Monza filler leak

[chrisk]

I have this leak in my Monza fuel cap, that I thought was coming from the fitting edge of the filler.

With tank empty, I closed petcocks and cap, and blew compressed air up the venting tube.
There was more air exiting the filler cap proper than I had expected.
On closer examination I note a (about 3mm dia) venting hole on the spring loaded sealing part of the cap proper.

Has anyone else found this problem on the Monza caps? I googled the problem, and  Triumph and Norton forums were talking about a similar leak on Monza caps, but nobody found a fix.

As I have a separate vent outlet, I'm contemplating sealing up the hole.

 

[piranha-bro2]

If you've got an adequate venting system elsewhere, I'd definitely seal up the pesky hole.

BTW, what exactly is a Monza filler?

 

[chrisk]

Here you can see the pesky hole: https://www.ebay.ie/itm/Monza-Filler-Cap-2-inch-with-alloy-adaptor-/371982736107

 

[jbr120]

Had a similar problem on a 1200 tank I converted to take a Monza cap so I made a tube that fitted inside the bore and extended down to about 10mm from the bottom of the tank. This stopped fuel surge hitting the underside of the cap and stopped the annoying dribble but did make the tank slow to fill. Never did stop the fuel escaping after filling up and the engine was warm. 

 

[Vince]

I had to add a hole to my Ducati flush fit large O ring sealed tank cap, if you fill the tank to where you cover the actual breather pipe over with fuel mine would self syphon out the breather hose. Even with a one way valve this still happened when the tank was filled right up. The final fix was adding a tiny hole on the very well sealed cap to break the vacuum. I tapped this hole and screwed in a Delorado main jet, 150.