1983 mk1 golf not running or starting properly please help

1983 mk1 golf gti injection problems… iv bought myself a 1983 mk1 golf as a non runner. i have managed to get it ticking over but to start it i have to hold the diaphragm up about 5mm. once it is ticking over you can rev it up to 3500rpm but once it needs more air it makes a weird noise… iv found a video on youtube which is the same problem as mine… Golf mk1 1.8 KR Problem - YouTube

why do i have to hold the flap open to start it? is this a problem with the vacume? i have look for air leeks and cannot find any?

i have replaced the fuel filler neck, fuel filter, injector seals, metering head gasket, inlet manifold gasket

im running out of ideas.

please someone help

Could be a split in the air hose from the metering head to throttle body, but could also be a fuel pressure problem.