70hp 1974 won't run at 15-25%. what can it be?

[blueboat1]

70hp 1974 Johnson. 17' aluminum fishing boat. I try to troll for tuna and cant. The motor will troll fine at just above an idle (6 mph), try to tap it up to 8 and it will run up to 12mph and settle. Try to come down and it drops back to 6 mph. When I move the throttle from idle to and stop it will gradually pick up speed till I get to 10. Could this be to much prop? 13 3/4 X 17? Any tips are greatly appreciated.

 

[The Marine Doctor]

Re: 70hp 1974 won't run at 15-25%. what can it be?

You might try a decarb process...this might give you more control over the rpm.<br /><br />TMD

 

[Solittle]

Re: 70hp 1974 won't run at 15-25%. what can it be?

The prop should not be part of the equation on this one. You also might try rebuilding the carbs in case you have some crud in there.

 

[R.Johnson]

Re: 70hp 1974 won't run at 15-25%. what can it be?

Sounds like you could use a snyc & link on this one. Those three carb's have to be fully closed at idle, and all open together. There is also an idle pick-up timing. You will want a service manual for this.

 

[blueboat1]

Re: 70hp 1974 won't run at 15-25%. what can it be?

I have checked all of those items. I'll keep looking and post the fix if i find it.