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Airspeed

85hp Johnson bogs when trying to plane

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Posted (edited)

First time here and glad I found a source for some help! I was recently gifted a McFast 5 bass boat with a 1976 85 Hp Johnson. I put fresh fuel in it, took it to lake and it ran great, took off like a rocket, pulled into shore drank a celebratory beer then went again. The second time it took forever to get on plane and would bog down on full throttle. If I back the throttle off a little it would start to rev and eventually go but still can’t get full throttle. 

        I pulled the carbs and they were super clean, I went ahead and replaced all the fuel lines as they looked cracked. Checked the filter on the fuel pump and it was clean. All the linkage seems to be functioning fine. Motor starts right up with just a tap of key and idles fine. The fuel bulb isn’t collapsing and the in line fuel filter is new and clean. Plugs all look good and I’m getting spark on all four. It’s pumping water and doesn’t seem to be getting hot, a friend suggested we let it sit for an hour with the hood off to see if it made any difference, after an hour we ran it again with no change

        It seems to surge as it try’s to go, I’ll feel good acceleration for a few seconds then it loses power but eventually will go but not in full throttle. I should mention it will use full throttle once in awhile but it’s rare. Top speed so far is about 35 mph (used gps on my phone) it seems like an 85 hp outboard should push this boat faster than that, had a 55 hp Chrysler push my last, heavier boat with similar hull up to 40 with ease. 

             I appreciate any suggestions! 

 

Edited by Airspeed
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Try a different fuel source.   The  kiss principle.  Good luck. 

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1 minute ago, Hop said:

Try a different fuel source.   The  kiss principle.  Good luck. 

Thanks for the reply! If you’re talking about a different gas station I don’t have that option unless I drive a forty mile round trip, if you’re talking about a different tank possibly, I have two plastic tanks, both are newer and very clean, does the same on either. 

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Just remembered something else, motor has electric choke and when I hit the switch it would speed up a little, this kinda confirms I have a fuel supply issue. Being that all the fuel lines and filters are clean could I possibly be looking at a fuel pump problem? I can get one for $20 but don’t want to start throwing money at it for no reason.  Thanks again! 

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Posted (edited)

Sometimes there is a tiny leak in the fuel line, small, but big enough to suck air into the system, it causes a shortage of fuel supply to the carburetors because it sucks some air instead. double check all your connections and hoses. As for speed, that may be a trim issue. If you can, do a compression test and see if the cylinders are within 10 % of each other. As for the fuel pump. It may be that the membrane is eaten up by ethanol or just gone leaky because of old age. Membranes are cheap ,so is a $20 fuel pump. They usually are at least twice that price. If you decide to take the fuel pump apart to replace the membrane, be very careful with the tiny springs and places where they are in. The suspects are false air and pump membrane.

Edited by rolmops

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44 minutes ago, rolmops said:

Sometimes there is a tiny leak in the fuel line, small, but big enough to suck air into the system, it causes a shortage of fuel supply to the carburetors because it sucks some air instead. double check all your connections and hoses. As for speed, that may be a trim issue. If you can, do a compression test and see if the cylinders are within 10 % of each other. As for the fuel pump. It may be that the membrane is eaten up by ethanol or just gone leaky because of old age. Membranes are cheap ,so is a $20 fuel pump. They usually are at least twice that price. If you decide to take the fuel pump apart to replace the membrane, be very careful with the tiny springs and places where they are in. The suspects are false air and pump membrane.

Thanks, I’ll double check fuel system today, think I’ll go ahead and order a new fuel pump, found lots on Amazon for $20, the ones I looked had good reviews so hopefully they’re not craptastic! 

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Posted (edited)
14 hours ago, Airspeed said:

Just remembered something else, motor has electric choke and when I hit the switch it would speed up a little, this kinda confirms I have a fuel supply issue. Being that all the fuel lines and filters are clean could I possibly be looking at a fuel pump problem? I can get one for $20 but don’t want to start throwing money at it for no reason.  Thanks again! 

I just read your report again and the cracked fuel lines and the electric choke do point at a possible other cause. Fuel lines not only crack outside, but also inside and tiny rubber particles are swept along in the fuel towards the carburetor where they can cause trouble in  2 specific spots. The first is where the fuel comes into the bowl. That is where a little inlet control needle is held in place by the float. the second place is the high speed needle hole.  I write this because the electric choke bypasses that whole setup of the carburetor and they enrich the fuel mixture straight into the horn of the carburetor.

Edited by rolmops

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Just because they look clean doesn’t mean much ... order the kit and tear em down .. be sure to remove the Welch plugs to get at every orifice... the kit will include new ones ..  also at that vintage plugs wires etc .. electrical items break down under load and sometimes your chasin fuel issues for no reason...

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I wonder what fixed the problem? Update please? Thanks. If you're still looking, try a portable gas tank and hose to eliminate the built in tank and hoses if applicable. Process of elimination is sometimes easiest way to troubleshoot.

Sent from my moto z3 using Lake Ontario United mobile app

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Had the exact same problem with a 2006 merc 115 4 stroke this past may.---Yamaha dealer looked at fuel filter with no water in it and said" It is probably Plugs"--

 

reluctantly, and believing my friend was wrong and it was a fuel issue, i replaced the 4 plugs---Ran like the 125 hour beauty she is.  I was shocked but very glad that was it.

 

good luck and good fishin'

 

coach

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Posted (edited)

If the problem was the plugs themselves would it run like H the first time he tried it out I don't know....? Seems more like fuel related (e.g. pump) or spark distribution problem....had that when the power pack was going bad on my 1979 Johnson 35 and thought it was fuel related...I don't know whether they used the "modular" power pack back in 1976.

Edited by Sk8man

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We have the exact same year motor doing similar. Motor ran great until I forgot to open vent on a plastic tank causing it to collapse. After that it would bog down under a load and not plane out. We changed plugs, power packs, seafoam thru carbs and replaced fuel line to tank, still bogs down. Any other Ideas?

Edited by Afdinsp164

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