Hi, I am Bosse, new to this forum and have had a 2002 640 LC4 and a 2008 990 Adv R. Both are sold, but I am looking for a 2003 640 LC4 Adv as it is a lighter more agile offroader than the 990, eventhough the 990 is a rocket. I like the big double tanks too, as it gives a long range to the bike. Wrt your problem, it seems to be a carburetor problem. Does it run well when cold or does it hack its way forward. Try to give more fuel by adjusting the needle, check the o-rings for air-leakage, check the sparkplug, adjust the opening if needed, clean the airfilter often, check that the airtube on the tanklid is working, change the fuelfilter (hmm don´t remember if there is one). Can´t think of any other stuff to check at this point. Good luck!// Bosse