Engine won’t start after engine swap

Thanks that’s really useful
I agree that immobiliser is unlikely as there was no problem prior to the swap
I was wondering about the crank position sensor on the new engine as a possible cause.
Would a failed sensor cause similar symptoms?

Morning - I had a similar issue after an engine swap (though it was just another M13A) - turned out all the jiggling of the wiring harness caused some of the older connectors that were corroded in place to straight up break off with the stress of moving about.

I had intermittent dropouts of RPM, and the immobilizer was quite unhappy at times, sometimes randomly kicking in while driving, especially over rough ground.

These connectors were under the fuse box, but I would go through and systemically check all your connections, just in case, the electricals can get a little worse for wear with time.

Image of connector (I cut the red and white one which IIRC was just a headlight, the other two were corroded off with an intermittent connection):

it is a fuelling issue.
I sprayed carb cleaner down the intake and it kept running fine.
I've used the Swift injectors rather than the original Jimmy ones...could that be the problem??

Tom

Stop trying to guess your way through this, systematic diagnosis is the way forward - get a gauge and confirm the fuel pressure is within spec.

Failing that, use a noid light and see if the injector drive signal is present beyond that first two or three seconds.

Sounds suspiciously like a a grounding fault. The fact the starter motor was being feeble. I would check the engine earth strap as the fuel pump might not be grounding properly either. 

Sure, check the grounding, but, since the fuel pump is in the tank and doesn't use the "engine earth strap", I'm fairly certain that's not the problem.