vented gas tanks

[Bob Hurst]

Picked a very clean 88 Johnson 9.9, runs great. My question is should the tank be vented? The guy I bought it from didn't know anything about the issue. My brief experience has been with everything from a 1970 merc 4hp to the 1982 20hp merc and a 88 nissan 5hp as well as a 2002 2 1/2hp nissan have had a vent screw you loosened before running the motor. One thing I did notice on the tank cap on the latest motor it looks like the cap has a cover over where the vent screw would be. Any reason why this tank wouldn't be vented? Thanks!

 

[tashasdaddy]

Re: vented gas tanks

most original OMC metal tanks, had the vent built into the cap, not like the mercs, with the thumb screw.

 

[Silvertip]

Re: vented gas tanks

Any tank must be vented or it would be impossible to draw fuel out of it. Ever try sucking on an empty soda bottle? The sides cave in right??

 

[MrBigStuff]

Re: vented gas tanks

Awhile back, I acquired an '86 6hp Yamaha along with the original fuel tank. No vent screw on the tank either. Neither the OEM owner's manual or the service manual speak to the tank and need for venting.

The tank can develop significant positive pressure as the gasoline heats up. Opening the cap produces a big whoosh of escaping air. This can occur even when the engine has been running for some time. However, it has never starved of fuel while operating so I think the cap is vented in one direction. It will never suck in air when the cap is loosened. The cap can rotate about 90 degrees and effectively open to atmosphere without coming off the tank but I never ran the engine with it in this position.

 

[jtexas]

Re: vented gas tanks

short answer: yes.
long answer: it has to have some sort of arrangement for replacing fuel with air, or else your primer bulb will get sucked flat, or the sides of the tank will cave in (plastic tank), until the fuel pump can no longer overcome the vacuum it's created and you starve for fuel. Maybe that cover you speak of isn't air-tight? it should be, though, so you can close it off if you need to, say, to contain fuel during transport, for example. Try it and see.....portable tanks are pretty economical nowadays.

 

[steelespike]

Re: vented gas tanks

I'm pretty sure the original OMC tanks have a vent that opens when you connect the fuel line.As I recall there is one or 2 pins on the tank connection
these open the vent when the line connector presses on them.

 

[jtexas]

Re: vented gas tanks

If you mean the old double-line pressure tanks, they were long gone by 1988. The 2 pins on the fittings now, one is a barb to hold it steady, the other is the fuel nipple.

 

[steelespike]

Re: vented gas tanks

If you mean the old double-line pressure tanks, they were long gone by 1988. The 2 pins on the fittings now, one is a barb to hold it steady, the other is the fuel nipple.

No I'm talking about the single line OMC factory tank.There is no cap vent.There is a pin on the tank valve that opens the vent.Clearly visable in illustration 2 on page 4-67 of the Seloc service manual for 1.5 to 40 hp.
56-70 evinrude/johnson motors.
They may have done away with these tanks sometime after 1970.Though
it seems to me my 88 Evinrude had one.