I have the jet ski described in the title, what it does is it idles fine out of water (that i can tell). when put into water the ski will accelerate but will not plain or go over 10 mph. i have replaced the fuel and looked/emptied the gas tank and the fuel lines. I have taken the jet pump apart from the ski and inspected the impeller and intake grate. wearing and the impeller look good. When it was apart i did not check the pump seal, bearings and the driveshaft and spindles (its been awhile since i have taken it apart, can't remember). i also did not check compression and fuel filter. I remember that the impeller cone did not have any grease in it. Does anyone know what could cause this or what the solution to this problem is? thanks

 

If the motor revs up, but the ski doesn't go, then it is a pump problem (cavitation due to blockage, or dinged-up impeller).
If the motor does NOT rev up, then it could be:
Fuel - plugged lines / clogged carb / weak pump (vacuum leak).
Compression - one or both cylinders.
Spark - way out of time, or fouled plug.
It takes almost no power to idle or even rev up on the trailer, so that doesn't rule out any of these problems.

 

Well i can start it in water it moves and idles in the water just fine, but when i pull the throttle it only goes so fast (10mph) even at full throttle. I remember that it was shaking more than usual and hearing a sound like a puff of air every few seconds when i had the throttle full.

 

I am assuming you are trying to describe that it does NOT actually rev up on the water. So we are down to those items I mentioned before. One way to get some more info is to strap it to the trailer, back down into the water and try to rev it up without going anywhere. Now you can remove the cover and try some things. Spray some fuel (even WD-40 can act as a substitute) down the carb... does it react with more power??? That might be a fuel problem then. Does the air puffs sound like backfire?? Does it sound like its in the exhaust (maybe spark timing, or fouled plug), or blowing back up thru the carb??( bad reeds). Check plugs for fouling, and check compression... might be trying to run on just 1 cyl.

 

I don't think that it is backing firing but not 100 percent sure. if i remember right it sounded like it was coming from the gas tank area. I am going to do some compression tests in the spring. What should the compression be? I will try your suggestions when spring comes around and let you know.

 

Compression should be 135 psi or more. It is important that each cyl is close to the same reading... maybe no more than 5 psi different.
Best readings are with a warm engine, throttle open, opposite plug INSTALLED during the test. But most guys don't worry about all the details & just crank it over with the meter and see "about" how much compression they have. A squirt of oil in the hole may raise the readings as well. Be sure to ground the loose plug wires while testing... loose plug wires can damage the CDI ignition module.
I understand the "wait for spring". My skis are under the tarp, plus 2 feet of snow.

 

I have taken my pump apart. i found that my vane guide has some huge chucks out of it. Would that do it? i also bought a new impeller tried to put it in but as soon as i start spinning it to go in it sticks and won't turn anymore (sometimes either direction). could this be the pump housing or the impeller itself?

 

I am still not sure if we are talking about an engine that won't rev up, or one that will rev up but won't push the ski very fast. Can you enlighten me on that? Did you do the compression test?
Big chunks out of the vanes can cause cavitation and reduce thrust. It will not stop the engine from reving up.
Does the new impeller (which won't cure the broken vane problem anyway) bind up on the threads of the shaft, or does it bind up rubbing on the wear ring. If it rubs on the bore of the wear ring you probably need a new pump (is this a salt water ski... they corrode behind the wear ring and make it swell up.). But you need a new pump anyway due to the broken vanes. If the impeller is binding on the shaft threads, then inspect them carefully and chase the threads if needed.... or maybe you need a new shaft... you could get that with the new pump. There are used pumps on the market that will be way more economical than new. They might need new bearings but that's a cheap fix.