question about "thermal valve" on rear carb of U20 engine with SU carbs

[TucsonJim]

I have a 1967 1/2 Datsun 1600 that has been fitted with a '68 or '69 U20 motor with SU carbs. The rear carb has an assembly that has a fuel inlet and a check valve operated by a bimetal strip. Looks like it opens when heated, porting fuel out the rear fitting of the assembly. Is that a correct assumption? That rear fitting was capped (very unprofessionally), rendering the assembly useless. What is the function of this assembly and to what should the rear fitting be attached?
As always, thanks for your help.
Jim

[spl310]

It's a return valve, but not heat activated. It is just to recirculate the fuel on late cars - it was an idea to reduce the chance of vapor lock. The 67.5 did not have the return line. The original carbs had just a fuel barb on the carbs rather than the banjo fittings.

[keith0alan]

It is heat activated. When cold it bleeds a tiny amount back to the tank through the return line. When the fuel gets hot is opens up allowing the hot fuel to be returned to the tank. As the hot fuel in the line is replaced by cooler fuel from the tank it closes again. This should reduce vapor lock. There should be a return line to the tank on the firewall. The supply line is 1/4' and the return is 5/16".

[TucsonJim]

It is heat activated. I know because I heated it and it bent back, allowing the valve to open. Since my 1967 1/2 gas tank does not have a fitting for a return line, I guess I’ll cap it off (with something better than a piece of hose with a bolt stuck in it).
Thanks for the responses.
Jim

[redroadster]

Also helps on carb icing... the throttle plate can become iced in place in cold and humid / clammy cond.
Like a airplane has the carb heat for landing