Jaguar Repair: xj6, fuel pressure regulator, propane torch

Question
I Have a 1983 xj6.I put another engine in it and now the spark plugs are white/Not getting hot so maybe too LEAN.It is also idling erratically and when I give it throttle is tries to die unless i roll on the throttle slowly.If i come to a stop it will die unless i stay on the throtle and keep the rpms up.
I have a new fuel pump ,new filter ,timing is correct, fuel pressure is 32-38 even when it is dying,40 PSI with vacuum removed from fuel pressure regulator.
I believe It is sucking air on the intake side. Sound logical? What are your thoughts, Thanks SID in Az

Answer
Hi Sid,
That is possible but a hundred other things are also possible. Try spraying a little propane from a propane torch into the intake to see if you can get the RPM to smooth out. Also have some one hold the RPM as low as it will run and check the out put of the O2 sensor. (.5v is normal and below that is lean and over that is rich.)
An intake leak can make it run lean but so can a bad coolant temp sensor or a bad airflow meter or the mixture adjusted wrong. Fuel pressure is correct so fuel supply is not the problem. How old is the fuel? Check manifold pressure and if it is unusually low check exhaust back pressure (must not be more than 1.5 PSI on a rev up.) If it will pull up a hill good you can forget testing exhaust back pressure. If it seems to be on some cylinders and not others it could be clogged injectors or bad injectors.
You should start from scratch and do a compression test and set the ignition timing etc.
Howard