K Series Poor Running

[The_Burg]

MGF 1996 vintage. Replaced the throttle body with a 52mm off a Trophy, (crashed by the wife).
Huge increase in performance, went brilliantly, for a couple of days.
Started missing a bit and rough running.
Tail pipe looked a bit black so assumed running rich. Fitted new plugs, filter and appears it was actually running weak.
Checked all connectors and fasteners, no issue spotted.
Trouble starting now and runs terrible. Did the throttle pot reset, even worse. Ticked over at 3k for a bit and settled to around 1500.

Any ideas? My guess is throttle pot. With no ODB difficult to check. Seems perfect from stone cold, so possibly temp sensor?

Which one though? How can you diagnose with no port?

Help!

[paintman]

Tried refitting the old one?

[George111]

Got an air leak ? Idle it with a blown out candle near the joints and see if the smoke is blown around.

[The_Burg]

Old one is gone, refitted to the dead car so it ran.

I believe the the TF has a 52mm body so will have trip to the scrapyard i think.

[The_Burg]

Seems to have started running better again.
Nothing at local scrappy, so no alternative.

[Mr2Mike]

The symptoms do suggest a vacuum leak, either a split/broken pipe or one that isn't attached to where it should be.