Topic: 70's gas tank vacuum or "no tank vent" question

[immortal]

Perhaps a silly question...but if anyone can put my mind to rest it would be greatly appreciated. My CB750 Cafe Racer has a "I believe" a 1974 gas tank that has no vents.  While building my project I used a speed boat or racing type quick release gas cap see in the picture below.

My mind gets to thinking...a dangerous thing....The cap and for that matter the previous one does/did not appear to have a vent of any kind. Would this not cause a vacuum that would affect the fuel flow? The tank has no vents.

Is it too insignificant to affect the flow? Does anyone know?  Thanks for any info on this!

[TwoTired]

Tank requires a vent.  The one shown is NOT stock for any SOHC4. (It IS pretty though.  )
  The original one does have a vent in the cap.

Cheers,

[immortal]

Thanks for the reply. Although I had the tank painted it was from a CB750 early 1970's. I striped off the blue paint and black/gold decals, removed the emblems and filled the emblem indents and replaced the cap.

How did these early tanks breath?...through the cap?...it looked air tight like this one....any one know? or any ideas on how to get this tank to breath after I invested so much time and money into it?

[immortal]

Here is the tank when I was working on it...am I having a brain fart over this?

[ekpent]

There is a small hole in the bottom of the stock cap and propably breathes past the seal with a simple little baffle system so as not to leak fuel.If yours is totally sealed that could be problematic. A vent tube like on some older dirt bikes would not look to swell,hopefully somebody here has tackled that issue and will report.

[immortal]

Thanks for that, I'm getting it now. I simply did not think of it when I was working on it and since I have cut off the cap ring and spent $85.00 on this gas cap...there is no turning back.

Looking under the new cap I see a spring loaded plate with a large flat "O ring", the top of the gas cap is clam shell over the top. When you close the latch the spring looks to have a tight seal over the tank. My thinking is to test the tank with a couple of gallons of gas back into the gas can. If the flow stops...I presume it will then I will drill a couple of tiny holes through the flat "O ring" and plate under the cap and test again.

Any other thoughts greatly appreciated.

[ekpent]

I think your test idea is a good one as it may have a built in vent of some type. I have read on here before of what a good flow rate for the petcock should be such as gpm etc but do not recall that info. Maybe Hondaman or someone will chime in on that one or may find it with a search.

[wildcatmahone]

That's the stainless marine gas cap that I will be running you definitely need a vent for proper fuel flow. I ended up emailing a supplier of that cap and got a response saying it requires an auxiliary vent. Here's how I plan to do it. Install #10-32 rivet nuts on my tank and fit the cap with some ss countersunk screws. To get around having to run a tube or such I'll drill a hole through the length of one of those screws and use some gasket material between cap and tank. Let me know how it works out.

-WILDCAT

[immortal]

Thanks Wildcat, 

I will keep that info on hand. First I am going to see how the flow is affected by timing/measuring the vented and unvented flow ...being the cap open. I know its not very scientific as the fuel consumption is way less than the gravity flow, so perhaps I will open the pet cock partial way and time and monitor the difference.
Assuming the difference will be major.....then I think I will drill a small hole through the plate and flat washer under the cap and then shake test the tank to see how partial it is to fuel spill.  Hopefully it works well then I can test the fuel flow.

My thinking is the fuel flow is so slow that you would only need a tiny vent.....does anyone have a picture of,or can tell me how big the air baffle is on those older caps so I can recreate?

If that fails I will move on to your idea WILDCAT...I have stainless screws but drilling out one of those screws is definitely an option.....I will do this test first and keep you updated

[immortal]

OK, So today I filled the tank to the brim with gas and clamped the quick release cap shut and switched to reserve and ran a clear fuel hose to gas cans.  The flow started great.....and it stayed great all the way to empty.....I thought I must of missed something so I ran it through again and monitored it all the way.

I even tried shaking the tank running it and listening for air suction but nothing.  My thinking is these caps must leak enough air or the underside of mine is leaking enough air though the screws that it allows enough air in to replace the gas pouring out for the tank to work.

Wildcat, before you go to work on drilling out screws I would test it first. There may be a factor riding that is different but I must say from what I did today I'm thinking it will be fine....

Just a note, I did use RTV silicone under the cap maybe there is a gap there or maybe its the O ring or screws...or a combination there of..but it works!

[Rookster]

Those caps do vent.  The rubber o-ring seals the outer lip but the rod running through the center with the spring does not seal and acts as the vent.  My Carpy Dunstall tank has the same cap and their are no fuel flow issues.
Scott

[immortal]

Hey Rooster, thanks for the reply that makes perfect sense and I now have peace of mind.

Thanks again!...hopefully this little topic will help someone else out too!

[wildcatmahone]

Great that's good to know your getting flow. Hmmm I recall toying with that cap and pressing the seal down when I first got it. I could of swore it vented through the middle as suggested but heeded the advice of the supplier. I think some sort of semi-formable gasket would be ideal for this cap, gonna see what Mcmaster has. Thx for the info!