40 HP will not run at full Throttle

Hi all. This is a great site just full of information. This is my first time, but I hope that In the future I will be able to help someone as much as I have already been helped by this site. Thanks in advance!! I have been lurking trying to figure out what could possibly be wrong with the boat I just got. Well, my father in law just gave me his old johnboat with a Johnson 40 on it. I think the engine is an '88. I need some help trying to get it running. <br /><br />A little background. It was not running when I first got it. Cleaned the carbs, changed the fuel, new plugs, I tried to follow what had already been covered here; and it started, all be it not well, but hey it runs. So took carbs off again blow air through all the little jets and holes and took it to the water. Got it started, a little rough, but it started. Got out into the lake and tried the give it some gas. It would not go to full throttle. So, with the throttle push to wide open it would run as if only at half throttle. I pushed the chock in, bam, up on plane and sailing so fast my hat flew off. Let the chock out back down to half throttle, all with the throttle lever at full.<br /><br />I don’t think it could still be a gummed up carbs, but.. So, I looked around and in the fuel lines it looks like there are air pockets and a few air bubbles coming out from the line closest to the carbs when the engine is off. There also appeared to be some air around the fuel pump. Is the fuel system supposed to be completely free of air, and would an air leak in either the carb or fuel pump or both cause the engine to only open up with the chock in. Also, when I tilted the engine it looked as if the remaining fuel in the bowl of the carb leaked out.<br /><br />Please help!!!!

Re: 40 HP will not run at full Throttle

Welcome to iboats. An air leak would definitely cause your problem, it could be causing a lean condition, as well a gummed up carb could cause a lean condition and if you keep running it lean you will smoke a piston. You need to take those carbs back off and soak them and clean them properly and put new kits in them, then make sure you have no air leaks, that's where I would start at least. While your at it, if that thing has been sitting a while you will want to put a water pump in it and take a look at the t-stats.

Re: 40 HP will not run at full Throttle

I agree with Cricket Too. Might want to replace all the fuel & oil lines also & use the correct calmps.

Re: 40 HP will not run at full Throttle

Thanks for the replies. I'll re-clean the carbs and make sure once the new kits are in that the gaskets are seated right. I hope this solves it. Thanks again for the help! I’ll let you’ll know.

Re: 40 HP will not run at full Throttle

One other thing: This is a used boat and used outboard. Check the fuel tank carefully for debris, and clean out or replace the fuel lines and bulb. <br /><br />Then, put a fuel filter in the fuel line between the tank and the bulb. One of the nice, big clear plastic ones from the auto parts store is perfect. Just get one that matches your hose size.<br /><br />The idea is that you want to clean those carbs, then not have to clean 'em again. Crud in the tank or in the lines will clog 'em up again, sure as you're born.

Re: 40 HP will not run at full Throttle

I just wanted to thank everyone that responded!!!! Put in the new carb kits and new fuel pump and the old 40 hp was screamin'. Didn't even need to choke it to get her to start. Made it to 32 MPH. <br /><br />THANKS!!!

Re: 40 HP will not run at full Throttle

Great, glad you got it running good again.