Fuel pump pipe size

when I re shelled my cabriolet had right carry on finding fuel tank to fit
basically I did proper bodge job.  Lol
could only get carb fuel tank I think ? did not have the pipe on bottom tank for electric pump gti model
so I just wired in tank pump up to high pressure pump ran ok 👌 like that
so was always in my mind that in tank packs in I'm stuck
so ive took it out replaced pump with pipe with the filter on end from the pump and removed the swirl chamber as well since pump would not suck from tank
so fitted new high pressure pump starts idles perfect
but can't drive it got fuel starvion dies and cuts out when give it big licks got drive just touching throttle
only thing can think of pipe in to the high pressure pump I fitted 8mm pipe as was size on tank sender unit
took bit getting on with plenty hot water     Do you think this be my problem not getting volume of fuel as don't want go back running 2 pumps 

Or I was thinking be fuel accumulator ?
But think it's good as holds pressure can turn car off.30 sec later still got pressure on system it never being changed so be 33 yr old.   But don't want throw money away just changing parts no reason

I suppose it could be that you are sucking the hose closed under throttle, and that at idle it is fine.

Or that it is too long, and sucking the bottom of the tank.

So did you use a hose from the sender down to the bottom of the tank, or a pipe.  

If you used a pipe, then how far is it from mating with the sender, and are there clamps on it.  

Before I removed the fuel pump I had marked out drawing round sender unit so know unit length was right so did not runout fuel. Used filter from bottom pump so know was right

What size fuel tank do you have and what year is the car smaller tank with full size spare or larger fuel tank and a space saver wheel?