After troubleshooting a sudden onset persistent bad misfire in the 1360, it looks like the head gasket is cracked between #3 and #4 cylinders.
I have already changed the entire ignition system 1 bit at a time, checked the fuel delivery, changed the fuel filter and also changed the fuel from V power to BP Ultimate with no joy.
#3 and #4 plugs (only) get black in 50 metres.. then it misfires.
I'll borrow a compression tester today to make sure, but it looks like it's off with its head.
Yes the gasket is a BK450, but was supposedly made in UK, not Belgium.
I can see a good s/h one going back on if this is the problem.
[edit]
Head gasket was fine. But I replaced it with a new one anyway
After doing all this with no result and speaking with GR, I discovered the cause of the problem:
Apparently when I gave it a big rev the other day, it had sucked the old `Finer Filter' foam pad against the RH stubstack of the Dellorto, = instant richness on #3 and #4 whenever it got on cam.
I have had this old filter on the 1412 for the last 6 months, as the Unifilter foam came out for cleaning. It's back on now.
Obviously the 1360 + RE282 cam sucks harder than the 1412 + RE13 cam did.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
