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What would cause a 78 Honda CB750K to pop a lot? It is a lot like small backfires.

Air filter is fine, fuel is fresh (if that makes a difference), plugs are good and I get a pretty blue spark when I take the plugs out.

Timing?

Fuel / Air mixtures??

Does this happen when closing the throttle? Under hard accelleration? All the time?

Could be 'decelleration popping' as they jetted bikes pretty lean back then. It is common. Honda XL's and XR's came with a wire mesh screen in the airfilter to keep sparks from this lighting the airfilter on fire. If it only happens with closed or partly closed throttle, it is no big deal.

Had that problem on a 93 Honda VLX and a 99 Vulcan 1500A - friend had that problem on a 04 Honda 1300VTX. All caused by the same thing - air getting into the exhaust system. Usually around the exhaust port.

Just a thought.

as Tommcatt said look for air leaks around the exhaust system.
if the pipes are rotted at all it will cause the back firing. not to mention idling problems and fowling of the plugs. if there is any rot at all replace the pipes it's a lot less of a hassle then trying to weld because if it is rotted out side then the insides are going to be even worse. eBay has good deals on 4 into 1 sets right now buy now as low as $115.00. Good luck and get out ridding.

It only happens at idle and decelleration, never during excelleration. The pipes are pretty old looking and plenty rusted but they dont appear to have any integrity issues.

One more question. The beast refuses to idle if it is not warmed up. Could the two issues be related?

At a guess, I would say it is the stock (lean) jetting causing this. It won't hurt anything but it can be sorted out with a jetting session if you so desire. MAny bikes from that era have a similar problem with cold idle and de-cell popping.

Sounds like an air fule mixture problem to me.