1100 EFE Tank creating it's own vacuum

[waterwagen]

When the tank is full she's rides sweet as a nut not a problem. When the needle hits half or the is more air in the tank than fuel she coughs and splutters like the carbs aren't receiving enough fuel. I've fitted a pingel on off tap recently as the vacuum had gone in the tap. 
I've figured it's the tank pressuring as as soon as I release the petrol cap there a big gust and she'll run perfect til the pressure builds again. ONLY at half tank. Full tank am guessing there's not enough air space to cause a problem.
I've stripped the petrol cap and cleaned everything as far as am aware the tank breaths through the key barrel? 
Has anyone had this problem? Solutions? Thanks in advance and sorry about the around the house explanation. 

[gpz1100convert]

clean the tank cap through. should sort it.

Had the same issue here with an aftermarket cap and ended up fitting a breather and a rollover valve in the end mind. Having said that the turbo can gulp down the fuel on boost.

Mark

[waterwagen]

Stripped the cap and cleaned everything. Problems still there.

[arnout]

Jun 17, 2015 22:48:35 GMT waterwagen said:

Stripped the cap and cleaned everything. Problems still there.

Hmm.. I'm not completely sure but I think the EFE fuel cap doesn't vent via the lock but vents to the space in the fuel cap seat onto which an internal tube is connected.. Underneath the tank a rubber vant/drain hose connects up to this tube.. Perhaps this route (either inside the tank or the hose) is blocked.. Peeking into the tank opening you'll spot the small hole in the fuel cap seat that -I think- is hooked up to the internal tube..