1986 XJ6 Sov motor into a 78 XJ6 ser2. It's still virtually the same motor except the donor has fuel injection. How hard should it be to do this swap? What are the traps & pitfalls?
Yes it may seem a strange swap but the ser2 is in great condition (except the motor) & the 86 Sovereign is good for (some) parts- inc the motor.

 

AFAIK, you could use the manifold and carbs from the old engine, but might need to set the carbs up with different needles and jets. The Series 3 head has larger inlet valves.

If it was me, I'd at least have the head off the 86 engine to take a look. It should be a slotted block, and this prevents cracking between the bores, but HGF is still possible at around 80k miles.

 

Fairly straightforward swap, but you will need a return fuel line. this means replacing your tank with a series 3, or get the return line connected to your current fuel tank.

If you have a complete donor car, just take the tank from it, including the fuel lines from the engine bay to the tank. And keep the donor untill you finished, just in case you might need something from it.

 

It's already a dead SerIII motor in the car, It had a very dodgy head job!!! done to it with many washers (lots) on some of the head bolts- I tried to fix it but seems either I shouldn't touch heads (was a newbie learner-failed head overhaul (twice)) or the engine needs piston rings etc- (can't do the head properly I'm not going to attempt a total rebuild). Main reason I was suspicious about this swap though was the old dead serIII motor had been stripped of the fuel injection too. Looking at the horrible job he did on the head though maybe he had even less idea than me- was certainly rough.