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70 hp johnson outboard wont go full speed


rock44

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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.

Sent from my iPhone using Lake Ontario United

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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!!

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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 by john1947
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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)

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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.

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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)

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  • 6 years later...
  • 3 weeks later...

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 

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