Hissing petrol cap

Hello one and all

I need your expertise desperately!!

MY wife’s bike (Bros 400) has a hissing tank. Every time you open the petrol cap, it sucks in air which would suggest there is a blockage somewhere.

I have checked the breather that runs from the top of the fuel tank and where is exits underneath the tank. All is clear.

I have checked the breather in the petrol cap itself by blowing an compressed air through it, and again, all is clear.

It is being a pain, because dspite the fuel being fed to the carbs via a fuel pump, after a while on the road, it will die if she stops and lets it run on tickover. Then it takes ages to fire up again. Fuel cap and pump are identical in looks to a 97 CBR6

I can’t find anything on the Honda Service Manual, Haynes manual or the Bros/Hawk forum. Have asked them the question, but for some reason no reply. Which is very unusual as they have been known to reply within minutes. (Different time zone as they are very popular in America).

Does anyone have any ideas as to the cause. The cap is idientical to my old CBR6, and my current VFR. I used to have a 650 Bros and when I swapped the cap over, the problem diappeared instantly so it is definitely the fuel cap.

I would preffer to fix itrather than buy a pattern cap, as this then means you need 2 keys to run the bugger. 0

Can they be stripped down and repaired, and if so, how?

Bit of a cheek asking you guys but I am well and truly stuffed! And the wife is giving me grief too!

i dont think its anything to worry about, i think its cos its warm out etc heating tank up, if its hissing then its a good thing.

my old badit use to do same etc.

Does it suck hard enough that the fuel cap does not pop open like it should ? What your describing is a symptom of blocked breather hose . Did you with the cap open …get your lips around the (down by your feet in the crud and crap) end of the breather pipe and blow back up through the pipe to the tank ? This is the absolute definative test device is yer own mouth … You may want to wipe the end a bit if your not into the taste of road crap ))