1956 25hp Johnson RDE-17 mystery problem

[osaga]

I'm hoping to get some help with a problem I'm having. I have a 6 gallon pressure tank. On the last 5 or 6 occasions I've taken this boat out, after running for about an hour, when the fuel tank level approaches 1/4 full, the engine starts to die. It will be running along fine and then within about 3 seconds, it will slow and die with a few sputters and jerks. When this starts to occur, if I quickly press the primer button on the fuel tank it'll keep running for a few more seconds. If I continually press the primer rapidly, it will continue to run but with some sputtering and reduced power. This has been happening with surprising consistency on the last several outings.

My first thought was that something was wrong with the pressure tank. But I've since rebuilt it thoroughly several times -cleaned everything up, replaced all the gaskets, the disc valve and even the fuel pick-up tube. When running, the tank builds and holds pressure very well. I haven't been able to determine if the pressure relief valve is sticking or not, perhaps building up too much pressure? but that seem unlikely.

-The engine starts and idles easily and it runs well WOT. (until it doesn't, when the tank gets to 1/4)
-When it dies at around 1/4 tank, I can start it again without much effort after pressing the primer button.

-After it dies, I can start it and it'll run for several minutes without having to press the primer. The second time it dies, it'll start and run for a couple minutes, the third time about a minute, until eventually I have to press the primer continually to keep it running. Seems to be exponential. And this is something that has occurred consistently.

-I've noticed some excess fuel in the intake manifold but thought it's likely just from tilting the motor.

-Carbs were cleaned not too long ago, and needle valve and float were replaced.

-I haven't replaced the intake manifold gaskets, or inspected the reed valves which are both likely original.


Any idea what could be going on here? Should I go after the tank one more time?

Thanks

 

[racerone]

Inspect pick up tube in the tank.----And 1955 was the last year for the green 25 HP motors.

 

[F_R]

Very interesting, and I really can't think of an explanation. It is on a boat, right? So it isn't a matter of tank being too low below the motor.

Pressure in the tank does the same thing as pressing the button---supplies gas to the carburetor, except by doing it manually, you may be suppling it at greater pressure. I think I'd be checking to see how many PSI pressure the tank is actually building. Thinking too little psi, not too much.

 

[tomhath]

What racerone said. It sounds like the pick-up tube has a leak that's letting air in. I'd look for it about 3/4 of the way down the tube...

 

[F_R]

But the OP said he has already replaced the pick-up tube.

 

[tomhath]

But the OP said he has already replaced the pick-up tube.

"cleaned everything up, replaced all the gaskets, the disc valve and even the fuel pick-up tube"

It sounds like you're correct, he replaced the disc valve and pick-up tube...or did he replace the gaskets and clean the rest?

 

[F_R]

I dunno. Nor do we know if the pressure relieve valve is leaking and if the tank is pressurizing enough. All we can do from here is guess.

None of us has suggested slopping some soapy water on it to check for leaks yet.

 

[osaga]

Thank you for the replies. I had the same thought initially, that there was a hole in the pick-up tube 3/4 of the way down. But I did replace it with a hard to find NOS tube, I inspected the old tube and didn't find any holes. I also thought perhaps the disc valves were sticking, but they pass the rattle and mouth pressure tests. The tank builds what seems like a lot of pressure, the fill cap is hard to turn to release the pressure, and it will hold pressure over a long period of time after it's been sitting. It seems unlikely to me that too much pressure would cause the engine to die, unless I hear otherwise. Also when the problem is occurring, I've released the pressure in the tank and starting it up again, which didn't seem to change anything.

Other theories:
Perhaps the needle valve in the carb is not the right one for my engine, and is becoming stuck, requiring the primer pressure to dislodge it? Seems odd that it would occur only at 1/4 tank.

Could the intake manifold or reed block gaskets be letting too much air in after the engine warms up for an hour? requiring extra fuel pressure from the primer to keep the engine running? Seems unlikely

 

[racerone]

If it runs for an hr there is nothing wrong with the motor I say.------Just test run with another tank.----There must be someone with a tank like that around the corner.

 

[oldboat1]

Tilting the tank doesn't help? That is, letting remaining fuel flow down to the pickup? I can remember getting back into the dock that way on at least one trip.

 

[Jdunham]

Is there any chance that the arm for the float is bent and there is less fuel in the tank than indicated?

 

[oldboat1]

^^That's a good suggestion imo. Doesn't seem like 1/4 tank should be a problem -- pretty sure when I was faced with rowing home there was less gas in the tank.

 

[osaga]

Thanks, I hadn't thought to check that but I just took the tank apart and measured the pick-up fill tube and it reaches to about 3/8" shy of the tank bottom. This is about 3" lower than the level of the 1/4 tank worth of fuel currently in it. So (unfortunately) it's not a bent tube or gauge float arm.

However, I may have found the problem.... I installed an inline fuel filter quite a while back, on the short connector line from the tank. It's one of those clear plastic with gold crusted filter material inside. I just took it off and tried to blow through it, it becomes more difficult to blow air through the harder try, to the point that almost no air passes through it when blowing as hard as possible. I also now realize that it was installed backwards. I've got a new one arriving Saturday and will report back. Seems like a 50/50 chance that's it

 

[oldboat1]

Does the '55 have a glass bulb and filter? That should be suffficient -- that and periodic tank cleaning (some solvent, and marbles or pebbles shaken around in the bottom!)