1978 OMC metal gas tank; need to open cap to vent?

[MadtownChris]

Sort of a newbie outboard owner here. I know they say there's no such thing as a dumb question but this seems like one.

I have the 1978 OMC gas cans (pictured) for my 85hp '78 evinrude. As far as I can see there is no vent for air to enter the can as gas is withdrawn.

Until I forgot today, I've been unscrewing the cap a half turn or so - the cap seems to have a setting that's made for this - it can be fully closed, the half turn seems to stick in a spot, or you can unscrew it all the way.

Today with the cap screwed in, the first 15 minutes were normal full throttle. Stopped for a while and the second 15 minutes, full throttle barely got on plane and RPMs not up to par (was also dragging the swim ladder like a dumb ass). In idle it died a couple times.

Only now did I realize I forgot to half-open the cap. I unscrewed it just now and it sounded like there was a small vacuum in there.

So my question is, am I supposed to vent the gas cap manually for these tanks?

Thanks!

- Chris

 

[Stumpalump]

I'd like to see the other side of the cap.

 

[Sea Rider]

What does the owner's manual says about gas tank ? Cap definitely doesn't seem to count with an air vent, a pitty it's not a 360 screw cap in which you can play with different cap heights. On that old gas tank seems it's the way to go.

Happy Boating

 

[MadtownChris]

No Title

If only I had the owner's manual. Here's the top of the cap. The small round center is not a vent. It's just sort of a rivet holding the other side on.

 

[MadtownChris]

No Title

If only I had the owner's manual. Here's the top of the cap. The small round center is not a vent. It's just sort of a rivet holding the other side on

Hold on a sec I was just tinkering after reading some other posts. There are two pins in the connector. On one of my tanks when I press one pin in, tank pressure forces gas out. When I pressed the other, a small hiss indicated air escaping and then no more gas with the other pin (since pressure released). The air pin makes a slight noise as though activating a valve.

On the pictured tank, same pins. Gas pin shoots gas out. Air pin seems not to work. No hiss and the pressure is not released (gas pin still shoots gas). I'm thinking the air valve in the connector is gummed up or broken...

- Chris

 

[Sea Rider]

Had same OMC plastic 6 gal tank with same issue. On tank's valve you have 2 pins, left was solid fixed where fuel line remained clip connected to, the other the fuel pin, but remenbered quite well that if my tank was left with built up pressure, fuel will be sprayed out when fuel line was connected. Was a real mess, ended releasing internal tank pressure previous connecting fuel line to avoid sprays. Those tanks are supposed to have an internal vent somewhere ?

Happy Boating

 

[Chinewalker]

Those are the best tanks going - FAR better than any of the plastic junk being offered today. And for less than $15 and about 20 minutes time you can rebuild a leaky top fitting, too! You need two #303405 core plugs and two #322598 o-rings. Pop the core plugs out of the back of the tank-top fitting. Slide the pins and springs out. Using a dentist pick, gently pull the old o-rings out, taking care not to scratch the inner housing. Slide new o-rings in place, seating them firmly into their seat in the fore of the fitting. Wooden dowel works well here. Slide pins and springs back in. Install new core plugs with a flat punch. And you're done! If you think yours might be plugged, you can remove the fitting from the tank and disassemble it further, blowing out passages with carb cleaner and compressed air. You'd then have to either make or invest in a gasket for the fitting-to-tank area.

These tanks rarely have the water-in-the-gas issues of the cap-vented tanks as they don't "breathe" like vented tanks do. They also don't leak when properly maintained. They don't blow up in the sun, either.

Here's a tutorial at Odd Job Motor with photos...
http://www.oddjobmotors.com/tt4_singlelinetanks.htm

 

[pecheux]

I have one of those gas tank (1981) still nice inside and out and cap is always fully closed when in use. I beleve they are called self venting tanks.