full throttle to half throttle without throttling down

Patricia K. Green · May 2, 2006

I have a 78 Johnson 70 hp.that will go from full throttle to roughly half throttle without throttling down. There have been days where this does not happen. When it does happen I throttle down and sit there for a few seconds and then throttle up. The boat starts to come out of the water only for a second then back to half throttle. I have replaced the spark plugs (3 times). I have replaced the fuel tank and lines. I have rebuilt the carbs and fuel pump. When this happens I can choke it and nothing happens. I can throttle down and choke it and the motor starts to flood, which is normal. Vent is open. The boat did sit up for two years.