Recently I did an oil change on the Firestorm and installed a stock filter and 15w-50 oil. Now the manual says to use 10w-40 however the service dept at Yamaha advised the 15w-50 wouldnt make any difference. Did a bit of reading since then and some riders advocate avoiding the 15w-50 while others say it doesnt matter. Since I did the oil change the bike backfires regularly, at times quite violently. It also wont hold a smooth idle and coughs every 1-2 seconds. Open up the throttle and following the backfire the engine dies. Also noticed this morning a knocking sound that periodically comes from under the fuel tank. I am running Micron pipes (not sure if that matters) and Im not sure on the oil that was previously in the bike. I do still have the old oil filter. Just after some feedback if anyone has experienced this or similar issues. Prior to the change the bike ran perfectly. Im inclined to check the old oil filter and see if it matches the new one I bought, as well as draining the oil and replacing with 10w-40. Seeing as I dont know when the spark plugs were last changed I might change these as well.