93' ski nautique surging issues

Hey guys, 

I apologize if some of this seems jumbled with information as I've tried literally everything so I'm trying to share as much as I can with you guys to get the best possible feedback. I appreciate any help that can be given to help me finally get my new to me boat working so I can enjoy the last little bit of the season.

I bought a ski nautique 1993 about a month ago and I've been taking it out at least 2 days a week trying to get it to run correctly with no success. 425 hours on the engine with fresh oil/transmission fluid. It seemed to run fine during the test drive with the previous owner, but after taking the boat home I noticed some fuel leaking from the accelerator diaphragm in a holley 4010 carb. After trying to replace that I noticed the previous owner stripped 2 out of 4 diaphragm bolts, so I went ahead and replaced it with a Quick Fuel M600 carb. Along with the new carb I replaced the oil filter, fuel filter in the Fram CCS1136, cleaned the sea water strainer (not much in there), and replaced the fuel lines from the gas tank to the fuel pump.

After all these changes I was taking the boat out and it randomly wouldn't start on the water. I noticed the carb was flooded with the secondaries constantly dumping at idle. After adjusting the floats, the boat would now start every time. The boat wasn't the fastest getting up to speed but it could maintain 20 mph no problem. Once I went to 30mph or so the secondaries would kick in for a few seconds and then the boat would stumble and I noticed the secondaries were empty. Upon working these issues out with a veteran forum member here, I replaced my pro tec (1 coil was oozing) with a DUI. Along with this I got a warranty replacement QF M600 carb since the secondaries seemed to be malfunctioning and completely draining themselves not matter how I adjusted the float.

Now I have the new QF M600, DUI, new spark plugs with .055 gaps, replaced the fuel pump, replaced the anti siphon valve, replaced the fuel tube in the gas tank, and blew out the vent tube from the gas tank to make sure it wasn't blocked. Also, while testing over the last few weeks, I tried multiple times to remove the gas tank filler cap to test for fuel flow issues as well. With the new DUI it seems to be running fantastic at idle, and during acceleration all the way to high rpm, initially. The boat starts up every single time instantly. I can accelerate to say 5 mph and run fine, once I go to 15 to 35 mph and try to hold it, it will go for about 10-15 seconds and then stumble and surge then go back in rpm, or it will outright stall. The choke is wide open the entire time. The odd part is it can run 35-40+ mph no problem and hold it without any issue. In the video of going 35+ that I'm linking when it decelerates, it because i'm pulling back on the throttle itself. You can see what's happening when I try to run 15-20 mph consistently. The DUI is wired directly to the keyed power with 12 gauge wire, I timed it to both 10 and 12 degrees at idle with no change. The alternator reads 14 volts when the boat runs. The voltage where the DUI connects doesn't seem to drop when the boat bogs. I traced all the wires from and nothing seems to be having corrosion or a short that I can see. I looked behind the dash where the gauges are and dont see any wires touching or connection issues.  I have a buddy watching the engine when it bogs, floats stay middle of the window for both primaries and secondaries. When the boat surges and stalls, the carb has the floats looking correct with fuel so I'm assuming it's getting fuel and I replaced everything I could fuel related. When it surges to a stall, I can pull back on the throttle into reverse and see the fuel squirting into the carb from the primaries like it should. When I bought it I pumped out the old gas and have been running 89 ethanol free with a fuel tank of fresh gas. While at idle I hooked up vacuum gauge to the PCV valve and the vacuum is reading correct 15-20in Hg at 750 rpm idle. I looked at all the exhaust valves/cylinders with a endoscope camera and they all seem to be fine.

I'm at a loss for what's going on, and I'm so frustrated taking my boat out for 4 hours a week, troubleshooting the entire time I'm out. I replaced everything I could think that would be causing the problem. Is there anything to check with the new DUI? I've triple checked the firing order and timing over and over.

My next steps were to put a fuel pressure reader into the fuel line, but with the carb having fuel when it surges/stalls and the fact it can run 35+ no issue, I have a hard time believing it's fuel. I also was skeptical of any vacuum leak that happens at the mid range RPM so I was going to remove the spacer between the carb and put another fresh gasket. Either that or replace the 4 hole gasket/spacer with the open square style to make sure nothing is sticking with the throttle plates, etc.

Vide of surging around mid rpm:

https://drive.google.com/file/d/1Qbhh0t4uvS-j5rB86ZAjkyNiwu6cBjyU/view?usp=sharing

Video of boat running like a champ at high speed:

https://drive.google.com/file/d/1Qg15FqAY7-xF3uZtWzNLR6jEpDzhQc96/view?usp=sharing

TLDR;

-Boat starts and idles great every time

-Can go 5 mph without issue

-accelerates very hard/well out of the hole all the way through the full RPM range, issues start when trying to hold a speed thats not close to WOT

-15-34 mph it will hold for 10 seconds and then cut out/surge/sometimes stall, but starts right back up instantly like nothing happened.

-35mph+ it runs fine without cutting out

-multi meter test shows alternator and keyed 12v aren't losing volts

-carb floats are where they should be when it cuts out/bogs

-fuel pressure 40-60 and engine temps 140-170

-replaced everything on the fuel side (pump, anti siphon, fuel lines(no kinks), fuel filter, fuel tube in gas tank, checked breather hose)

Ken kneels down in front of the king.  King taps Ken on both shoulders with a sword and says: “ I dub thee Carb Whisperer”

+1

Can't begin to describe how grateful I am for all the advice he's shared on this forum in the few years that I've been active on here. The depth of his knowledge and the time he is willing to take to help others here on CCF is simply unbelievable. I for one am deeply indebted.