A minor and rare ignition idling problem...

[Bob]

Good tip, or the Bongo Shop sells locking ones which go straight on.

[cmm303]

Yep, if the vent is blocked a vacuum will build up in the tank and stop the pump delivering fuel.

This is most likely the 'Sucking' sound you hear when releasing the cap.

If the tank is fairly empty (so fuel can't slop out) it might be worth a short run with the cap removed.

Or slacken the cap when parked downhill and see if you no longer get the starting problem

[BongoBongo123]

Yep, if the vent is blocked a vacuum will build up in the tank and stop the pump delivering fuel.

This is most likely the 'Sucking' sound you hear when releasing the cap.

If the tank is fairly empty (so fuel can't slop out) it might be worth a short run with the cap removed.

Or slacken the cap when parked downhill and see if you no longer get the starting problem

The only thing observed so far is I started the Bongo with no issue at all. I filled up later in the day and heard some slight pressure sucking when I unscrewed the cap so slight pressure or suction is building for sure. I imagine it should not be.

I looked at the cap and could not really see how it comes apart, the is like a 3 blade centre hole with a extremely fine nylon mesh semi transparent over it... about 7mm wide that is the only mechanism by which I can see air passing.

I just had 10mins free so washed/soaked it in very hot tap water with lots of Fairy in for 5-10mins and shook it vigorously in the suds, then shook it a lot to get the water out and tissue dried. I will see how things go.

As you say I can maybe just not tighten it as hard it won't be going anywhere it is behind a locking flap.