"All: This one's a mystery to me, and I've got a pretty good engine background, so any insight would be appreciated.
History: 1980 Welcraft Nova 250. Single merc 260/pre-alpha drive. Boat sat in a barn for at least 10 years before I purchased it (last sticker expired in '97).
Following items were done as a matter of course before first time I hit the water:
Ignition system: new plugs/wires/cap/rotor/points/set points/set timing.
Fuel system: Remove q-jet, tear down, 2-day soak, rebuilt Q-jet. I knew she had sat 10 years so I tried to to a very thorough job cleaning out the carb, but I am human, so there is a possibility I missed something.
Completely drained old fuel from tank. replaced water sep fuel filter, purged fuel lines, etc. Re-installed carb, with new fuel.
Initial start had motor that would run fine at higher RPM, but wanted to die at idle--sounded really lean as RPM dropped off, and idle mixture screws provied no real help. All the classic signs of a vac leak with a q-jet (pissy idle, with man vac/RPM surging.
Confirmed she was running lean by slowly closing choke plate manually, and the engine really liked the extra fuel. Hunted all over for a vac leak, including running around the base of the carb and intake/head interface with a propane bottle. No help.
Out of frustration, I pulled the carb back off, looked all over inside for root cause. finding nothing, I raised the float level 1/4" to richen it up across the board (I know people are rolling their eyes, but this was a desperate moment).
Overall, idle improved somewhat (like it will actually "run", though badly in Neutral @800 RPM. Idle mixture screws have a very MINOR influence on idle quality now. She's still running lean at idle, but off idle great--It was "good enough" that I was willing to risk sea trials, which have thus far proven successful (other than this niggling issue).
Off idle/wot perf is actually pretty good (for an engine with 950 hours). Spinning a 4-blade 18" prop, With 4 people and half tank of fuel, I turn in 42 MPH at 4200 RPM WOT (on the low end, but not too far off the mark). Also, Fuel Economy is well within acceptable range.
So, now if I can JUST make this thing meter fuel correctly at idle:
Last observation--when actually got a couple hours not hauling kids around on a tube yesterday AM, I started trying (again)to sort this idle thing out.
At base idle Neutral (800 RPM) looking down into the carb, fuel is spraying out of the main venturi on Port side (not really supposed to at idle), but nothing coming out of stbd main venturi. If I kick open the throttle to about 1500 RPM, neutral, I get some visible fuel flow through stbd side, (and lots more through port side).
So now, here I am with a side/side issue fueling in the carb, and the whole system biased a little rich with the raised float. Off idle is OK, and performs quite well, even running only on the primaries, prior to the secondaries opening.
Sure feels like an air leak, but I'll be darned if I can find it.
Anybody want to take a stab at this one? Any ideas would be greatly appreciated.