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15 hp johnson fuel pressure 1979

A

antlers

Member
When trolling with a 1979 15hp Johnson outboard, Should the fuel line primer bulb have pressure in it?
How is the best way to check the fuel pump? thanks.
 
Well yes and no. There MAY be pressure in the bulb at any time due to pressure build-up in the tank. The tank is vented to let air in, but not let air out. So, as you can see, there may be pressure present. (This is assuming you have the original tank or one like it.)

Now for the rest of the answer. If there no pressure in the tank, for instance with the cap loose, then there is a suction condition present inside the hose as the fuel pump draws fuel from the tank.

How to check the fuel pump?? Does it run at all speeds? If it does, the pump is working. If not and you suspect the pump, can you keep it running by squeezing the bulb? You are doing the pump's job as you do that. The REAL test though is with a fuel pressure test gauge T'd into the line between pump and carburetor.
 
kimcrwbr1 said:
Have you cleaned the strainer/screen in the cap on the fuel pump? The best bet is to just rebuild the pump and replace the fuel lines then your ready to rebuild the carb if necessary. Be real careful pulling the pump apart and take pics as there are several different ways to put it back together wrong.
http://www.ebay.com/itm/New-Fuel-Pu...088-/271927178307?hash=item3f501f7443&vxp=mtr
Click to expand...

Hi, Yes I have cleaned the screen, also the carb and installed a kit.And it trolls perfect. But when I gave it gas to cross the lake it acted like it was running out of fuel, And I pumped fuel in with the bulb. I was fishing in a no wake zone so I really could not try it out at hi speed. Assume its the fuel pump??
 
I have cleaned the carb.1979 15hp Johnson.It runs fine on the test stand. Can one tell me the correct way to set the rich,lean screw when I go out on the lake. thanks.
 
I finally got to try out this 15hp Johnson on the lake. I cleaned the carb.with new kit,replaced the plugs.Turned the mixture screw out 1-1/2 turns as a start. IT runs smooth and beautiful at idle and trolling speed. But at full throttle it cuts out and acts like its running out of fuel. Turning the mixture screw out one more full turn,it will not cut out any more,but will not reach full power and plane the boat.Also it runs rougher at idle speed with the mixture screw turned out. Any ideas,thoughts appreciated before I take it to a shop.
 
My '79 9.9 developed a similar issue. I had chased cooling issues for 2 years and finally nailed it. I took it out fishing and it ran like a top then the next weekend it wouldn't run. It would start with when the bulb was primed and choke applied but die after that. I was fed up after thinking I had conquered this motor and took it to a shop. They put a rebuild kit in the fuel pump and it still didn't run well so they replaced the entire fuel pump. The motor runs like a dream now both at idle and at high speed.

KJ
 
The mixture screw is for idle mix only.--------No need too adjust it at full throttle.-----------Operate the manual pump at full throttle to see if that helps.----------------Check for leakage in the fuel line , connecters and INSIDE your fuel tank too.
 
High speed mixture is controlled by a fixed jet ("orifice plug"), and by its very nature is non-adjustable. It is a precision part and must be absolutely clean. Even a trace of any contamination will make it run lean at high speed.
 
Thanks for the info guys. I did not realize the mixture screw was for idle and low speed only. But that makes sense to me now. And I feel more and more like I have a weak fuel pump. Sure hope one is available for this 1979 15hp Johnson. Have replaced fuel lines and connectors and it now runs unbelievable smooth at low speed.
 
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The fuel pump used on that motor is very common and available all day long.----------Might just need a $4.00 diaphragm.
 
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