Ace hoget and Dave
I agree with "medium warm E )Dave on this one.
Back to basics.
I think the clue here is it runs on 18 degrees , just, and then dies at 8. ???????? . Are you sure that cam timing is correct..????Backfires come back up the inlet runners when the spark fires with inlet valve still open.?????????? If you have checked the crank at 8 and fired off a spark to No. 1 with ignition on, then something else on the checklist is to blame.
Keep on diggin.
P.s Does your oil pressure vary much from summer to winter Dave when cold as i have noticed a winter start up pressure of 80psi. I have a 1000miles on it now. In summer it was 70. I run 10/40 with extra zinc as i dont have a roller block. I also run a Melling HV oil pump. Pressure drops by about 15 psi when hot.