912ULS

When you say a backfire, do you mean combustion occuring outside of the cylinder, i.e. an explosion in the exhaust system?

If that is the case then you have unburnt fuel arriving in the exhaust which could be due to a range of problems that would probably also result in rough running at other engine load rather than just at cruise power settings.

Here are my thoughts: If the problem is only in the cruise then I would start by looking at the parts of the carb that control the part-load fuelling. The main jet, the needle, the diaphragm and the slider. I would have thought that a problem in this area would just give you rough running rather than a backfire.

I think we need some more to go on:

Have you got any other symptoms?

When did the problem start?

How many hours are on the engine?

What was the last maintenance/repair activity that was done to the engine?