My BMW r850r motorbike back fires when I hold the throttle steady at 3000 rpm?

SOURCE: Back firing and doesn't fire on on cylinder until warm

Typical of xv,s itcould be a fuel blockage or damaged diaghram in carb! check all hoses!

SOURCE: rebuilt all four carbs cleaned and replace needle

With Mikuni carbs, the idle circuits affect engine performance up to 3000 rpm under light loads. If the idle circuits are restricted (by fuel gum, rust dust from the tank or dirty fuel, the results will be 1) the engine will run with "choke" on. 2) poor or no throttle response below 3000rpm,3) back firing and missing on the effected cylinder(s). Cure is to blow out the idle circuits and idle screw adjustment jets with several applications of carb cleaner and compressed air. Failing that, strip the carbs.

SOURCE: my gs500f is back firing through the CARB. the

Lean mixtures burn very slowly, at times slowly enough to continue burning through the power and exhaust stroke, causing a backfire when the intake valve opens, and that flame gets a shot at the new mixture charge.

In normal operation, as the engine slows, the fuel delivery from the main circuit falls off, and the idle circuit is supposed to take over. If the idle circuit flows insufficiently, that becomes a transition to fuel starvation.

You can try pointing an unlit propane torch into the inlet air, and see if you can get closer to an idle while supplying a supplementary fuel source. You will need to do this in a way that gets propane to both carburetor inlets, maybe rigging a Y with vacuum hoses and electrical tape...

This started with work on the carburetors, so the fuel system would be the most suspect. That, and the fact that it will run at higher RPM would seem to rule out fuel delivery.

I was looking around at photos while developing this answer (needed to know whether this was a twin or a 4-cylinder), and one resource said the idle speed should be 1,200. I don't know if that's right, but maybe 1,000 RPM is too slow for this motor to keep it together. (I do doubt that, though.)

When I wrote that last sentence, I started to second guess myself, thinking "What if the fuel shutoff(s) is/are vacuum operated, and as the bike approaches idle there is insufficient vacuum to hold it/them open?" But the I rejected that, because there is even less vacuum at cranking speeds, yet the bike starts.