PLEASE HELP 1995 searayder with 120 hp Mercury jetdrive can't get over 10 mph

[C2CMARINE]

my 95 searayder has a 120 hp mercury jet engine, i just bought it and the seller said it ran great, started up fine, I bought it and took it out on the water, it starts fine but at about 7-10 mph it stops going any faster, the engine sounds good, it revs up and keep sending power to the jet pump but the boat won't go any faster, I checked the ride plate and it seems to be ok, and I checked the impeller and wear ring. The wear ring has gouges around it, but there doesn't seem to be a lot of clearance but I guess I wouldn't know, I did order a manual, I'm an I/O guy and this is my first jet boat. I will attach pics of everything and I'm hoping one of you can help me. Is the ride plate not flush enough? To much clearance on the wear ring? What can I do? Thank you so much in advance for any help you can offer me!

 

[Luke Lapham]

Need to look in front of the impeller in the wear ring .. look for pitting and holes more likely on he bottom just inside near the intake grate.. clearance looks fine.. if it was too tight it would sieze up the engine.

I had the same thing happen on my merc M2 .. took off the wear ring and found deep pitting .. the pitting will cause cavataion and it feels like your "slipping" motor reeves up fine but wont go over 5-6 mph ... i found that jb weld works great as a bonding agent to fill the pits and then sand it back down to spec... done it twice and it held up for years..

 

[tdonoughue]

I guess the first place I would start would be inspecting inside the intake to make sure there is nothing blocked up there. Not likely, I will grant you, but...

Then, on the ride plate, on mine all of everything is smoothed over with silicone. If your bumps are causing dirty water into the intake, that will limit you quite a bit.

I can't speak to the clearances (even on a Yamaha, which I have). Others will have to chime in there. But it impresses me that if there are scratches, you probably are in for a wear ring replacement. But I don't know that would limit your speed...

 

[Luke Lapham]

The speed is limited by the cavataion. With the holes or pits in the wear ring act as a "void". These pumps have to be smooth as the Preashure is pushed like vacuum.. that being said if there are voids they create the air/bubbles that will cause cavataion and it won't create thrust.

 

[renrab]

I had a 90HP 17' Dynasty Jet Storm, same setup.

First thing I replaced was ride plate seal, night and day difference.

http://www.boats.net/parts/search/Merc2/Jet/120 (POWERHEAD) 1995/0E095088 THRU 0E141088/DRIVE HOUSING COMPONENTS (SPLINED DRIVE)/parts.html

Part#55 is the seal, if this is old and tired it will allow MAJOR cavitation.

Next I would recommend using JB weld (not quick dry normal formula) to build up the wear ring, you want as tight a tolerance as possible between impeller and wear ring. I also had my impeller sent to impros and reworked but I had major dings in the impeller when I bought the boat.

I went from a painful 3 min plane out to an instant pop out of the water.

Have you done a compression / spark test to see that you are firing on all cylinders as well?

I can be of some assistance but I know the 90HP's a bit better.

Matt

Edit1: When reinstalling the ride plate and new gasket it will be an EXTREMELY tight fit, you can use a car jack with a 2x4 cut the width of the ride plate to slowly jack the ride plate and gasket up while tightening the screws, just fyi.

Edit2: After re-examining your pics you can see how bad your gasket is, completely worn and tattered around the corners, i'd be willing to bet the ride plate just falls out when you loosen screws, with a "good" gasket the ride plate should take alot of force to remove and install.

Edit3: Your screen inlet part #45 in the link I gave looks like its cracked, this would cause major issues as well I believe. I feel like in your picture I can see broken casting on that aluminum screen, I may be wrong though.

 

[Cobra Jet Steering LLC]

If your pump is messed up I still can not understand why only 10 mph, cavitation can cause slow hole shot and lower speed, however I would be checking the spark plug wires, those old force wires break down internally , they work for a bit and then stop firing , the inside of the wires turns to dust , usually the engine tries to run when it is cold or on the trailer but in the water it drops to very slow speed quite quickly, 10 mph sounds like you are dropping a cylinder or two in the water, I know how to replace the plug wires without replacing the coils, so if that turns out to be the issue let me know.

To check your wires remove the plug end of the wire, Use needle nose pliers to pull on the inside core wire, if it is good it won't budge if it is bad a short piece will pull out and then there will be nothing .

 

[Hector]

My BIL had the same issue and it was the cable that drops the bucket over the jet nozzle. It was partially dropping it while trying to go forward.

 

[Floyd]

It looks like the middle bar of the intake grate is missing and the base has lost some bits as well. Those parts may have been ingested and damaged the front of the impeller...which would cause cavitation. Take a look with a flashlight.

Also, in the photo that shows the transom drain plug, the casting above the hex bolt-head looks like something has broken off the casting?