Topic: Things that make you go Hmmmmm......

[jsaab2748]

If a gas tank vent is blocked completely or even partially, it can cause a vacuum in the tank and can affect fuel flow to the carbs, right? Couldn't it also cause pressure to build in the tank due to hot weather conditions/fuel slosh? And couldn't this latter condition cause unnecessary pressure on the carb needle/seat assys. (too much fuel right?), during parked OR while riding? I'm chasing a ghost with my 750 k5.  I have noticed better performance with the gas cap unlatched. Accelleration seems stronger and cleaner, and idle seems to be smoother and more consistent. Cruise seems to be stronger as well,  I don't have to "prod" the engine to run at highway traffic speeds anymore. I know my bikes gas cap vent does in fact breath, but suspect it may be partially clogged as I can hear it hiss sometimes when the tank is sloshed, and the driveability "ghost" comes and goes but it isn't severe.  Does anyone know how to disassemble the gas cap to check the vent path? The spring loaded underside of the cap looks to be rivitied in place and unscrewing it was impossible. Sorry for the rant, just want to cover all the bases to get opinions. Sometimes I amaze myself with the uncanny amounts of time I waste thinking about this kind of stuff.. thanks jim

[douglascoolgrey]

I don't know about the 750s, but my 550 has a breather seperate from the gas cap itself. This is 1/8" sized hole that sits behind the gas cap. I've also got an overflow tube that leaves the tank just above where the tank sits on the frame.

Yes, you can create a slight vacuum in your tank. Yes, this would create the symptoms you've described. Yes, a partially clogged breather could still cause these symptoms. Do you hear a swoosh sound when you open your gas tank to fill it after riding for a while?

[jsaab2748]

No noise when the cap is opened that I can hear, just the long hiss when the tank is moved a bit with the cap on and latched. Just thinking that if pressure inside the tank doesn't equal atmospheric even for just a short time, it could cause problems. Not sure if the gas cap can be disassembled.

[Terry in Australia]

I had a similar problem with my BMW K100RT 15 years or so ago, it had a fuel filter inside the fuel tank that hung off the fuel pump, anyway, the filter was blocked and the only way I could keep the bike going on my way back from a long trip was to release the fuel cap to relieve the pressure in the tank, which would allow the bike to be ridden for a mile or so before it started to die again. I've never experienced this with a CB750 before, but if you can hear it hissing, I guess that may well be the cause. Cheers, Terry.