I've got a QJ on a 1987 Mercruiser 350 engine - casting number on the carb is 17080561. The boat/engine/carb is new-to-me, so I'm not sure of it's history. I rebuilt the carb over the winter using a kit sourced from Cliff and followed his book and a thread on a boat forum that was put up by a guy who also followed Cliff's book. I did pull the idle air tubes out and cleaned under them, then put them back.
The engine has all fresh ignition components and the timing is correct. It has a new water/fuel separator filter, new fuel lines from tank to fuel pump, and before starting I removed as much of the old fuel as possible and have run at least 2 full tanks (37 gallons) through the engine.
For a description of what it's doing: on cold start it fires right up with throttle about 15-20%...if it's sat more than a couple of days, I give it one full throttle pump before cranking and that seems like enough. I can let it run a few minutes then pull the throttle back to idle and it is happy at about 600 RPM. After driving the boat for 30 minutes or so (at roughly 4,000 RPM) and pulling back to idle, the engine wants to die. If I get back into the throttle, it's fine (maybe some very minor lower RPM hesitation that it works through) and it will run for as long as I want it, but coming back down to idle, it wants to die again. If it dies, it becomes difficult to get started again - sometimes holding the key over for a long period (10 seconds) can get it to gradually catch and start running again, but I have to get off idle quickly to keep it from dying again. It did this last weekend when putting the boat back on the trailer and while attempting to re-crank it, it smelled like it was flooded (I did not pump the throttle during this attempt).
The next day I ran it on the hose in the driveway and started it with the spark arrestor off, and it started fine. While messing with it, I was moving the choke plate by hand and if I started closing it, I got the same symptom where the engine would want to die. When I did the carb rebuild, I did not replace the divorced choke thermostat, and honestly it looks kind of rough - wondering if I should replace it? Anyhow, I zip-tied the choke linkage for the primaries open on the carb and ran it the next day. It maybe ran better (maybe it was in my head), but when putting the boat back on the trailer again, I got the same symptoms of it wanting to die. Another thing I noticed when running it in the driveway was that with the boat at idle, holding the choke open and looking down in the carb with a flashlight, some fuel would dribble out of the center of the venturis. Guys on the boat forum said this is likely what is causing my issue - what are your thoughts?
I've got tomorrow off work and plan on pulling the carb off the boat and opening it back up. Based on the boat forum's suggestions, I'm going to check the float height and also look for any trash that may be preventing the needle from sealing on the seat. Appreciate any guidance or thoughts you may have to help me solve the issue.
