1977, 70hp Johnson not opening up

[triton1973]

I have a Johnson 1977, 70 HP when I run the motor at home I get 5600 rpm at wide open throttle and it sound great throttle at 1100 rpm. On the water is a deferent story I would put the throttle all the way down it planes and run great but I can tell its not running to its full potential then I would hit a small swell it would just get up and start running wide open 5600 rpm and I would have to back it down but would not get back up until I hit another good wave and if I get it up there it can stay there if its flat, my GPS tells me I got 27.6 knots at full speed. My boat is a 16 foot fiber glass Lone Star Triton. Eney help is appreciated

Thanks in advance

 

[HighTrim]

Re: 1977, 70hp Johnson not opening up

I have a Johnson 1977, 70 HP when I run the motor at home I get 5600 rpm at wide open throttle and it sound great throttle at 1100 rpm. On the water is a deferent story

You are not saying that you run the RPM's up that high in the driveway are you?? If it goes into thermal runaway the consequences could be catastrophic. Have you checked the spark to ensure it is running on all cylinders? Boat not water logged?

 

[triton1973]

Re: 1977, 70hp Johnson not opening up

You are not saying that you run the RPM's up that high in the driveway are you?? Yes 55 gallon drum.

Have you checked the spark to ensure it is running on all cylinders? Yes.

Boat not water logged? No.

 

[iwombat]

Re: 1977, 70hp Johnson not opening up

55 gal drum or no, you're in for a rude awakening one of these days.

Keep it under 2k w/o proper backpressure.