Simplest issue would be that you sucked something into the pump and it just needs to be cleared.
Pull the pump. You'll need and impeller tool to remove your impeller. Check it for damage, check the wear ring for damage. If the person that you bought it from has not replaced the wear ring then plan on replacing it. You''ll need to cut the old one. I used a chisel and busted up my knuckles a bit. I'm sure that there is and easier way, but once the ring is cut it will come out easily. Freeze the new one and it will slide right in with little resistance. You'll need some synthetic gear oil to refill when you are done. Take the cone off and drain the oil. Pull the pump shaft and make sure that the bearing and shaft are in good shape. If the previous owner hasn't rebuild the jet pump then it is probably time. Rebuild kit are about $40 on Amazon or $105 with the pump shaft if it is worn and needs to be replaced. Tapping or pressing out the bearing and seal and replacing them is about a 20 minute job if you've done it at least once before. I just rebuilt mine and the job went easy. I wish that I had done it sooner so I didn't need to replace the shafts.
If all of this checks out good then check the through hull fitting and carbon ring. If these aren't sealing well then when the jet pump revs up it will pull air through these and you will lose power.
Check the drive shaft splines and make sure that they are in good shape and that there are no metal shavings in the grease. Damaged drive shaft or pto splines can cause this issue.
Good luck!
