Backfiring from one cylinder

[Merridew]

Hi
My 1975 860 GTS is backfiring from the front cylinder particularly when I am throttling off.
And occasionally if at a constant speed. The exhaust as a result is getting very hot including the rear muffler.
It does not seem to be oil as it’s not smoking.
It running with foam air intakes. I have ignitech ignition operating with a lost spark.
Any suggestions?

Chris

[Ianhood]

A faulty plug can cause that issue. The other think I would check is to see if the exhaust is sealing properly at the head. The threaded ring can come loose and that will give the symptoms you describe.

[Merridew]

I already changed the plugs.
But will look at the exhaust thanks.