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

The fuel pump operates from the crankcase !-----A positive pressure pulse and a slight negative pulse.----------The crankcase compression is NOT the same as compression in the cylinder.
 
My neighbours motor had me scratching my head for months. When hot would rev down and conk out and not restart. Had spark, had compression (hot and cold) and had a carby full of fuel. Only out of frustration did i bypass the fuel pump and the problem went away.
I can only assume that once the pump got hot under the cover did it leak fuel into the bottom cylinder causing the engine to drop a cylinder and run like a bag of crap. I did notice that pumping the bulb did have an affect (actually improved a flooding condition) that lead me to bypass the pump.
When hot try spraying crc or some solvent around the motor to see if its sucking air when hot.
Bypass the fuel pump, just gravity feed the carby from a bottle and see if that fixes it.
 
If I recall your post, you were saying that the primer bulb goes flat just before the motor hiccups. If you had a restriction or air leak BETWEEN the primer bulb and the carb, then I doubt the bulb would go flat. So your problem must be between the check valve of the primer bulb, closest to the gas tank, to the liquid gas in the tank and of course the venting air above the gas is critical as well.

I can't remember if you have tried a new tank but that would be my next step if you haven't.
 
So the primer bulb was not going flat then, at least not more then normal. One or the other observation is wrong. He might want to find out. I don't make up the physics.
 
The fuel pump operates from the crankcase pulse. That's the info I was seeking. Will do a compression check to see what it is. Then will do a fuel pressure check. Then wait for the ice to melt off the lake for the try out. Racerone....really nothing in the crankcase that could weaken the pulse? Can one assume that?
 
Leaking / worn piston skirt / piston rings / bad lower seal / labarynth seal / leaking gaskets / bad reed valve. ---These items all have an affect on the crankcase compression.----------Is the pump mounted the correct way ?
 
Pump is mounted correct. But I have always felt the pump was not putting out enough fuel. Not that the new pump is bad. But maybe what makes it work. So its time to rig up fuel pump tester and see what is going on. We back flushed the tank and pickup tube, and a friend of mine tried it out with new fuel line on the river with his evinrude....ran fine up river. That is why we ruled out tank and fuel line at this time. Could have been something in the pickup tube I guess, but doubt it.
 
Take a look at the gasket that seals the fuel pump to the powerhead. Any air leak or problem in that will reduce the pulse power of the fuel pump.
 
Will do that. Not much left...fuel pump and or carb. And have already rechecked the carb. Just hope its not of Racerone's list...could be.
That might mean a new 4 stroke$$$$$$$$$
 
OK. Found my fuel pressure tester. I am going to put a T in the line between the fuel pump and the carb. Run engine in a barrel of water and check pressure at idle, Half speed, Full speed. Appreciate any help as what the readings should be. Also I have to keep in mind this motor would only run out of fuel under load....pushing boat. That is the way I would really like to check the pressure...on the lake. All kinds of lakes close by ice and 3 feet of snow. Might make it down to the river before it blows out. But maybe something will show up on first test. Thanks.
 
antlers,wait to run it on the open water, running her in a tank without a load will tell you nothing,except idle fuel pressure, I would not suggest running engine much above idle speed in the barrel without a load,Parts could go flying and get expensive!
 
It is normally a go-no go senario. The fuel pump is not a pressure pump they are low pressure high volume and I guarantee you if it works at idle speed it is working at high speeds. Your symptoms are more fuel flow issues. Di you replace the fuel lines? My guess is the fuel fitting on the motor from the fuel tank is gunked up or rusted. Pull the fuel line on the fuel pump in off and see how easy it is to pump the primer bulb. If it is difficult to pump the primer replace that fitting they do get old. It should be real easy to pump the primer until the float bowl fills up. .
 
Here in WA we get those little mud dobbers that build their nests in all kinds of small holes if you have a telltale fitting go ahead and clean it out as well.
 
The only fuel fitting or connector that has not been replaced is the one mounted on the motor. Yep, the connector you are showing is the only one that has not been replaced. Nothing really to go bad on it. But worth cleaning at this point. I agree should not rev.motor up in test tank. First decent day, Will check fuel pressure at idle. We can go from there.. Thanks to all.
 
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Finally got on the water with fuel pressure gauge. Pumped the bulb the gauge showed 6 lbs pressure, The motor started right up and the pressure did drop to 1lb or less. Turning the motor on could never get the fuel pressure above 3lb. explanation why it will troll all day. But will not run at high speed. Seems the crank case pressure that operates the fuel pump is a real thing .And not worth fixing on a 1979 motor?? Only real puzzle it came on all of a sudden. Let it be a lesson...start with the gauge first.
 
I know the fuel pump is mounted correct. The motor runs strong when you pump fuel to it by hand. One would think only a sick motor would not purge the fuel pump?? Is it possible there could be a restriction in that port??? One season this motor ran full speed. The next season it would only run at trolling speed. Would not think the motor would lose crank case pressure from one season to the next?? But it seems to be the case.
 
If the motor runs 100% when the primer bulb is squeezed, I would doubt there is a crankcase problem. Crankcase leakage would also result in significant power loss.
 
I agree with above post. The tank set up may work on another motor but it depends on how much lift is required. I have also seen problems show up when the hose is too long and/or too soft. A soft hose will absorb pulses and slow fuel flow, lowering pressure. Omc in those years used a hose with greater integrity but became brittle and stiff in time. I replace these with as short a hose as needed and using a high quality 5/16 automotive type fuel hose. Ethanol will soften hoses and cause them to absorb fuel pump pulses as well. Ethanol has no place in most outboards. SeaFoam and other products may treat the ethanol but of course don't eliminate it. If it wasn't for ethanol. I'd probably be "flippin' burgers". I love the stuff.
 
It could be the pulse port has a chunk of gasket in it just push a chunk of fuel line up tight and see if you can blow through it easy? You may need to put that piston all the way down so the ports for the cylinder are open.
 
I fought a fuel delivery issue on my modded 15hp small block. I have been in a lot of carbs in 45 years and I will tell you the opening on the needle seat is too small in my opinion. Most small carbs when pressure fed have a 1.5mm opening and a 2mm for gravity fed. I think the port or opening on mine was .040 or so. So I carefully drilled a 1.5mm hole and finished or polished the seat for the needle with a toothpic on a drill. Set the float level and the problem never reoccurred.

I also pulled out the primer bulb springs as they are not needed and add to total head.
 
This motor ran fine up until last season, When it would not run at high speed. It runs fine now if you feed it fuel. The fuel pump is only pumping enough fuel for trolling speed. Proven by fuel pressure gauge. Looking at the old fuel pump removed, Does not look like there was any thing wrong with it. If its not lack of pulse from the port. I do not know what to try next.
 
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