Mercury 90HP SportJet - No Speed

[dannyh83]

My name is Danny and this is my first post here. I have a '94 Dynasty with a 90HP SportJet Mercury Engine that will not accelerate past 10ish mph but the motor will sure crank up (high RPMs).

I've read through old posts and cleared the intake grate, checked the ride plate and seal, checked the impeller which looks good. One more suggestion I keep coming across is to check the gland seal and inspection cover for air leaks but not sure what exactly these are or if there is one on a Merc Engine...? Any help or suggestions would be appreciated. Any other places air could be getting in? Thanks!

[CharlieB]

Is this a new problem or did you acquire the boat with this problem already?

Impeller clearance to the pump body is critical on a jet, once this clearance grows there is increasing 'slippage' of the water and RPM rises and thrust reduces.

Can you remove the intake grate and use a set of feeler guages and tell us what the clearance is between the impeller and the pump housing 'ring'?

[dannyh83]

Yes. I've looked inside with a flashlight and it looks very tight but I will measure with feeler gauges in the morning and post back. Thanks for taking time to help me out!

[dannyh83]

Just checked the impeller clearance and it is hard to get a matchbook through indicating to me that it is about .020. I will double check with feelers in the morning but don't think that's the issue (from what I can tell). I aquired the boat in this condition; motor runs great and boat moves in water just not fast. Are there any other common issues I should be looking for when I measure the clearance?

[CharlieB]

Go here to download a manual for a jet drive, it may not be the exact drive but the impeller clearances and leading edge of the impellers are the same.

http://rapidshare.com/files/21324034...t-50-55-60.rar

This link was pulled from those posted in another thread Merc Manuals ***Sticky Please*** http://forums.iboats.com/mercury-mariner-outboards/merc-manuals-sticky-please-415403.html
which is the topmost thread in the Merc forum.

Remember the password 'ribforum' as you will need it to unpack the compressed files.

Study section 6c which details impeller clearance and refinishing the leading edge of the impeller, a worn leading edge will cause your current problems of high RPM and lost thrust.

[dannyh83]

Thanks for the help! I was not able to download that link (file does not exist) but I found another link to a manual..

I measured my clearance and it is very low. 0.008 is the largest that will slide in and out while the next size I have (.011) just barely fits but it gets stuck so the actual size is somewhere between the two, definitely less than the 0.020 clearance specified.

Is there something else that could be causing the boat to cavitate or not pickup speed? Attached are some photos. See anything?

[CharlieB]

I just tried again, go here
http://forums.iboats.com/mercury-mariner-outboards/merc-manuals-sticky-please-415403.html
Click on the link from aChris on the next to the last post
http://forums.iboats.com/showpost.ph...11&postcount=3
Almost in the middle of the 2 stroke lists click on the link
http://rapidshare.com/files/21324034...t-50-55-60.rar

You will need to remember the password 'ribforum'

While this is for an outboard jet pump and the clearance IS larger than in your pump, pay particular attention to section 6c Worn (Dull) Impeller, is the same style leading edge on the blades of the impeller.

As this edge gets worn from sand/rocks, the water cavitates, creating air bubbles, and the pump revs because it looses its 'bite' on the water.

You will need to remove the impeller, either carefully file those leading edges or take it to a prop shop that can. Alternately, replace the impeller.

[kend301]

You are beyond impeller clearance here if your top end is 10 mph , is the ride plate bent ? Also post up in the PWC section that is where you will find lots of sport jet threads !

[kend301]

Post up some more pics if the impeller , the pitch seems to be off , I do not know if it is the angle , mark one of the impeller blades and then rotate it and take a pic of each blade at the bottom as they pass .. think the impeller is bent.