Johnson outboard idles high rpm need help

[DinkMasterr]

Just bought a used 14ft alumacraft with a 2000 25hp johnson tiller with electric start. Put a new starter and selenoid on it. Fired it up. Upon starting it it would rev up very very high rpms and not come down to an idle rpm. The throttle on tiller handle is on low idle too but still wants to rev at high rpm. Anyone know how i could fix this? TIA.

[Tennessee Boy]

Hopefully,  someone qualified will answer your question.  Until then I’ll give you my answer.  ?

 

If it’s idling really high then I would think you need to adjust the throttle cable.  If it’s idling a little higher than normal you should be able to adjust the idling speed on the carburetor with a screwdriver.  If’s it a modern computer controlled engine you need to consult someone smarter than me.

 

I just noticed you said you put a new starter and selenoid on it so you probably already know everything I just said.  ?

[Way2slow]

Maybe I can help you.  I've never worked on this model or or any 3cyl 25hp Johnson so I can't get real specific but I should be able to guide toward the things you need to check.

First, take the air silencer off and look at the butterflies in the carburetors, make sure they are fully closed and the cam mechanism that opens and closes them is not holding them partially open.  When the throttle is fully in the slow position, there should be a very small gap between the cam and the roller/follower.  If not, something in the tiller or the fast idle stop adjustment is holding them open.  If that is good, then you probalby have a problem with the timer base linkage not pulling the timer base all the way back.  

On top of the throttle linkage on the side of the motor, there should be a linkage connected to it and going back under the flywheel.  This linkage connects to the timer base and is what advances the timing as you give it gas.  Most motors, the first 20% of the rpm comes from just advancing the timing so if it's not fully returning, it's going to idle extremely high, even if the carbs are fully closed.  That is usually a ball and socket connection so you can pop it off the linkage, pull it forward and see if that stop the high idle, in fact, it may not idle at all if it's all the way forward.  

NOW! don't just start adjusting linkages to try and fix the problem.  All that stuff has to be synchronized together.  There are little alignment arrows that have be right and all three carbs adjusted the right point of opening when they are.  You will need to do what is referred to a link and sync and you should find you a manual that tells you how to do that if you have never done it.  If it's not right, you can have all kinds of rough idle problems and acceleration problems.  Usually on the Johnson, just to adjust the idle speed, you don't do it with the stop screws, that timer base linkage is usually adjustable and you adjust that for the proper idle speed.  Using the stop screws throws off the synchronization that I just mentioned had to be just right.

[airshot]

The prior post is about right and you need to get a " factory service manual" not an aftermarket manual as the syc service is critical !!  The only issue other than that, running on high speed with the throttle closed could be a very lean fuel issue !!  Check you fuel supply, hose and fuel pump etc etc.  If your fuel supply is not restricted then look carefully at the syc issue..

[DinkMasterr]

@Way2slow its a 2 cylinder 25 horse with one carb and 2 spark plugs. When starting it it goes full speed but the butterfly is all the way closed with a very small gap as if it was in idle speed. Its open just a hair as if the roller on the carb linkage is just starting to move as the cam follower comes by to open the butterfly. Very puzzling

[Way2slow]

OK, I didn't realize they still made that motor in 2000, so that throws everything in the previous post out the window, none of that applies to the two cylinder.  That also complicates things a little.  What would be considered the timer base is that same plate with the cam action that opens the carburetor, so there is no way it can advance without opening the carburetor.   So, with that said, it leads to believe a serious vacuum leak or something in the ignition.  Something is causing a serious lean condition.  This is one of those situations where I almost need hands on and a little head scratching, trial and error.  One thought comes to mind if someone has previously worked on it, they might not have the timer base in right.

[thomas15]

I agree with obtaining a factory service manual. As mentioned the primary adjustment is probably engine timing and there is probably a dimension for that gap in the cam/roller assy.

 

If this were my motor before I did anything else I would do a compression test just to confirm that there isn't a problem there, make sure you ground your spark plug wires. I would also buy a carb rebuild kit and do that. And also change all of the fuel lines under the hood, make sure there are no vacuum leaks. 

[Bird]

Adjust the idle screw.

Previous owner probably couldn't keep it running at low rpms and turned up the idle.

 

[Way2slow]

A couple of thoughts come to mind.  The timer base/cam is adjusted too far advanced and someone adjusted the cam follower that opens the carbs to compensate. That beginning of that cam arm should completely clear the follower. I think there is an adjustment at the motor where the tiller connects that is used to just the low speed, make sure it's not all the way in.  Also at the pivot of the tiller there is a gear, make sure someone has not put it one tooth off.  I would back the cam follower completely off the cam and make sure the butterfly in the carb is completely closed, throttle plate touching venturi.  If it still revs too high, look at undoing the linkage from the tiller to the timer base and see if you can goofball it the timer base, pushing the cam clockwise and try it again.  What we are trying to do is isolate the cause.  If neither of those help, we're back to an extremely lean condition or ignition.   If it's not turning more than about 2,000 rpm, it's safe to run it for the next test.  Anything more than that and it's possible for it to go into runaway.   With it running, take a can of WD40 and spray the base of the carb, intake, hoses, case halves, basically the whole intake side of the block.  If there's a vacuum leak, the WD40 should pin point it by causing it to do some serious smoking and change rpm.  Also, if the carb has adjustable low speed jet, check the presets on it.  I don't remember it.  If someone tried to adjust the carb on a hose and not in the water, that could be way off.  It has to have the back pressure sitting in the lake causes to adjust the carb.