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1979 15hp johnson , runs out of fuel at high speed

A

antlers

Member
Have rebuilt carb,replaced fuel pump and all fuel lines including bulb. Trolls great all day long. But will not run at high speed. IT helps to pump the bulb? After market plastic tank. Do they have a filter screen in them? I see no way into the tank? any help appreciated.
 
If it runs better when you pump the ball check the fuel pump. Did you thoroughly clean the carb? What did you set the float at?
 
Sounds like an air leak somewhere. Do you have another fuel tank and fuel hose you can use for a test? It could be a problem with the pickup tube in your tank, bad fittings on the hose or a bad fuel bulb.

KJ
 
You say "It helps to pump the bulb."...... That really doesn't tell us the story. What exactly happens when you pump the fuel primer bulb? Does the rpms increase to where the engine hits full throttle?

In pumping the fuel primer bulb, you're acting as a manual fuel pump.
 
OK, here is what happens when I turn on full throttle. Just about the time it tries to plane the boat it runs out of fuel, pumping the bulb will help but never will reach full speed. If I do not pump the bulb it just goes flat, and motor stops. Then I can prime the bulb and the motor will start right up and run fine at slow speed. Have checked the carb. again, it is spotless inside, high speed jet is clean, float level is correct. The fuel pump is new. The bulb with line and connectors are all new. Only thing I can come up with now is the pick up tube in the tank. See no way in to it. Maybe try to blow it out with air. Untill I can find another tank to try??? any other ideas welcome. Wanted to add. This will not happen with motor on test stand. It will run full speed. It has to be in the water pushing the boat under load.
 
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Some will argue, but that pump may have an arrow with ----UP ----on it.---------Easy for you to check if it is mounted correctly.---------------Some cheap plastic tanks are just that, very cheap / unreliable.---A hose for the pick-up tube and have seen that hose cracked / loose.--------I prefer the old steel OMC tanks.
 
YES, I am looking for a clean original tank. After cleaning the carb. this motor runs and trolls so good. Just got to get the high speed going.
 
antlers said:
OK, here is what happens when I turn on full throttle. Just about the time it tries to plane the boat it runs out of fuel, pumping the bulb will help but never will reach full speed. If I do not pump the bulb it just goes flat, and motor stops. Then I can prime the bulb and the motor will start right up and run fine at slow speed. Have checked the carb. again, it is spotless inside, high speed jet is clean, float level is correct. The fuel pump is new. The bulb with line and connectors are all new. Only thing I can come up with now is the pick up tube in the tank. See no way in to it. Maybe try to blow it out with air. Untill I can find another tank to try??? any other ideas welcome. Wanted to add. This will not happen with motor on test stand. It will run full speed. It has to be in the water pushing the boat under load.
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That's a pretty good explanation. The only confusing part is how you could pump the fuel primer bulb if it indeed goes flat if you do nothing.

However, it is obvious to me that you have a fuel restriction... and that is your line of thought also.

If you were using a built in fuel tank, I would suspect the anti siphon valve that would exist where the rubber fuel line connects to the built in tank.

It appears however that you are using a portable (6 gal?) fuel tank that has a restriction to it.... either a clogged fuel intake screen or a unopened air vent valve. To check for a unopened or faulty air vent valve, simply loosen the fill cap.
 
I have tried it with the cap loose. Makes no difference. It is a 6 gallon plastic tank. Because of health problems I am now just getting back to the problem. Its been going on since last summer. With help I am determined to find the problem. And will let every one know what it was. Appreciate all the help.
 
The inlet screen has my vote,antlers Try using a known good tank from a friend and retest. Have you visually looked in tank for debris,Perhaps reverse flush pickup tube and dump contents of tank
 
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antlers said:
Have tried to look in tank with light and mirror, Just can't see any thing. I will try to back flush the pick up tube. Thanks.
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When you pump the fuel primer bulb... does it snap back to it's original shape OR very slowly return?

Can you obtain another tank to try?

The fuel primer bulb.... sometimes one of the valves will come apart, the inner portion will drop out, eventually turn around and get pulled back in backwards which effectively creates a shut off valve. When holding the bulb horizontally, can you feel anything laying inside of it?
 
I am looking at some original tanks on ebay, Hope to find a good one. The primer bulb once it goes flat does not snap back.It slowly comes back.
 
I am looking at an old Johnson fuel tank. good shape, clean inside. But opposite of the filler cap, on the other end of the tank. It has another filler cap for oil reservoir. Anyone seen this before? And how does it work? The fellow selling it did not know.How does it mix the fuel?
 
antlers said:
I am looking at an old Johnson fuel tank. good shape, clean inside. But opposite of the filler cap, on the other end of the tank. It has another filler cap for oil reservoir. Anyone seen this before? And how does it work? The fellow selling it did not know.How does it mix the fuel?
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Avoid that tank even if it's free!
 
Have another question....I just realized that I did not replace the fuel connector that is mounted on the motor.It feels solid and looks like it is held in place with a bolt? Is this a special connector? And is it available? Have replaced all others.Might as well replace this one. While waiting for tank to try. Thanks.
 
On the plastic tank have you just tried screwing the fitting on the tank out there should be a screen on the end of the dip tube going down into the tank. Do you have pics of the plastic tank?
 
Sounds like a special connector? Maybe just remove it and clean it. To make sure nothing inside restricting fuel. The screw that holds the fast...slow...idle linkage in place, also holds the fuel connector. Have had the plastic tank for years no problems, has a vent on filler cap. But I see no way to remove pickup tube to check.
 
Have now ruled out tank and fuel line as cause of fuel starvation at high speed. Brings on a new question. That no doubt should have been one of my first questions. I understand the compression on this 1979 15hp Johnson should be around 100psi and not vary between cylinders over 10 to 15 psi. So my question is. How low can the compression be, before it effects the purge of the fuel pump? I know how important compression is to properly tune an engine. But have no experience on boat motors. And reading all of these treads on fuel starvation, one hardly ever mentions compression and fuel pump purge. Just wondering where the line is there? I know compression test will be my next test. Despite the engine seemed to run strong before running out of fuel. Thanks.
 
antlers, any more than a 10% fluctuation between cylinders is normally unacceptable for compression readings,but iv'e seen engines run with low compression #s
 
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