noisey missfire?! UPDATED, Problem solved!

[Marty8v]

Just got a very nice bright red 1991 8 valver but its just not running right.
it starts eventually and seems to be running on 3 cylinders. BUT when revved it makeds a horrendous tapping sound and then cuts out when the throttle is taken off.
sounds expensive to me , does anyone have any ideas?
Many thanks

Had a mate round who is a mechanic and almost straight away established that the head gasket has gone between the no 1 cylinder and the next one along! Common problem caused when incorrect fuel is used , someone not been using super unleaded 98 octane, just plain old premium 95 octane. all the info was in Golf + magazine the other month and described exactly what has happened to my car, getting it fixed today though 

[Organisys]

Check Timing first.

[rubjonny]

you can safely use 95ron fuel in a 91 GTI, because they have a knock sensor which allows the ECU to retard the ignition timing when the engine starts to knock.  You'll just loose performance!

Check out my digifant tuning guide in the maintenance section, check the timing as organisys said, and also look at the idle switch to make sure it clicks when the throttle is closed.  Also make sure the knock sensor is present and connected, its bolted to the front of the block.

If it looks a bit dodgy, this may be the real reason the engine has blown its head gasket if that turns out to be blown!

[Idle Duffer]

Sounds exactly like head gasket has gone between two cylinders.  Causes horrendous pinking ( Pre-ignition )..  Usually between  cylinders 3 & 4