115 hp johnson wont idle

D Pon

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Apr 5, 2011
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17
Hey everyone I recently bought a boat with a 115 hp johnson outboard. It idles rough on the garden hose but wont idle at all in the water. I bought it from a friend knowing about the problem because other than that it runs great, I really dont have much knowledge with outboards so any help would be greatly appreciated,thanks.:eek::eek:
 

D Pon

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Apr 5, 2011
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Re: 115 hp johnson wont idle

Well the boat is a1977 and im pretty sure the motor is the original and its been recently rebuilt and my friend had the carbs rebuilt when he bought it so im really stumped.
 

DargelJohn

Petty Officer 1st Class
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Feb 14, 2010
Messages
337
Re: 115 hp johnson wont idle

Does the boat have a tachometer? On the water muffs, it should be reading 900-1000 rpm's. In the water it should read ~850-900 rpms.

Under the cowling, there's a big screw on the left front side (facing the front of the motor) which is part of the throttle linkage (see pic). If it is idling too low, then make a 1/4 turn clockwise and check the results. Continue this adjustment until rpm's are correct; should not take more than one full turn though.
 

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emdsapmgr

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Dec 9, 2005
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Re: 115 hp johnson wont idle

Any outboard needs to be in top shape to idle in gear in the lake. There is a lot of backpressure from the water, preventing the exhaust from excaping out the prop. As such, you need to tune the engine so that it runs well in the water, not just on the trailer. A poor idle can be caused by a restriction in the idle passages inside the carb. Easy for them to get restricted with dried fuel over a winter storage period. These carb passages must be cleaned out with a gasoline type cleaner, like a Gumout product-something that will dissolve dried fuel. Also pull the idle jets out for a visual inspection/cleaning. Set the idle speed rpm's between 650 and 750 rpm's when the boat is floating in the water and it is in forward gear.
 

bigpoppakdog

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Jul 9, 2008
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Re: 115 hp johnson wont idle

I guess assuming all components are ok, I would say you need to clean the carbs and work on the idle adjustment screws while your motor is in the water. When I was tweaking the idle adjustments, I backed my boat down on the ramp low enough to drop my motor down in the water and then did my adjustments from there. That way I was getting good back pressure on the motor and my boat stayed in one spot.
 

SparkieBoat

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Re: 115 hp johnson wont idle

clean/rebuild the carbs...do a compression test and a spark test with a spark gap tester set at 7/16
 

D Pon

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Re: 115 hp johnson wont idle

I forgot to mention that even while you have it at 1/2 throttle or so just cruising around you have to mess with the trim before it will even let the RPMs come up and run like it should but once u find its happy spot itll run all day but still wont idle
 

bob johnson

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Feb 25, 2009
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Re: 115 hp johnson wont idle

Id go ahead and take some base line measurements.... for your self and for other helping you.

first make sure all the plugs are firing!!! spark tester is good for that...

second go ahead and take compression readings....

almost all motors idle lower in water than on muffs...BECAUSE of exghaust back pressure...the idles adjustments are based on IN WATER IDELING!!!

if you have spark and good compression then we can move forward with maybe trying to help yopu.

good luck

bob
 

D Pon

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Apr 5, 2011
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Re: 115 hp johnson wont idle

I did a compression and spark test and its good on all four cylnders. After it been running I took the plugs out and all of them are sopping wet. I ordered the carb kits and hopefully that takes care of it but if anyone has any more ideas id be happy to hear them.
 

Monte1961

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May 8, 2011
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Re: 115 hp johnson wont idle

For what ever reason/s carb/s are dumping to much fuel on engine. A few causes are, float/s out of adjustment, low speed jets to big, low and intermediate carb position not in sync. Are the correct plugs installed? But am inclined to agree with the earlier posts! Keep us posted.!
 

Subdriver

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May 27, 2011
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Re: 115 hp johnson wont idle

Sounds like carb/carb adjustment problem. When the motor is trying to idle, in the water, the exhaust backpressure will make it harder for engine to breathe, the carbs have to be adjusted to account for this. Also plugged or restricted idle ports can cause this. Once the throttle plate opens and the idle circuit is nullified, motor runs fine. I'd focus on the carbs first. Hope it helps.
 

D Pon

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Apr 5, 2011
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Re: 115 hp johnson wont idle

I did check the plugs and they are the rite ones but I tried some NGK plugs instead of the champions and it made it almost impossible to start for some reason but im gonna rebuild the carbs and go from there thanks.
 

Monte1961

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Re: 115 hp johnson wont idle

And 1 more thought. If it has cork floats, get new plastic ones! Those cork floats can drive a man insane!
 

D Pon

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Apr 5, 2011
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Re: 115 hp johnson wont idle

The parts guy said they are plastic so im good to go there
 

D Pon

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Apr 5, 2011
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Re: 115 hp johnson wont idle

Well I rebuilt the carbs and theres absolutely no change it still runs the same and im stumped
 

cdn-pk

Petty Officer 2nd Class
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May 20, 2009
Messages
162
Re: 115 hp johnson wont idle

what about the timing? if your sure about the carbs..... gotta be the timing?? unless the it's dumping too much gas before it's advancing the firing angle. did you do a link n sync?
 

D Pon

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Apr 5, 2011
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Re: 115 hp johnson wont idle

No I havent done that yet im not really sure how too.
 

D Pon

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Apr 5, 2011
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Re: 115 hp johnson wont idle

Would that have any effect on my idle issue or just wot
 
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