i need of some assistance. I have transplanted a ef xr8 motor into a 67 mustang. After laying around for just over 1 year i finally reached the day when I could actually start the engine. First time I did the thing ran like an absolute pig..would idle etc etc.. loaded in some injector cleaner and the thing can now at least idle. The funny thing about it was if i kept on pumping the accelerator the engine would rev up nicely as if it wasn't missing, although the revs wouldn't go all that high.

Just today I decided to have another crack at it. The engine managed to idle enough for me to hop out of the car and stand next to the engine bay to rev the engine. Now I don't know if I bumped something but all of a sudden the car ran like an absolute dream..idled without missing a beat and revved nice and hard. I turned the engine off and re started it and the little f**ker went back to how it was. Could this be as simple as cleaning the ISC? For the life of me can't figure out what I bumped but it was in the area of the ISC.

Any help would be greatly appreciated.

I also have a question about a vacuum line. It runs next to the fuel lines at the front of the engine where the upper radiator pipe is. Seems to disappear under the upper intake manifold. I can take a piccy if it would help.

I think we need more details there Dazza but, the line you refer to sounds like the Canister Purge line. This goes to your CANP solenoid on the charcoal canister. The CANP solenoid controls the ingestion by the engine of the excess tank vapours.
ISC is unlikely to stop an engine from revving but will certainly bugger up the idle if it doesn't work. Have you done a self test? You can also do a wiggle test where, in self test mode you wiggle the connectors and wiring harness and the EEC will log a fault code for a dodgy connection if there is one. Check also that your TPS is working ie; changing voltage as you open the throttle. Also check you don't have snapped t-body shaft....you could be moving the throttle linkage without the blade opening.
Hows your fuel pressure?

Pete.

sounds like your maf is playing up, as for the vac line i was gonna say it could be the egr line but come to think of it i dont think ef's had egr????

are you running the EGR? if so are you running the EGR regulator? i disconnected mine as i was told it was part of the cruise control and hooked the vacuum line straight to the EGR valve and it idled very very lumpy so i took off the vacuum line to the EGR and it idled fine....so for now i just blocked it off untill i fit the EGR regulator again....
have you got a pic of the engine in the car?