Repetitive stall at 3/4-WOT high RPM.

1996 Tohatsu M50D has developed worsening problem over the last year. At first though it was water in fuel and repeatedly drained fuel filter secondary (Racor) bowl and chained filters. Discovered a bad tank and replaced but problem persisted every now and then. Now runs fine at low and mid-range but at high range to WOT it seems to slightly down throttle over 1-3 seconds then dies. (see video attached). It cranks right back every time without hesitation or need to reprime bulb.

I have tried the bulb pumping testing as it shut down twice. The weather was bad and rough so we gave up. I will do that again. We do think we have eliminated electrical issues. Stop Circuit test was good and the CD unit has been replaced. I need to retest fuel intake side. I replaced the tank with a WEST Marine tank because the old tank did allow water to get into it. The shutting down was happening with the old tank so naturally I thought that it was water. Could the compensation venting on the new tank be faulty?
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I am leaning more toward vacuum system failure. If I am correct vacuum negative pressure increases as rpm increases and that pills more fuel in the system. Wouldn't a small leak manifest itself at higher negative pressures and stop leaking a those pressures decrease? If that leak suddenly caused loss all negative pressure fuel flow would stop!

What about the reed valves? If they leak/fail at higher rpm wouldn't that stop the fuel flow without emptying the carburetor bowls and then allow the motor to crank once then vacuum pressure lowers and the leak/failure closes? I have read on a few other power sports forums that reed valve failure can indeed do just that and that reed valves need replacing very 100 hours (that's a bit extreme and Im sure is meant for racing considerations).

I am trying to attach a .mov file of the motors behavior but the site will not accept the format I guess. Does this forum only allow still pics??

Rob

You have some possible issues. Yes, fuel is the most common problem, so VERIFY no air leaks in the entire system from tank to carb. Fuel starvation occurs at WOT, but not at lower speeds, as there is less demand at those speeds.

2-strokes have alternating vacuum/pressure in the crankcase. No, vacuum is not necessarily higher at higher speeds. Evern so, there is a lot of fuel in the fuel system if the pump were to run less for some reason. I would definitely disassemble the pump and verify condition of the valves and diaphragms.

Reeds are solid on these. I have never ever seen a bad one.

Although you swapped out the CD, did you test the exciter and pulsers when it was hot and failing? Those can test good cold and fail hot.