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curiousgeorge

'91 skier surging

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curiousgeorge

I have a '91 skier with a 5.7 mercruiser motor. The boat starts and idles great. When I get to about 20 mph it begins surging and will not accelerate. The more I throttle the less power I have. If I hold it steady at about 20mph it is fine.

 I have rebuilt the carburetor 2x. I have replaced the fuel pump and fuel filter. Anyone ever run into this? 289 hours on the motor. 

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curiousgeorge

Believe it is a cracked head. Water came out when spark plugs removed. 

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Woodski

@curiousgeorge:  bummer, that is  not a good sign.  Water from all 8 cylinders or just a few?  Those symptoms don't sound a lot like a cracked cylinder head, more like a bad anti siphon valve at the fuel tank so if after the big problem is resolved, you still have issues, check the anti siphon valve located on the top of the fuel tank.  If you have to go the head replacement route, there are several options open to you to bump the power level up at pretty much no extra cost.  The newer generation (post '96) Vortec cylinder heads breather much better than the ones originally installed on your engine, basically a bump from 265 hp to 310 hp.

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curiousgeorge

@WoodskiI have read some posts about vortec heads. Don't they require an exhaust manifold swap? I admit I am a mechanical idiot. I am relying on friends and family to help me fix this boat. 

Edited by curiousgeorge

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mcgreg

Tyro here. Did it ever overheat? Impeller go out? Water in adjacent cyl holes or one hole? Maybe it is simple blown head gasket? Guess you will find out when the head comes off. Check out compression before hand. Pray it aint the block. Good luck on that!

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Woodski

@curiousgeorge:  Not necessarily, it depends on exactly which head you end up with.  The bolt pattern is the same so not issue there, some manifolds may not exactly align on the port to port clearance.  When I upgraded to fast burn cylinder heads that have a D shaped port, I needed to simply grind a bit off the top of the exhaust manifold (inside) to match up the ports.  Gasket choice is important also for a good match.  More importantly on the cylinder head is the orientation of the intake manifold bolts, there are two styles, perpendicular to head or vertical to earth axis.

Leak down test should show you a lot and pinpoint if you have a bad head gasket or even a cracked cylinder head.  Good luck.  If you need to go with a replacement cylinder head, the Vortec (post '96 version) will bump hp from 265 to 310 and is a nice upgrade & easy to do. 

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