Quote:
| Originally Posted by ells bit of a thicko question but when the petrol leaves the petrol tank how does the air get in to replace it?
Cant see that even if these two ball bearings in the cap mount were working where the air would come and go to easily!
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The ball-bearings are (should be!) held in by a spring and a worm screw. It operates as a valve, allowing air to flow in to the tank.
The ball bearing should be pressed up against a hole by a spring. It's all held in place by the worm screw. What happens is that the pressure inside the tank builds up, it sucks the ball bearing away from the hole, effectively opening the valve, and allows air inside. Once the spring overcomes the pressure (the reduced pressure) it pushes the ball bearing against the hole and seals it again.
The worm screw is used to hold the spring and bearing in place and also to set the preload tension of the spring. It may be that the spring is wound in too tight so the valve will not open correctly ie, the pressure difference cannot overcome the spring to move the ball bearing and open the valve.
If the fault lies with the fuel cap then you may find that adjusting the valve is all that is needed. Drilling it out is not a good idea, it just needs setting up properly.
As for how the air gets in - the fuel cap itself is not air tight, so air can get in through the cap, either via a dedicated air passage in built into the cap or through the thread of the cap itself, depending on the design.
HTH!