15hp with fuel starvation

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JMichael

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I'm trying to help a friend with his outboard. I've got it narrowed down to knowing the issue is fuel starvation because I can keep it running if I squirt fuel into the throat of the carb. The motor will start and idle, but as soon as you start to open the throttle it will bog and die. The only other abnormal symptom is that it will be sitting there at a high idle running great, then suddenly it will (cough/sneeze/backfire thru carb) and die. Although I have not soaked the carb in a vat, I've cleaned it thoroughly twice now with carb cleaner and compressed air and I've checked the float height. The primer bulb stays firm at all times, all fuel lines are in great shape and the fuel filter bowl (clear) shows no sign of air in the fuel system. I've checked the reed block and it appears to be OK. I have noticed a puff of smoke when it coughs that appears to be from the base gasket leaking, but I'm not sure if that can cause the symptoms I'm seeing here.

Now for the part that will make everyone close this thread. It's an 88 15hp merc. I know most of you are not merc guys but an outboard is an outboard when it comes to diagnosing mechanical issues. So does anyone have any ideas that I haven't covered?
 
lckstckn2smknbrls said:
Fuel pump.
Well I thought about that and tried to find if there is a way to test the fuel pump on a merc where the pump is molded to, and is part of the carb body. I mean you can't really check it's output, at least I don't know of any way and can't find any info on doing it. Then there is the part that I'm pretty sure that I read where Pappy stated that if you keep the primer bulb pumped up and hard, it eliminates the fuel pump as a suspect. Now maybe that only applies to johnnyrudes or maybe it was only pertaining to a particular motor, I'm not sure. But I know this thing has me stumped so far.
 
perchjerker said:
if you keep the bulb pumped up hard you are basically bypassing the fuel pump. The bulb is doing what the pump is supposed to do
That is what I think I remember Pappy saying and the reason I tried that. It made no difference in the way the motor acted. And the bulb never lost it's firmness during any of the run testing.
 
One other thing I noticed that might give someone other ideas. No mater how far I turn the mixture screw out/richer, it never seems to get so rich that it starts to affect the running of the motor. It will however cause the motor to cough/sneeze if I turn it in much more than 2 turns out from lightly seated.
 
Fuel hose inner wall separation. I have seen this happen on more than one occasion so this is something I factor in when dealing with no fuel conditions. There will be a crack form inside the hose and then as fuel passes the crack it tears the the inner wall away from the hose. As the inner wall extends down the inside of the hose it will start flapping inside and then all of a sudden the inner wall lining will blow up like a balloon blocking all fuel to the carb.
 
perchjerker said:
I would say your hi speed circuit is plugged from the sound of things
My thoughts exactly and the reason I didn't hesitate to pull that carb apart for the second time. Main jet and emulsion tube are clear, the carb is spotless.

KMixson said:
Fuel hose inner wall separation. I have seen this happen on more than one occasion so this is something I factor in when dealing with no fuel conditions. There will be a crack form inside the hose and then as fuel passes the crack it tears the the inner wall away from the hose. As the inner wall extends down the inside of the hose it will start flapping inside and then all of a sudden the inner wall lining will blow up like a balloon blocking all fuel to the carb.
Considered that. Disconnected the fuel line from fuel pump (there is no other fuel line from there) and squeezed the primer bulb and gas squirts like a super soaker water gun.
 
perchjerker said:
the jet and tube may be spotless but how about the passages inside the carb housing?
Blew carb cleaner followed by compressed air on all passageways and verified I had good flow out the other end of each one,.......... both times the carb was apart.

I keep thinking that somehow the fuel pump has failed in a way that prevents fuel from passing through it. It's really got me scratching my head.
 
perchjerker said:
is there fuel in the bowl when it acts up?
Hard to know as there's no way to check other than pulling the carb off and seeing how much it has. The only thing I know for sure is the bulb never needs to be pumped up after initial start and the bowl was full the two times I've removed the carb. So I've been going on the pretense that it's full up to this point.
 
Did you actually pull the float needle seat and check for an object blocking the float needle seat? That would starve the float chamber and give you these symptoms while maintaining a hard bulb. The float then would refill during low fuel demands like slowing down before turning off.
 
well I would at least take the fuel pump apart and check it out

real hard for me to say I am not familiar with that motor at all

the only other thing I can think of is can you try to lift the tank over the motor and see if it gravity feeds the fuel?

I know you opened the tank vent so I wont ask that 8)
 
Sinkingfast said:
Did you actually pull the float needle seat and check for an object blocking the float needle seat? That would starve the float chamber and give you these symptoms while maintaining a hard bulb. The float then would refill during low fuel demands like slowing down before turning off.
Pulled the needle out and blew carb cleaner and compressed air through at the same time I did the same thing to all the other passageways in the carb body.

perchjerker said:
well I would at least take the fuel pump apart and check it out

real hard for me to say I am not familiar with that motor at all

the only other thing I can think of is can you try to lift the tank over the motor and see if it gravity feeds the fuel?

I know you opened the tank vent so I wont ask that 8)
Even took the lid completely off the fuel tank for a little while during some of the testing. I pulled the pump apart and gave it a visual inspection and cleaned a couple of small particles from it. Did not inspect the diaphragm for pin holes as I was going under the assumption that pressure on the primer bulb had eliminated the pump as the possible culprit in this instance.

Is there any way one of the gaskets like the base powerhead gasket could cause symptoms like this? The weak link in my abilities lies with the internals and mechanical portion of a 2 cycle engine. I'm not comfortable with what/how things go on inside the crankcase as far as drawing fuel in, expelling burnt gases and the whole reed block and other things that make them different than a 4 stroke.

I'm off to try my luck with the crappie this morning so I'll check back on this later today.
 

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