Typically when you push in the ignition key switch you get a fuel enrichment. While I am not certain how that works on a 70-HP, it often operates an electrical solenoid valve that lets a lot more fuel flow into the intake manifold, completely bypassing the carburetors. If that action causes the engine speed to accelerate, then the problem of limited engine speed is likely due to a fuel restriction in the carburetors or the hoses which feed the carburetors.
An engine made in c.1982 may very likely NOT have rubber fuel hoses that are ethanol-gasoline resistant. Replacing the smaller rubber hoses in the engine may be a good step toward prevention of future problems. Maybe you can buy a length of the proper-type rubber hose from the Evinrude dealer and cut the hose into individual sections to replace each segment of the fuel distribution system under the cowling. That would be a lot cheaper than buying the many hose segments as individual parts.
Evinrude usually uses Oetiker clamps for securing fuel hoses, so probably should invest in a bunch of new Oetiker clamps and a crimping tool for them, if you decide to replace a lot of hose in the fuel system. The archive contain a very good discussion about Oetiker clamps, where to get them, and where to get crimp tooling. See
Oetiker Hose Clampshttp://continuouswave.com/ubb/Forum3/HTML/020693.htmlThere is probably an under-cowling fuel filter in-line somewhere. That would also be something to replace periodically.
New spark plugs on the exact recommended brand and model and with proper spark gap will also help improve engine running characteristics.