70 hp johnson outboard wont go full speed

[rock44]

With my 70hp my boat will top out at 30mph and now i put the throttle down and it goes 9mph wide open. I can get it to 2.8 to 3.5 mph trolling speed and it runs a little rough while trolling and if i go any slower it will stall and while trolling i have to hit the choke every 15min or it will stall or as it feels like its gonna stall i throttle up a hair. Any thoughts on whats going on? Just changed the plugs and run non ethanol gas.

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[nygooseduster26]

My 25 hp is doing the same thing. It's a 1992.

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[samncrzy]

Always start with the obvious.... Check all your fuel lines and fittings. Look for dry rot and/or cracks in the lines. Check the connector that goes from your fuel line  to the motor, for a few $$ change the fuel filter.

Sounds like you may be getting an air leak somewhere.

Pump the primer ball slowly while looking for gas pushing out from the fittings and carb.

Next issue, if the above checks out, may be gunk in the carb. Try running some seafoam through it to see if it unsticks. (usually a needle and seat in the carb sticks)

If none of the above "easy" fixes work, then you will have to have it diagnosed by a mechanic.

Good Luck!!

[rock44]

Thanks. I'm burning a ton of gas also if that helps with figuring out the prob.

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[john1947]

my 120 evinrude did the same.After 2 mechanics couldnt fix it i found a stuck float.It sounds like gas which is very common today.

If its a carb youll know if you put your hand ober the intake and alot of extra gas on your hand

Using alot of extra gas leads me think a stuck float

My motor would act very inraticly and surge also.

Edited August 17, 2014 by john1947

[LongLine]

Before messing with carbs, make sure your sparking all your cylinders.  9 mph full throttle is classic when you've only got two cylinders firing.  (Did it once i.e. loose plug connection)

 

Tom B.

(LongLine)

[rock44]

Yeah it kinda feels like its missing also at fullspeed so i guess that makes sense

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[Has Been]

How old is it ? If it is a 3 cyl & has  3 carbs it sounds to me like one or even 2 are clogged. Or it won't switch over from the low speed to the high speed jets.

 

 I had a 1970 55 hp  3 cyl 3 carb John that I wpuld turn the jets in to troll w/ and had to turn them back out to cruise home.

 

 Also had a friend buy a boat w/ the exact same motor & it could not get on plane. They put some marvel mystery  oil in and they were  plowing water out to their fishing spot & all of a sudden it took off like a bat out of hell& been good ever since.

[rock44]

Its a 1996

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[rock44]

While trolling when its about to stall i choke it and it purrs like a kitten but it doesnt last long and runs rough again

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[LongLine]

Get yourself a spark plug light at Harbor Freight. (cheap)  Make sure you've got fire on all 3 cylinders. 70's are very thirsty engines.  If your using a lot of gas,  sounds like you're just not burning it properly.

 

Tom B.

(LongLine)

[bigCOHOnes]

Clean jets in the carbs if you are choking motor to run.

[Tim G.]

I'm having this exact same issue. What was the solution. I also have a 60 horse   johnson 1996. 

Edited August 24, 2020 by Tim G.

[bluegiller]

Check out my post on Tim he's post "help with my 1996 60 horse johnson

I have loads of power and all kinds of problems . Fuel line was the problem.

Good luck

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[ffejb111]

I have a 1958 murcery 650 that full speed 7 mph. Dont know what to do?

[tuffishooker]

Have the same engine maybe the same problem ? As posted start with the obvious ! Clean all electrical contacts then apply dielectric grease esp. the bolt that holds the power pack ! When we ran the engine in the dark saw sparks by the power pack , the bolt had backed off just enough to create  an intermittent problem ! If not the carbs may have to be rebuilt . A few nobrainers ! Use ethanol few gas and still use a fuel additive , I always filter my gas before putting in the tank . 

Check for spark at each cylinder ! I made my own spark tester ; mounted 3 spark plugs on a steel plate bolted the plate to the engine hocked up the coil wires to the plugs [ do not mix them up ] crank the engine , you should see spark at each plug !

Again let us know how you make out 

[Tim G.]

I've rebuit the carbs and have spark 

[tuffishooker]

As a last resort #1 drop the lower unit and check for a blocked exhaust . #2 Check the compression using  gauge that goes directly into the cylinder head not the one with the short hose !

We have to see the diagnosis !

[bluegiller]

Have you tried a remote gas tank? Maybe the tank has a small blockage in the puck up?

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[Rusty]

CHECK YOUR IGNITION SYSTEM...COILS, PLUGS WIRES ETC

 

[Starcraft75]

Had the same problem with my 115 Evinrude turns out coils were bad. One wasn't firing and another was intermitent. Replacing all 4 to be safe. Thought it was fuel pump, replaced it and decided to throw the coil tester on it. Bingo

[Tim G.]

Its in the shop  now.. 

Edited September 18, 2020 by Tim G.

[tuffishooker]

Let us know the diagnosis so we can all learn from it !

[Tim G.]

My mechanic said it has some internal damage from over heating.  He's given up and said not worth investing  anymore 😕 

[bluegiller]

If it has good compression it's fine nothing else from overheating would make it run bad. I would not give up. I still sounds electrical and fuel related .

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