[Caliente Pocket]

On most vehicle (cars, motorcycles, whatever), removing a sensor like this will cause the ECU to go into limp mode. It won't run as well as it should, maybe even worse than with the stock muffler. You may get trouble codes, but I don't know for sure since I've never done it. If anything, I'd only recommend using this as a very short term fix. If it's going to be a long time before you can afford the fuel computer, consider putting your stock exhaust back on for now. Running too rich for a long time can cause problems with your engine just like running lean can, and without the O2 sensor in the circuit, your ECU has to guess about the right amount of fuel.

That said, if you're going to disconnect the O2 sensor, leave it installed in the pipe and just unplug it. Make sure you protect the connector from getting wet and grimy while it's unplugged. You're going to want it again when you do install a fuel computer.

 

[Caliente Pocket]

"Afterfire" is as good a term as any. I've heard it called popping, backfire, etc. It's pretty common on the 950s if you do any exhaust mod at all without adding a fuel computer.

Anyway, in this case it's actually an indicator of a lean condition. Manufacturers tend to run the bikes just a little on the lean side in order to make it easier to meet emissions standards. But on an air-cooled bike, you usually want it just a little on the rich side for better cooling. You're better off a little rich than a little lean on this bike, but you really don't want to go too far in either direction.

If this only happens when you close the throttle, and it doesn't bother you too much, and it's not extremely loud (sounds like popping but not gunshots), you're probably okay for a little while, but it means your bike is running hotter. You don't want to do that indefinitely.

Edit: By the way, have you tried disconnecting the battery for about five minutes? Supposedly that forces the ECU to relearn the right fuel settings, and it might correct this for you. If you're running a slip-on and stock intake, the difference in airflow won't be too different, and the stock ECU may be able to compensate.

 

[Caliente Pocket]

Just the negative terminal will do it.

 

[Caliente Pocket]

Doesnt sound like gun shots, more like small fire crackers

I don't think it's a big problem, then. More of a nuisance. Especially if it was doing that with the stock exhaust and the mechanic said not to worry. The slip on makes it more noticeable because it makes your exhaust louder.