'96 2330 SST.. 5.7 Mercruiser, Alpha One Gen II. Engine rebuilt top and bottom at 900 hours (2014). Now has 1312 hours. I think the carb (Weber/Carter 4 barrel) is original and nothing was done to the carburetor at the time of the rebuild.
The boat has run perfectly since the rebuild, but last weekend when I opened it up to 4000... RPMs would not hold steady. Every so slowly and slightly... down to 3950, then back up. After this fluctuation happened over and over for about a minute, I pushed the throttle forward.... didn't bog... but didn't speed up either. Then suddenly the RPM and speed would go up, but again not hold steady. This problem only happens above 3600 and it "feels" like a gas supply issue. No popping, no backfiring. The engine doesn't die either. I can cruise at 3400 (about 30mph) all day and there is no change in rpm or speed... 3400 is my usual cruising level, but I will typically take it up to 4000 to 4200 for 5 minutes each time out. No problems until last weekend.
My mechanic is great, but can't get to the boat for 10 days. In the mean time, I'm on the boat every 3rd day or so... so I'm kind of curious if any of you might say... ah HA... I had the same problem. I'll probably wait for the expert, but I might change the fuel filter and or the anti-siphon valve in the mean time.