Topic: 75 cb400f gas tank vent

[cmorgan47]

question:  where is the vent on the tank vent on a 75 cb400f?  is it in the cap, or elsewhere.

reason for asking:  i've sorted out most of the issues with this bike and it's been running pretty well for a couple of weeks now.  however, after first adjusting the carbs, i filled it up and soon thereafter, it died--like it was out of gas--and i couldn't get it started again.  by chance, a friend pressed the gas cap, heard a sucking sound, and then was able to start it.  same thing has happened a couple of times since and seems to happen most right after i fill up--thereby leaving very little air space in the top of the tank.  wondering if i'm getting some sort of vacuum locking and i've read of similar issues here.  don't know where the vent is and i don't see it in the parts manual.

at any rate, thanks....nice forum.

[Lumbee]

...yep, pretty sure its in the cap...look under the cap and you should see a little hole in it.  Poke some wire or something in the cap to see if you can clear any obstruction out.  I ended up just taking a small drill bit and just drilling clear through the hole to just underneath the cap itself to be sure the gas is lented.  To check if its venting or not you could unhook the tubes going to the carbs, and turn the petcock on, and let the gas drain into a container.  Give it a few minutes and just see if the stream weakens any.  If it does, then open the cap and see if it starts again.  You could also remove the tank, and just shake it up a little, then open the gas cap, if it pops from the pressure, then you'll know its not venting...

[cmorgan47]

yeah, i found that hole after posting and figured that must be it.

good diagnostic advice, i'll try it.

thanks

[JHansen]

careful if you are gonna drill out that hole any bigger!

Nobody wants metal shavings in their tank!