Welp, I've searched and didn't find anything on Barnacle Buster; so I thought I would ask. Hopefully I'm not opening up a can of worms here.

Long story short, bought a boat. Boat runs well and idles all day long. However, when I get over 4500 RPMS, alarm comes on and powers down the motor. I can immediately crank back up the engine and let it idle to cool down, and the alarm will go away and I can continue on my way.

I was thinking about a Descaler product, but am being told to take all Zinc and thermostat out of motor before I flush it ... any recommendations? Or preferably previous experiences ?

I am quite certain you are chasing the wrong problem that doesn't exist. You need to diagnose what's actually happening and why, not randomly lock onto a "fix" that likely won't be a fix.

You are describing a classic fuel starvation (flow restriction) issue. There are many possible reasons for this, some extremely simple. Example: A fuel filter in need of replacement can cause this. Enough fuel can trickle through to support idle to low RPM, but not 4500 RPM (in your case). An obstructed fuel tank vent, a stuck/clogged anti-siphon valve, a failing fuel pump (pinhole in the diaphragm,, for instance),

Is the alarm system actually shutting down the engine? Is it emitting steam from the telltale streamer when you restart? You never said what year, brand, or model motor you're even talking about! Consider it may be going dead due to the fuel starvation, not the alarm itself. Is this a motor that can store error codes? I think you're better off changing fuel filter(s), checking for fuel line liner degradation (the chance of this depends on the boat age you did not provide).

Have you considered using a shop for problem diagnosis instead of muddying the water by taking random action?

Apologies for lack of information. I have a 1999 Yamaha OX66 V6 150HP engine. (OLD, I know)

I can run the engine all day at idle, or below 4k RPMS (about 35 MPH), however shortly after I go too or over 4k RPMS the engine throws an alarm (steady tone), there is no reaction from the indicators on the digital gauges and the motor will run for about 4-5 seconds after which it idles down slowly/smoothly until the engine shuts off.

I can immediately fire the engine back up and it will idle, the tone (beep) is still there for a short period of time where it is cycling water and then the tone will go off and I can role on until I go above 4k RPMS again ...

The pee hole never stutters or stammers, and always seems to have an excellent flow; EXCEPT for a moment when I am around 4K RPMS; it is honestly "almost" unnoticeable, but there is a small lack of flow slightly before the tone comes on ... leading me to believe there is a water intake, flow, or output problem. Appreciate the quick reply and help.

Sorry, to answer your other questions. It doesn't emit steam form the telltale streamer when restarting. I am not sure if it is a motor that can store error codes. I have the receipt where the water pump was replaced in 2018, however I am not sure about the fuel pump, etc ... Degradation of lines were a concern ...

Two things I had on my Yamaha 150 if the water pump is for sure good. Poppit valve and a pinched fuel line. Had my boat at fun n sun twice and they didn't fix the overheat. Pulled the cowling and found a fuel line that bends in a 90* curve that was pinched off. Bought a new fuel line and the overheat was fixed.

Some might be thinking "Huh, how does a pinched fuel line cause an overheat problem?" ....The leaner an engine runs, the hotter it will run as the oxygen/fuel ratio is altered beyond the intended point for the engine's design. It runs the hottest just up to the point that there's no longer enough fuel for the burn, at which point ignition dies. This is why I suggested the OP investigate some cause for fuel starvation via restriction.

This phenomenon can actually burn up high performance engines (such as race engines) quite easily. Piston aviation engines are run through a range of altitude that affects the mixture ratio for a fixed fuel flow. Engine mixture is adjusted based on altitude and engine temperature. It's so critical that a good engine monitor tracks the exhaust gas temperature of each individual cylinder!

The additional info is great. Some Yamaha owners may have some wise advise. Some of this applies to all. You should discover what fuel filters you have. There's often one on the engine; I'll let yammie guys speak to that part. In the boat, do you have another filter for the fuel system (often looks like a spin-on oil filter)? Replace all that apply - it's probably needed anyway. Get a 6 gallon portable marine fuel tank, and run from that as a test. Use as little of the boat's fuel line as possible for this test, and find out if the shutdown still happens. This will do a divide and conquer to answer the question if the problem is hanging off the transom, or inside the boat.

There are specifications for fuel pump vacuum pressure, and tools to measure it. If you rule out the above items, it may need to go to a mechanic to have the fuel pump tested. A weak fuel pump (on its way to complete failure) will often supply fuel, just not enough at higher fuel flow demands. The test will help determine if this is the problem, but you should rule out these other things first.

I have an infrared thermometer I would use to check the cylinder head for heat (though I'm not thinking this is your issue). I'd run it for a while at 4000+ (where it won't shut down), remove the cowling, and check the cylinder heads. THEN, run it to make it shutdown, and check that temp again. I bet they won't be much different. If they ARE vastly different, then you may have a malfunctioning cooling system (think poppet valve as suggested by another post).