No Start/Stutter Condition 40hp 3cyl 4stroke

[RocketFox650]

I'm a SeaDoo mechanic and maintain 2 Stroke Mercs, having one in the same era, but recently while restoring a ski on the lift dockside, one of my lakes marina users asked me to look at his zodiac. My SeaDoo reader doesn't hook to Mercury readers yet, but the complaint is the engine turns over but doesn't start. It will fire for one cycle, but then just crank. If I use the mechanical idle advance, I get one spray of fuel out of carbs 2/3. Obviously I've verified fuel flow up to the carbs. By pressurizing the fuel bulb, I can watch fuel leave the output strong and steady. I imagine I could possibly manipulate the bulb and crank it if I could reach both, possibly getting it to fire. I've had the mechanical pump apart to trace flow, and basically because I'm an engineer I just tend to tear **** apart and teach myself. If I manipulate the diaphragm plunger by hand, I get strong flow. Because it's a customer engine, I stopped taking things off after I pulled the carb block to download the manual and teach myself the systems. I'm thinking about trying to pressure test the pump today. Right now, my theory is that A) Carb 1 has a stuck float/plugged jet, probably related to ethanol fuel. B) The diaphragm in the pump is too weak to maintain fuel pressure and is instead leaking back into the crank case. But there's also the possiblity of it being related to manifold pressure or a bad line elsewhere that's sucking air and preventing operation. I've never actually seen a mechanical pump fail on a boat, but something is preventing fuel pressure from being maintained, and I did observe fuel in the water, which would be a symptom of the pump leaking into the case, but I also had been pulling lines and squirting small amounts of fuel out and mechanically advancing the carbs repeatedly trying to clear #1. I did get voltage readings off the enrich sensor, but didn't have the service manual to reference, so I didn't check resistance and unfortunately "15VAC" doesn't get referenced anywhere, so it's essentially a junk reading. Besides, that shouldn't account for this level of issue.

Summary- if I mechanically pump the carbs, I get one jet of fuel and the boat fires for that cycle. With the pump off, mechanically manipulating the plunger sends fuel out. Priming the bulb with the pump on does indeed send fuel through the out port. Because it's mechanical, the engine should be engaging the plunger fine, and if it works in my hand it should work when anything else pushes it. I don't quite understand how it can continuously feed the jets when I advance the throttle by hand, but not feed them as it cranks unless it's a vacuum/pressure related. I deal with ethanol issues daily, so I'm not that surprised to have a completely separate issue with a single carb. The only other thing that occurred prior to the condition is a change of fuel filter, which wasn't bled on the bench, but I'm getting fuel, so the air has passed through the line. Is there anywhere else I should be looking for leaks/air sucking at speed? Oh, maybe it's different with these, but with seadoo's it's a running joke "it ran great on the trailer". Even though the prop isn't engaged at start, it didn't begin to have this issue until it was in the water.

Also- besides tearing apart carb 1, does anyone have any suggestions for clearing it? Air pressure, carb cleaner into the jet, seafoam, etc?

Sorry, a lot to read, I'm an electrical engineer with ADHD and haven't slept in 72 hours. I always communicate like this, can't help it.