Re: 1979 Evinrude 140 carb issue?
Make sure that the throttle linkage between the two carburetors is as it should be..... that both throttle butterflies start to open at the same time and close fully at the same time. You do not want one butterfly closed and one ever so slightly open.
The carburetors of course has all fixed jets. The fact that the engine runs okay at both full throttle and idle indicates that the carburetor is okay. There are no intermediate jets in those carburetors that you could get mixed up with the idle jets. Also you do not mention any surging at full throttle or idle so it's safe to assume that the fuel pump and crankcase pressure is okay and that no fuel restriction exists.
The first thing I would look at is the cam roller pickup in relation to the scribe mark on the metal cam. The throttle butterflies should just start to open when that scribe mark is dead center with the cam roller.... not before or after or weird things will happen such as what you're experiencing.
The idle timing when that scribe mark is centered with the roller should be 5 degrees. If it is not, adjust the linkage between the cam and the vertical throttle arm.
One other thing..... The cam roller on your engine, and all of the OMC engines for that matter, should be a one piece roller and have a diameter of about 3/8".
On later models, the roller is a two piece (bad) design (small black diameter inner layer with a larger clear outer layer. The outer surface cracks away in time which reduces the cam roller diameter to about 1/8" or so and there is no way it can be adjusted properly. If your carburetor has one of these later model rollers on it with the outer surface cracked away, replace it.
