Boat runs well but dies after a few minutes - check my logic before I spend money

[Gas Giant]

Hey guys,

Engine is a '78 Johnson 85hp motor, red plastic above-the-deck fuel tank, new primer bulb/hose.

To make a long story short, the engine runs well, but only if there is fuel in the carbs (imagine that). I can prime them with the primer bulb, and it does get hard. I did that and checked for any leaks in the fuel lines leading to the carburetors or fuel leaking out of the carburetors from stuck floats. I didn't see any. I should mention that I pulled both carbs off and cleaned them out about a month ago, but didn't try to run it until today.

Boat fires right up after that, and runs great - until it burns up the fuel in the carburetors, at which point it not surprisingly dies. It won't restart again unless I prime it again. (It will sort of sputter, but not run, if I don't prime it)

Since I can't find any leaks in the fuel lines and the primer bulb does get hard, I'm thinking I don't have a primer bulb problem or a fuel/air leak problem. I'm thinking I have a fuel pump problem - the fuel pump isn't sending fuel to the carburetors, so after it burns up what I sent to them with the primer bulb, it dies.

Would you guys agree, based on what I've told you, or is there something else I could be overlooking? I often overlook simple things when I get frustrated, so sometimes it helps to have a 2nd opinion from some folks who aren't frustrated.

Thanks!

 

[GA_Boater]

Classic bad fuel pump symptoms. After she starts, gently squeeze the bulb. If it keeps running, rebuild the fuel pump.

 

[emdsapmgr]

You can do a test. When the engine is running, constantly prime the fuel hose bulb. If the engine runs fine throughout the rpm range as long as you constantly prime the fuel hose bulb, you have a fuel delivery problem. Usually, that comes down to a weak, or non-functioning fuel pump, but it could be a fuel restriction, too.

 

[rookie456]

If it is a newer plastic gas tank, it maybe a ventless tank. If so, try loosening the gas cap in order to vent the tank, I had to replace mine with a vented cap to get my motor to run right.

 

[Bosunsmate]

yes as above or very rarely loss of crankcase compression

 

[racerone]

Check for leaks on the pick-up tube inside the fuel tank......Diaphragm for fuel pump is cheap and about the only thing that can go wrong with the pump.

 

[Gas Giant]

Thanks guys, I'll give them all a look this weekend.

 

[Tim Frank]

How did you check for leaks?
The leak you are looking for with these symptoms is a suction leak which is no where near as obvious as a leak on the pressurised side.

You can test the fuel pump output with a fuel pressure gauge. That way you aren't guessing.

 

[Gas Giant]

I was only looking visually for leaks.

I put in a new fuel pump diaphragm, but it has no effect on the problem. Which I found surprising.

I also determined that the engine will stay running if I act as the fuel pump by constantly refilling the carbs with the primer bulb as its running.

I pulled the sending unit off the fuel tank and looked it over. No blockages or restrictions in the pickup tube.....not sure how to tell if it leaks.

One thing I did notice is that my primer bulb doesn't seem to like priming itself. I had to disconnect it from the engine and have it oriented almost vertically before it started pulling fuel from the tank. And it doesn't seem to work too well in any other orientation. I think the problem may be here. What I'm not sure about is whether this is a sign of a bad primer bulb (it's almost brand new) or a sign of a problem in the sending unit.

I'll see if I can dig up a fuel pressure gauge and check the pump that way....but if not I'll give it the old pump-into-a-beer-bottle test

 

[oldboat1]

might be a carb vent issue that wasn't addressed during cleaning.

 

[Tim Frank]

I was only looking visually for leaks.

That won't find a suction leak

I put in a new fuel pump diaphragm, but it has no effect on the problem. Which I found surprising.

Doubt it was the fuel pump at all....it usually isn't. A ton of fuel pumps get replaced or rebuilt when it is a fuel line issue. As someone said "Classic fuel pump symptoms"....and he was correct. But, there are several other things that will also cause those symptoms; these fuel pumps are not that strong on the suction side, and even a small amount of air ingress can kill the suction and stop fuel flow.

I also determined that the engine will stay running if I act as the fuel pump by constantly refilling the carbs with the primer bulb as its running.

Try a "known-good" tank and line if you can. Otherwise, try removing the fitting from the fuel line at the engine end and connecting directly to the fuel pump. If no change, do the same at the tank end. You can clamp the fuel line directly to that male end on the tank. That will isolate which end is the problem. Then either replace the fitting, or you CAN replace the o-ring (they are cheap, but not always available at every outboard shop) that provides the seal. You could do that one at a time instead of removing the fitting if you can get the o-rings.
The other possibility here is that you have only "similar" to OMC fittings. Is that an OEM fuel line or an aftermarket? Some of those look like OMC but aren't. I've known people to grab one off a rack 'cause it looked correct and then have problems exactly like you are.
I've personally bought aftermarket fittings that are supposed to be OMC compatible that simply do not work.

I now will not use anything but OEM.

I pulled the sending unit off the fuel tank and looked it over. No blockages or restrictions in the pickup tube.....not sure how to tell if it leaks.

The pickup tube is not a likely culprit since you can pump fuel with the bulb.

One thing I did notice is that my primer bulb doesn't seem to like priming itself. I had to disconnect it from the engine and have it oriented almost vertically before it started pulling fuel from the tank. And it doesn't seem to work too well in any other orientation. I think the problem may be here. What I'm not sure about is whether this is a sign of a bad primer bulb (it's almost brand new) or a sign of a problem in the sending unit.

The bulb itself could be backwards. Make sure the arrow is pointing at the engine. In theory (I think... I've never opened one up) the construction should not allow you to pump up fuel at all if backward...due to check valve orientation; but I've had a couple over the years that I connected in reverse, and they "sort-of`worked", but caused similar problems to what you are getting. So it`s worth checking.

With the info you've posted to this point, my gut would lean towards the tank-end fitting having a major air leak....especially if that is the end that you routinely disconnect for filling. A new o-ring should sort that out.

I'll see if I can dig up a fuel pressure gauge and check the pump that way....but if not I'll give it the old pump-into-a-beer-bottle test

You already know that it will not pass that test. You need to hunt down (ruthlessly ) the source of air ingress and eliminate it. Then you can test the pump.


My 10 cents for a Sunday am. Sorry for the novella.

 

[Gas Giant]

With the info you've posted to this point, my gut would lean towards the tank-end fitting having a major air leak....especially if that is the end that you routinely disconnect for filling. A new o-ring should sort that out.


You hit the nail right on the head.

Turns out I had another one of those quick-disconnect tank fittings in my garage. Removed the old one and installed this one, and the problem went away. Good call!

 

[Tim Frank]

Great news.
Save the old fitting and find a source of o-rings and you can fix it.