Can someone please tell me what the reasons can be for back firing. MGB 1800 engine High compression.It was running fine, but after tuning the engine, it now runs fine on idle but back fires when reved up and under load. Checked leads etc.

Hi Hugo
It sounds to me like the timing is out. I assume it is actually backfiring and not just misfiring. If it is just misfiring it could be over fuelling - i.e. too rich. I think the timing should be checked.
Chris H