I have a 115hp Johnson 4 cyl 2 stroke engine. It will start and idle in
the driveway but not in the water?

I recently acquired Neptune 1800 with a '94 115hp Johnson (J115TLERC),
and I have had a lot of fun playing with it. Obviously I'm no pro, but
I'm not too bad with mechanics. I have taken the boat out on the water
about 8 times, and always had to struggle to get it started. Usually
once I recharged the battery and fooled around with the gas lines
(swapped out the primer bulb, new hose clamps, replaced the filters,
replaced sparkplugs, played with the choke) it started up and ran fine.
It's been a few months since I had the time to work with it, and now I
can't get it to run reliably.

The engine usually has to be started in the driveway and let idle for
10-15 mins to clear it out and get it running consistently. I've found
if I put it in neutral and leave the throttle forward slightly to give
it a little gas before I start it up, it runs quite well. It will idle
in the driveway indefinitely.

When I put the boat in the water, the engine would start with the extra
gas, but if i eased up on the throttle, it would die at 1000 RPMs (just
a bit from idling) and wont start without the gas.

What can i do? I suspect the fuel line is responsible for it being such
a struggle to get it to start on dry land but I imagine theres another
problem thats keeping it from starting in the water. I took it into the
shop in November to have them do a small amount of maintenance. They
cleaned the carbs, tested the pressure & sparks and adjusted the
linkage. Compression and spark was listed as 'Okay'.

Now, the engine will start if I am giving it gas, even in the water. We
had it running at 1000rpms for 10 minutes or so at times before we'd
slowly try to drop it into neutral, but it would die when the rpms went
below 1000.

Anyone have any ideas?

Also, how much money am I looking at if I were to take it into a
service shop?