40hp johnson lacking power

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ozzyoutlaw

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Jan 1, 2013
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hi im very new to this site trying to find a answer before i go spending money on a sevice if i can do it myself i will ive a 40hp johnson not sure age mabe 70s was working fine when i took it out but when i went to start it again all the revs were there but i could get out and run faster than it ive pulled carby apart cleaned it new fuel only thing havent done is changed plugs im pretty handy with all motors but this has me stumped
 
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boobie

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Re: 40hp johnson lacking power

A model # would be good. Can't really help you with out it.
 
nwcove

nwcove

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Re: 40hp johnson lacking power

welcome to iboats ozzy!
as stated, the model number is an important piece of info. you say it revs as normal, but wont get up and go?? sounds like a spun prop hub, but more detailed info would sure help .
 
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ozzyoutlaw

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Re: 40hp johnson lacking power

boobie said:
A model # would be good. Can't really help you with out it.
Click to expand...

were would i find that??? i found the model number wich is 4030-d number after the zero is worn cant see it well enough
 
1946Zephyr

1946Zephyr

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Oct 21, 2008
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5,556
Re: 40hp johnson lacking power

That should still have the old universal style ignition system in it. Take a look at the coils and make sure they aren't cracked. Originals will all be bad now, unless they have been replaced. Excellent old motors though. Tune it up nice and it'll run for years.
 
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ozzyoutlaw

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Jan 1, 2013
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Re: 40hp johnson lacking power

only real info i can give is when we first got it it went fine went out for a fish drifted for awhile started fine as went into foward fine but not enough power to move it as quick as it went out running on a 14fter has aqua wings on it im not to much into boats to know sorry
 
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tx1961whaler

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May 31, 2008
Messages
5,197
Re: 40hp johnson lacking power

2 things to start with:
Check the compression with a compression gauge. A parts store sometimes loans these out for free.
Check the spark with an adjustable gap spark tester. 6 bucks at the part store. It should jump a 3/8" gap with a healthy snap.
 
kbait

kbait

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Nov 13, 2007
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2,486
Re: 40hp johnson lacking power

If it acts like you pushed the clutch in (rpms increase but speed doesn't), you may have a spun prop hub, as stated ^. Engine will over-rev until you slow the rpm's, and the prop hub grabs again (slow). Will idle around fine, but if you give it throttle, it'll start to go faster, then over-rev again when the prop hub slips. Hope this helps. If this is indeed your issue, and new/used prop or a re-hub of your current prop will solve your problem. Good luck!
 
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boobie

Supreme Mariner
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Nov 5, 2009
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20,826
Re: 40hp johnson lacking power

Pull one spark plug wire at a time to see if it's running on both cyl's or not.
 
ACAMS

ACAMS

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Joined
Sep 7, 2011
Messages
40
Re: 40hp johnson lacking power

If it is a 73 40 hp it has a shear pin prop hub, they are not thru hub exhaust, but the prop is still rubber mounted and can really fool you.
 
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ozzyoutlaw

Cadet
Joined
Jan 1, 2013
Messages
6
Re: 40hp johnson lacking power

thank you guys so much havent worked on it yet but something else came up in my travels for answers im Australian btw forgot to tell you all that but fuel we get now might be a problem aswell but im leaving it for the pros to look at think thats the best bet :)
 
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