I have a 86 SeaWay Northstar with dual Volvo Penta I/Os with AQ145B engines (gas). These are 4 cyl engines with 2 single bbl carbs on each.
Prior to launch, I replaced all plug wires, caps, rotor, points, condensors and plugs on each engine. I also drained as much fuel out of the tanks as possible and put in about 15 gals of 93 octane into each tank and replaced the water separator fuel filters. I ran both engines several times for 20+ mins out of the water and had them idling at 900 rpm. Since I launched the boat 2 weeks ago, I haven't been able to keep the port engine running at or near idle speed. If I go below 2500 rpm, it stalls.
Since I encountered the problem, I completely dissassembled both carbs cleaned and reassembled with new gaskets. Everything looks good, gas sprays in both when I hit the throttle. Although the carbs really are in better shape now, the problem didn't go away.
I also adjusted the air/fuel mixture in both directions a quarter of a turn at a time with no change (other than making it worse). So I brought the screws back to original position....starts, runs at high rpm...and stalls at low rpm.
I also cleaned all the connections to the coil and tested all the plug & coil wires with a timing light....all looked good.
With the assumption that I had some water in the tank, I replaced the water sep fuel filter last night with a new one (which I put about a cup of fresh fuel in)....but I still have the same problem.
So, the engine ran fine in my backyard, but will not run at low rpm in the water even with the I/O drive pulled out of the water. The other thing that I noticed is that even when running at higher rpms, it runs rough and often surges back and forth. It appears to be a fuel issue....
Last thoughts....run a fuel line from the fuel pump to a fresh container of fuel to determine if a fuel line issue? If still runs the same...could it be the mechanical fuel pump?
Sorry for the long story....but I've owned boats and cars for years and do much of my own maintenance on them...but this issue has me at wits end and I have 10 lobster traps I need to get in the water!
Just thought I'd let you know where I am with my issue. I ordered a new mechanical fuel pump ($70) and will be installing that this week. Before I install though, I will run a fuel hose from the existing mech fuel pump to a gas can full of fresh fuel and see what happens...
Any other thoughts/recommendations would be appreciated.