77 70 hp Johnson having issues at top end

[hatboat]

Hi all, looking for some thoughts on what's possibly happening to my 70hp Johnson outboard. The motor starts find, plains fine, and when first run operates well at full throttle. After it heats up, at full throttle, engine starts to bog in and out...it doesn't quit but you can feel and hear the engine go in and out. When I pull back on the throttle it stops and runs smooth. As soon as I put it FT it does it again. Any thoughts on what's going on here. Any help appreciated. Thanks

 

[interalian]

Sounds like it wants more fuel than it's getting. Try pumping the primer bulb when it's acting up (best if you have a helper to drive the boat or pump the bulb).

 

[boobie]

It could also be an ignition miss when it warms up. Put a timing light on each cylinder when it's acting up, look into the light for any black spots which will indicate an ignition miss.

 

[hatboat]

Thanks for the feedback. The first thing I tried was take on a ride again. At FT it did the same thing, in and out, then had some one squeeze the ball and it stopped doing it. Needed more gas and when it got it ran fine. The funny thing was that when I throttled down it stopped doing it. Only happens at FT. But again, when squeeze the ball it stops. Any thoughts on next steps? Fuel pump? (This was actually replace already last season),

 

[interalian]

The motor uses the most fuel at WOT, so throttling back letting it run OK is fuel starvation. If the pump is recent, look for any loose hoses that might be letting air into the lines. Or the tank hose o-rings.

 

[hatboat]

Thanks, I'll take a look and see. The tank is built in so I'll check all of that. Thanks again

 

[hatboat]

Hello I checked all the hoses and the tank o ring all is well. It's still doing the same thing. I've attached a video. At 19 seconds you can start to hear it get fuel starvation. At 37 I throttle down and it works fine. Any other thoughts? Many thanks.

 

[interalian]

So if you can make it stop doing that by pumping the bulb, you must be starved for fuel. Have you rebuilt the carbs? Fuel pump? That's where I'd go next, especially the fuel pump if it's original. I just rebuilt mine and while the diaphragm wasn't leaking, it was quite stiff compared to the new one.

 

[Gary H NC]

Original fuel tank? Fuel pickup could be partially clogged..tank vent clogged or like said the fuel pump.
Does it suck the primer bulb kinda flat when it acts up? That is a good sign the vent is clogged.

Fuel pump kits are easy to do....I agree on doing that next.

 

[hatboat]

Thanks for this. The bulb doesn't go completely flat when it happens. The fuel tank is built in about 5 years ago, before I bought the boat. Is there anything on a built in that I should look for? I haven't done the carbs and that's a place I might go next. Would you start with the rebuilding of the carbs or go with a new fuel pump first? Again, many thanks.

 

[racerone]

Simple trick.----Test run with a known to be good portable tank and hose !

 

[ondarvr]

And running it while it's lean can destroy the motor, so do it as little as possible, just do it enough to see if it's running right.

 

[hatboat]

Great suggestion and I just did that with the same result. So I guess I can eliminate the tank. Carb cleaning would be the next logical step, correct? I haven't done it myself before, is there an easy tutorial or is it worth it to bring it in to get done. I'm in Canada and hourly rates are $90/hour, so if I had the option I would do it myself depending how tough it was. I'm fairly mechanically inclined. Thanks again.

 

[interalian]

I'd do the pump first, but since you'd need to be 'in there' anyway, carbs too, especially if you don't know when/if they've been done. Where in Canada?

 

[David Young]

This is a video i used to go by when i rebuilt my 1977 70 hp Evinrude. Its not the same motor but shows you how to undo the linkage and other things like that. I also made a lot of pictures on where everything went before i started. My rebuild went perfect. I ordered a factory rebuild kit with everything needed and it came with new floats too https://youtu.be/VDn9fAn7sjo

 

[Fed]

You might want to check the filter in the fuel pump first.

 

[hatboat]

Excellent, I appreciate it. I'm in Peterborough Ontario, East of Toronto. I'll take a shot at this and update later. Thanks again for all the advice.