Bogging down - 87 Cabriolet

Hi everyone, I'm really in need of some help! I've got a 1987 Cabriolet 1.8 GTI which does run but when accelerated past a certain point it bogs down and will not rev. It sounds like its not running on all 4 cylinders could this be down to timing? Any common problems? Please help! 😔

A bit more info on what you have done so far may help with an answer…
For starters how old is the spark plugs, HT leads, rotor arm, dissy cap?
Which fuel are you using?
Has it just started to happen?
Any other noises problems before this started to happen?
Fuel filler neck ok, any rust holes?
Does the main pump make more noise when running as it could be the lift pump in the fuel tank not working so the car is not getting enough fuel.

There's lots of questions to get you started.  

Helllooooo sorry it's been a while… we found the problem! The fuel filler neck had decided to rust away and rust and water was falling into the tank! Thanks for your reply!

Glad it's sorted, did you change the fuel tank as well?
If not have you left a magnet in the fuel tank to catch any rusty bits left floating in there, works well and is easy to do, just don't drop the magnet near the fuel level float in case it sticks to the magnet!