Battery-wise, as you probably know the Rotax is a total loss system. The battery is used for all running electrical. A bad battery can lead to misfiring and bogging. When I was new to Rotax, I used non-spec batteries as they were far less expensive, and I had all sorts of issues with the engine missing and bogging coming out of corners. Finally swapped to a YUSA Rotax-spec battery and the engines ran consistently well.
I had a fuel pump go bad once. At first the symptom was the kart was simply running poorly. As the issue progressed, the the kart would hardly run at all under load out on the track. After trying everything else, the light-bulb went off and I checked the diaphragm. It was fairly stretched from use. I had about two seasons on that diaphragm. Its such an easy thing to check and correct. I now have a separate rebuilt fuel-pump on stand-by just in case, and I visually inspect them regularly.
I’m not running DD2 (I’ve got a pair of Seniors), but the principles are the same. I do know the DD2 typically is jetted far richer than the non-DD2 Rotax engines. In the beginning, I was taking my kart to a near-by track that didn’t have many pro karts there. So I was basically all by myself. From reading the forums, you’d think Rotax was basically Voodoo when it comes to jetting and carb set-up – lots of threads about problems. And of course when you’re new to Rotax you’re not sure what is too rich or too lean. But I have to say once you spend some time with it, it really is a very robust package. These days I basically check the weather, run the data into a jetting program (like John Savage’s Jet Tech), install the specified jet and plug, and hit the track.