with the idle jet needle. Usually set at 1 - 1/2 turns from full 'in'.
Idle jet controls bypass air (to the 'emulation tube' - to control the exact needs of fuel/air ratio) for correct idling.
Water, from phase-separation of ethanolated fuels, usually show up 'first' with idle problems in these old Evinrude/Johnson OBs.
Suggestion: remove the idle and main jet screws ---- exactly
count the number of turns and partial
turns to remove the needles. Rotate the engine and dump out any fuel (and water) thats inside the carburetor jets and 'air bleeds', then dump the fuel in carburetor bowl. Squirt in some 'rubbing alcohol, into each needle bores / let sit a while / slosh / then dump. Reinstall the needles to the exact 'turns' as for removing, fill with fuel, spritz a small amount of starter fluid into the carb intake, pull the choke, and start the engine (engine's gear case submerged into a LARGE water filled bucket, so you don't destroy the impeller). Then adjust the needles for best obtainable rpm at under 'full load' then at idle, and 'hone in' to the best rpm between between the two adjustment on the two needles.
Evinrude parts: http://shop2.evinrude.com/Index.aspx..._id=0&siteid=1
Need model number and YEAR.
These pages include 'blow-up' / expanded parts diagrams.
Once you get the problem corrected, NEVER use any ethanolated gasoline that is over about 30 days 'old'. Be SURE to add a suitable 'stabilizer' in any 'fresh' gasoline'; but then, store it only containers that tightly SEAL OUT the atmosphere. Then when shutting the engine down, ALWAYS shut off the fuel tank valve and run the engine to completely 'dry'. For longer than 1-2 months OB lay up .... dump the tank; use the fuel in your automobile.