Agree with all of the above, but check to make sure there’s no water in the cylinders or leaking gas through the fuel pump diaphragm. I’ve had both issues in the last couple of months on 9.5’s (and other motors for that matter, too). The motor will run erratically to say the least with either issue. Sometimes good, sometimes bad. I had one run OK up to mid range, but be all over the place at the top end.

Make sure you have done a ‘link and sync’ too. Pretty easy on a 9.5…if the plastic follower is stuck under the magneto cam, the motor will run really erratically because the fuel/air mixture is wrong at all ranges.

You already said you’re using new J6’s…there is a situation where plugs that are the wrong heat range can cause surging power (J6’s are fine for that motor, just be sure they’re good plugs of course gapped correctly, wrong gapping can do this too).