This is really long... sorry, but it's the only way I can describe the 2-stroke oddities that are part of how these boats work. Settle down with a cup of coffee and bear with me.
The 2-strokes (particularly the LS/LX series) are a little different than other boats... I want to clarify a few things before digging in and making you chase stuff that may not need to be chased.
Note: All RPM values I use are "guestimates". There will always be subtle differences for every one of these 2-stroke boats... my 4000 RPM may act like your 4300 RPM. If the numbers I quote below are a few hundred off of yours, don't sweat it. Once you get how these motors make their power, it'll all make sense.
First, if you ran on one engine faster than "no wake" speeds, you may have forced water up the exhaust into the non-running engine. Top priority is getting the water out if that happened. If it didn't happen, we get to deal with your 4000RPM issues.
For future reference... To get the water out... remove the plugs from that engine, remove the safety lanyard, crank the engine for about 20 seconds, watch oil gas and water spray all over the place.
I understand you've managed to start and run the second engine... so you don't need to do this, it's already done. But, chances are
very high that the spark plugs got fouled at least somewhat while burning off anything. Take them out again, check them, clean them, replace if they're nasty (I'd check both engines while you're in there). If the engine didn't get damaged, we get to tackle the 4000RPM issues now...
The LX/LS boats are weird. They will not run happily from 4000-5000 RPM. This is by design due to the tuned-port exhaust system and it's just the way they are. Before testing, clean out your air filter. Based on
@holidayshoreslsboater1 symptoms, I think it's either wet or has too much oil in it. Once that's done, here's the test of whether or not you actually have a problem...
Run both engines up to 4000 RPM. This is probably enough to get on plane if the boat isn't overloaded. Once it's on plane and settled (or as much as it's going to get), goose the throttles and run her as far as she'll go on the tach (but don't go over whatever the max is). This shouldn't take more than a 30 seconds to get the RPMs up. Let them sit there at that max RPM for another 20 seconds or so, then back down to 5500 RPM. If it happily runs at that RPM
and you can now smoothly throttle up to max and back on both engines (without goosing it), you probably don't have a problem.
If you back down the RPMs to 5000, it will start to run rough or just keep dropping the RPMs until it comes off the pipe (the exhaust sound changes drastically and suddenly). It will probably not run at 4500 RPM at all. It'll always try to slow down or speed up the RPMs in that middle range. This is normal for these boats... Don't run at that "middle range" that it doesn't like.
If that's not what happens with your boat, describe what happens when you try the test described above. Even better, have someone with a video camera sit in the front facing backwards and give us some video, or edit your profile to show your location and one of us might be able to come take a look at it. It's possible you hydro-locked it (tried to compress water in the pistons), but I don't think so based on your previous descriptions.
If you're not used to the tuned-port exhaust, it will seem broken because of that huge "dead spot" in the RPM range before it gets "on pipe". It's just the way tuned-port 2-strokes work.
I know this was long, but I just wanted to make sure you aren't fighting a problem that doesn't exist.
