evinrude 4 hp litetwin (high rpm problem)

I have a 1969 evinrude 4 hp litetwin that after running it for about 20 minutes will not run at high/full throttle. It runs fine at the mid to low range but bogs at high rpms after she's been running for a while. Any suggestions to get me started on ttrouble shooting the problem? Maybe clogged fuel line, air, etc? Thank you for any and all help.

Respectfully

Re: evinrude 4 hp litetwin (high rpm problem)

First you need to eliminate the possibility that it might be running on one cylinder (when it is acting up). Remove and ground one spark plug wire and see how it runs. Then do the same with the other wire. If one of those duplicates the problem, but won't run at all with the other one disconnected, you have a one-cylinder situation.

However, back in that era they made some cheap fuel pumps that did not work very well. It is possible that you might have one of those. When it is acting up, give the hose bulb a squeeze and if it picks up and runs for awhile, you have a fuel problem. Maybe the fuel pump, but possibly something else. Could be a stuck tank vent too. See if it will run right if you loosen the cap.