Bike Fuel Pump - Tank Connection

Having some issues trying to work this out - Fuel pump set-up for bike carbs on a GTI, Using a bike pump

i've got my head round most of it, main issue I'm having is connecting it to the tank itself, as the outlet is 15mm.

Tank Outlet - 15mm
Pump In/Outlet - 10mm
Existing lines - 8mm (?)

How the hell do I go from 15 to 10, can't seem to find anything

They make adapters, that change the sizes of hoses, they are called reducers.  

ie:
5/16" x 1/4" Stainless Steel Barbed Reducing Coupling | U.S. Plastic Corp.

Erm,

The in tank pump on the GTI set up might be a problem as usually bike carbs run at very low pressure.

But assuming thay isn't a problem.

Where are you mounting your pump and which one are you using? I only ask as I've been through the ebay £20 specials and it failed. I also went with a Facet 'small block' pump which also seems to be having trouble and have now bought a Suzuki 1100 original bike pump to try.

Either use reducers

https://www.ebay.co.uk/itm/Metal-Hose-Joiner-Barbed-Steel-Connector-Reducer-Fitting-Air-Fuel-Water-Oil-Pipe/121691704476?hash=item1c55640c9c%3Am%3AmDELar2qZsJzhLwxxjumPGA&var=420694380568

https://www.ebay.co.uk/itm/Brass-Hosetail-Reducing-Connectors-Pipe-Repair-connectors-Reducing-Hose-tails/121368740932?hash=item1c42240444%3Am%3AmCVPUFrcg9lK2KZifJn5zFg&var=420311591046

Or alternatively you can use re-useable fuel filters

https://www.ebay.co.uk/itm/Inline-Low-pressure-petrol-fuel-filter-classic-car-6mm-8mm-10mm-1-4-5-16-3-8/121225309652?hash=item1c39976dd4:g:534AAOSwWiBaz1zo

By buying a couple you can reduce as you wish but not have lots of reducers around and filter the fuel too!

I'm surprised the tankl outlet is that big as I would have expected it to be the same as the fuel lines.

I only know non GTI tanks and lines and they all have 6mm lines and outlets from the tank so I can't help there I'm afraid

Ian