Unlikely timing else it'd be doing before...!
Weak mixture > hot exhaust valve edge, burnt valve, hot s-plug (damaged)..
Hole in exhaust, hole in inlet, air-leak ? weak mixture... usually - hence hesitation, esp. straight after choke comes off.
So, backfiring through inlet or exhaust? Very different things?
Backfiring & Air Cooled Specialists round Stockport
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1.9TD Syncro Doka / Syncro Kastenwagen / 16" Kombi Camper
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Thanks... the answer
It turned out it was simply a loose petrol line to a carb... so back on the road and for only a couple of pounds!
Yippee!!
Yippee!!
It turned out it was simply a loose petrol line to a carb...
But good to get that sorted before worse happened...

The 80-90 Tech Wikipedia Your 1st port of call

1.9TD Syncro Doka / Syncro Kastenwagen / 16" Kombi Camper
Syncronaut No. 1