The MK3 Is Fuel injection as standard. Although it is only single point
There is no such thing as a GTI Carb, ALL Golf GTI's are Fuel injection but they are Multi point injection
There is no way of fitting a GTI injection system to a 1.4L engine. I think hes been fooled! and if its running bad, there is a problem with the injeciton system.
These have one of two engines.
ABD Single point engine (Pre 95 models)
AEX Multi point engne (Post 95 models)
the AEX has an air filter which makes many people thing it is carb'd if you dont know what your looking at and some people will fit bike carbs or twin choke webbers to get more power but you would know if that was the case!