Author Topic: Vented petrol caps  (Read 149 times)

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Drew

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Vented petrol caps
« on: 22 April, 2022, 08:43:47 »
Hi all.
My BSA is living in a large shed. After only a few hours the smell of petrol in there is very strong despite there not being a leak as the tank is currently off the bike and no drips. So, the petrol smell must be coming from the petrol cap and I don't recall this being a problem in the past. Can anyone shed some light on how the vented caps are supposed to work? Are they designed to only let air in (to replace the petrol being used) or are they supposed to allow vapour out in the event of pressure building up in the tank, or both?

Is it time to replace?

Thanks in advance,
Ian
Things to do:
1. Learn to start a B31        *DONE*
2. Get a motorcycle license  *DONE*
2b. Rebuild carb & dyanmo
2c. Fit indicators
3. Hit the open road...

JulianS

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Re: Vented petrol caps
« Reply #1 on: 22 April, 2022, 08:56:02 »
You may see fuel residue, a small brown stain around the vent. The vent works both ways, the fuel may expand a bit in hot weather and pressure will increase with frothing when in bike in motion so the vent relieves this to.

My A10 has a slight petrol smell despite having no leaks and I think it is more noticable since using E10 fuel.

spaceman

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Re: Vented petrol caps
« Reply #2 on: 22 April, 2022, 09:55:39 »
The petrol tank vent is basically there to let air in as the petrol level drops. However, inside the tank, you have a petrol saturated vapour pressure of about 10 psi and, because of the vent hole, this vapour will inevitably escape into the atmosphere. I would be surprised if the vent hole itself led to much of a smell of petrol but, if the petrol cap is not sealing properly because the seal has degraded, this could lead to a larger amount of petrol vapour venting. Therefore, worth checking the state of the seal.


Drew

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Re: Vented petrol caps
« Reply #3 on: 22 April, 2022, 10:59:03 »
Cheers all, I'll check the seal.

Ian
Things to do:
1. Learn to start a B31        *DONE*
2. Get a motorcycle license  *DONE*
2b. Rebuild carb & dyanmo
2c. Fit indicators
3. Hit the open road...

Drew

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Re: Vented petrol caps
« Reply #4 on: 22 April, 2022, 17:01:39 »
Gasket checked.
No gasket.
Problem solved!

Cheers all.
Ian
Things to do:
1. Learn to start a B31        *DONE*
2. Get a motorcycle license  *DONE*
2b. Rebuild carb & dyanmo
2c. Fit indicators
3. Hit the open road...