I need some advice for setup i`m preparing!
The "ingredients" are a gx head with no double valve springs, some flat top pistons, SSS hitachi 1 1/2" carbs and standard 1200 camshaft.
Please tell me what your opinion and experience.

Someone??

i would get a better cam shaft at least, you wont need double valve springs with a stock cam

just noticed you individual su manifold set up, nice...

I got a lot of work still! My problem is that the carbs are too much for this head! GX carbs are 1 1/4 inch and this ones are 1 1/2 inch!

yeah will over fuel on stock cam easy, also will run way too rich, washing the bores almost. At least put new needles, conehead has a link some where for them

It might actually run to lean. Remember, carbs rely on a pressure drop to pull fuel from the float bowl, the larger the carb the less the pressure drop, and the less fuel might be drawn into the engine. You might even get better fuel consumption if driven sedately with these...

Should i keep them then or should i should i put it some 1 1/4 " SU`s?

You should trade for the 38mm Hitachis. They will work very nicely between 4,000-6,000 RPM with the stock camshaft.

big stick dual springs good port job
dazza

Forget the 'big stick' & go with a cam profile more suited to the use that the engine will see. SU's & Hitachi's don't like the low vacuum that 'big sticks' provide & you won't like the bad low end performance either.

Dual valve springs were standard on the GX head so singles are a step backward.

With twin SU/Hitachi's, porting a GX head is not required as they have the largest stock A series ports anyway.
For an all out race engine perhaps, but for street, save your money on the port job & buy the springs instead.