2001 Johnson 115 runs rough at low RPM's

I just purchaced a 24 foot pontoon boat with a 2001 Johnson 2 stroke 115 HP. The engine starts easily and idles at about 850 RPM. It runs a little rough between idle and about 1800 RPM. At speeds above 1800 RPM it runs as smooth as a sewing machine. It has never died and the acceleration seems even through the entire RPM range. I have a 30 day warrenty on the engine but I called the dealer today and he said that my problem is normal for my engine. It doesn't seem to be a serious problem but if it's not right I will hold them to their warrenty so what I need are some second opinions. <br /><br />Thanks,<br /><br />Emerson

Re: 2001 Johnson 115 runs rough at low RPM's

I think on some motors the manufactuer had decided that it would be best for design to disable a couple of the cylinders at specific RPMs. <br /><br />Supposedly it made it better in the longrun if used at sustained trolling speeds.<br /><br />It's quite possable that it might be normal.

Re: 2001 Johnson 115 runs rough at low RPM's

Thanks for your help. I have talked with a couple of others who have also confirmed what you said although they said the problem was more often noticed on Mercury OB's. I will be satisfied as long as my engine starts fine and doesn't die at low RPM's.<br /><br />Emerson

Re: 2001 Johnson 115 runs rough at low RPM's

There are a couple of things you should look at. First, when sitting at the dock, how close to the water are the exhaust reliefs and overboard indicator? If they're with-in 2 to 3 inches, you could probably benefit by having the engine raised a couple of (mounting) holes. This would provide a little less back-pressure and possibly help to ease the roughness you're feeling. The other thing that needs to be confirmed is your wide open throttle (WOT) rpm. That engine should be up around 5500 rpm - on that 24' pontoon boat, if it's not running close to that it could be lugging (overloading) the engine. This will contribute to the roughness too. If the WOT rpm's aren't close to 5500 you'll need to change to a smaller pitch prop. This will help the engine's ability to accelerate that large boat as well as get it into it's powerband.<br /><br />The last item to look at is your idle rpm. It may be too high. The engine in forward gear should idle @ 650 +/- 50 rpm. If it's idling at 850 in gear, that's too high and could cause shifting troubles or damage the gearcase in the future. The idle speed needs to be verified with an accurate shop tachometer - the in the dash tach's on most boats can be very inaccurate and shouldn't be used for setting the idle speed.<br /><br />Two points to keep in mind:<br /><br />1. The Johnson motor do NOT drop cylinders for idling, all 4 run all the time (the Mercury 115/125 drops the 2 bottom cylinders below 1800 rpm).<br /><br />2. All carbureted motors have a transition point where the fuel circuits change from idle to off-idle and then to mid range. This covers up to around 2000 rpm, depending on the application (boat type) and how it's propped. This could very likely be what you're describing as roughness. Also, if it's still being broken in and is running on double oil mix, that will contribute to some rough running too.<br /><br />Let us know how things work out.<br /><br />-John

Re: 2001 Johnson 115 runs rough at low RPM's

Thanks John - Unfortunatly I do not know what the exhaust reliefs or overboard indicators are. I do know that I don't think I could raise the engine any as I have a small amout of cavitation/ventalation at near WOT. I would think raising by a hole or two would make it worse. My WOT RPM's are pretty close to 5500 RPM's and my speed is about 28 MPH. I will check tomorrow what the in gear idle speed is (with in dash tach) to see how close to 650 it is. The engine is not being broken in and I am running normal oil mix. Thanks much for the help.<br /><br />Emerson