaust manifold and fill the engine with 8 psi through the pulse nipple. It should hold exactly 8 psi for a minimum of 10 minutes.
 

mikidymac

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#7
I would check the easy stuff first, throttle cable, idle screw, carb base gaskets.

Also the 951 is really lean and perfectly fine ones have been know to runaway on the trailer if you rev the engine up too much. If this is happening in the water then you have another issue.
 
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Water Crafts
2000 seadoo rx
#8
I didn’t mess with the idler screw but I’m wondering if the previous owner might have cut off a lot of the fuel with the idler screw to make it idle with 80 psi compression
 
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Water Crafts
2000 seadoo rx
#9
Throttle cable is definitely slack and the carb base gasket is brand new not saying it’s impossible for it to be bad just wouldn’t suspect it but I’ll check
 

mikidymac

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#10
That is very possible.

I would back the screw all the way off until it isn't touching the throttle arm then turn it in 1 full turn after it touches. This should at least let it sputter to life on the trailer until you can adjust it closer.
 

mikidymac

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#11
If it ever runs away even on the trailer just pull the choke until it floods out and quits.
 

mikidymac

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#14
No, you have to take the carbs off and the exhaust manifold off and block off the openings, I have some rubber block off templates from back in the day that get sandwiched between the manifold and carbs to make an air tight seal. Then you put 8 psi into the crankcase nipple with a gauge and schrader valve to keep it air tight. You then make sure the gauge doesn't drop, you do not keep pumping air in, you want a closed system.
 

PwcRyan

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96 Seadoo XP 787 and 1988 650sx modified
#15
No, you have to take the carbs off and the exhaust manifold off and block off the openings, I have some rubber block off templates from back in the day that get sandwiched between the manifold and carbs to make an air tight seal. Then you put 8 psi into the crankcase nipple with a gauge and schrader valve to keep it air tight. You then make sure the gauge doesn't drop, you do not keep pumping air in, you want a closed system.
Is this a test for case leaks? I'm learning too. Lol
 

mikidymac

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#16
Yes, it will test for leaks at all engine gaskets and seals. It should be done on any old engine or engine that has been apart and especially after a rebuild.
 
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Water Crafts
2000 seadoo rx
#18
No but it didn’t have the issue before doing the engine so I wouldn’t think they’d suddenly go bad I did the job with the motor in the ski
 

Chester

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#19
No but it didn’t have the issue before doing the engine so I wouldn’t think they’d suddenly go bad I did the job with the motor in the ski
This doesn't make any sense. Your first post says you bought it with low compression already. How do you know a seal wasn't leaking and caused the first seizure? You have no idea if there was an issue with the seals unless you pressure test the engine or visually inspected them for wear.

Chester
 
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Water Crafts
2000 seadoo rx
#20
If the seals were worn out before had would it not have runaway with bad compression and now suddenly with compression be running away?
 

PwcRyan

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Water Crafts
96 Seadoo XP 787 and 1988 650sx modified
#21
This doesn't make any sense. Your first post says you bought it with low compression already. How do you know a seal wasn't leaking and caused the first seizure? You have no idea if there was an issue with the seals unless you pressure test the engine or visually inspected them for wear.

Chester
If the crankshaft seals were bad enough to cause a vacuum leak of that magnitude to cause a 6000rpm spike, wouldn't he would have bigger issues? I could see them contributing, but I would start small.
 

PwcRyan

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Water Crafts
96 Seadoo XP 787 and 1988 650sx modified
#22
If the seals were worn out before had would it not have runaway with bad compression and now suddenly with compression be running away?
Rebuilding the top end and improving compression will also increase vacuum throughout, so in my opinion it could happen.
 

jhjesse

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#24
You are sucking air somewhere. Check you carb base gasket. Might have slipped while installing your carbs.
 

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