Fuel Pressure problem in Johnson 90 hp Tank

[Rtychynski]

I have a 90 hp Johnson outboard
Approx vintage 1985
I built an aluminum fuel tank for it
It is a 1 hose system
It works when I prime with the pressure bulb
If I crank it wide open, it collapses the primer bulb and the engine stops
If I am running at lesser throtle, I can feel the bulb trying to collapse

I guess I missed the boat somewhere in my tank design
What do I need to do to correct this

I am told that this should be a pressurized tank system
I have a vented gas cap filler
Should this be unvented
What do you suggest
I would appreciate any help from any of you who have knowledge in this sort of thing
My expertice stopped at the sheet steel end
Thanks
Rudy

 

[Joe Reeves]

Re: Fuel Pressure problem in Johnson 90 hp Tank

It IS NOT a pressurized system. The tank must be vented.... gas cannot get out if air cannot get in. Your vented gas cap apparently is not open or it is faulty.

 

[ebry710]

Re: Fuel Pressure problem in Johnson 90 hp Tank

The line is the pressurized system, not the tank. The tank needs to displaced used fuel with air. From the bulb to the engine is pressurized by you squeezing the bulb. Then the fuel pump takes over.

 

[kenmyfam]

Re: Fuel Pressure problem in Johnson 90 hp Tank

Sounds like not enough venting to me. However does it run O.K. at other speeds ?? even trolling speed ??
What size fuel line are you using ?? if too small it may not be able to deliver enough to the motor.
Take and leave the gas cap off for a quick test (unless the tank is too full) and see if it still does it.
Good Luck.

 

[Rtychynski]

Re: Fuel Pressure problem in Johnson 90 hp Tank

I have tried it without cap on
No difference
I have almose decided that the bulb collapses to easy and might try a bulb bypass to use after starting ?

It IS NOT a pressurized system. The tank must be vented.... gas cannot get out if air cannot get in. Your vented gas cap apparently is not open or it is faulty.

 

[Rtychynski]

Re: Fuel Pressure problem in Johnson 90 hp Tank

Fuel line is 3/8 brand new . I also upped the pick up line size in tank to 3/8
It opperates fine at lower speeds, but constantly seems to pulsate the bulp ( I hold it in my hand to feel the response at different rates of throttle)
I have tried it with the cap completely removed (thought that it might be creating a vacuum in tank - but no change)

It IS NOT a pressurized system. The tank must be vented.... gas cannot get out if air cannot get in. Your vented gas cap apparently is not open or it is faulty.

 

[Rtychynski]

Re: Fuel Pressure problem in Johnson 90 hp Tank

Fuel line is 3/8 brand new . I also upped the pick up line size in tank to 3/8
It opperates fine at lower speeds, but constantly seems to pulsate the bulp ( I hold it in my hand to feel the response at different rates of throttle)
I have tried it with the cap completely removed (thought that it might be creating a vacuum in tank - but no change)

 

[jtexas]

Re: Fuel Pressure problem in Johnson 90 hp Tank

There's really only one possible explanation for collapsed bulb -- the pump is pumping but something is preventing fuel from entering the line.

Only three possibilities:
* an obstruction in the line or at the pickup tube.
* vacuum from inadequately vented tank.
* faulty bulb.

 

[kenmyfam]

Re: Fuel Pressure problem in Johnson 90 hp Tank

There's really only one possible explanation for collapsed bulb -- the pump is pumping but something is preventing fuel from entering the line.

Only three possibilities:
* an obstruction in the line or at the pickup tube.
* vacuum from inadequately vented tank.
* faulty bulb.

Pretty much sums it up !!!!
Have you tried it since with a portable (and vented) tank ????
That way you could possibly eliminate everything except the tank.

 

[ebry710]

Re: Fuel Pressure problem in Johnson 90 hp Tank

There's really only one possible explanation for collapsed bulb -- the pump is pumping but something is preventing fuel from entering the line.

Only three possibilities:
* an obstruction in the line or at the pickup tube.
* vacuum from inadequately vented tank.
* faulty bulb.

I agree. If the bulb is installed in right direction, then it sound like a restriction.

FYI 3/8" is a normal line and easily handles my Johnson 120. My system is:
1) tank
2) 3/8" hose
3) fuel/water separator
4) 3/8" hose
5) bulb
6) 3/8" hose
7) mini filter
8) 3/8" hose
9) VRO fitting
10) 3/8" hose
11) fuel pump

 

[kenmyfam]

Re: Fuel Pressure problem in Johnson 90 hp Tank

Oh Yes.....check the arrow on the bulb is pointing in the direction of your motor.