i bought a tw 200 4 my son at a good deal but doesn't run right starts and idles fine but at 1/4 throtle starts to break up went thru carb and every things fine ie main jet piolt no air leaks so i'm down to ignition what would cause it to run like this checked as many connections as i could changed plug so i'm bafeled also dealer says yamaha no longer makes service manual for this bike any tests i could perform on my own i would like to , thanks.