How do i fix a bike that keeps backfiring after having it in a shop?

thank you for more information. Now I can suggest close answers As I stated the back fire is from combustion fire getting back through an open inlet valve ( back fire is when the explosion is out of the air inlet ---fire out through he carby/s as to an explosion in the muffler which is not back fire--just so that I am talking at the right end)
as I stated it could be from incorrect valve clearance should be around .012" hot for the inlet and .0104" for the exhaust hot
have a compression test done to see if there is indications of a loose valve seat insert which will give the same result
next the ignition module is that item which produces a current at the right timing that flows through the coil/s to produce the spark for the plugs
I am not totally familiar with the bike as it may have points and not electronic (CDI) ignition
Assuming that it has points , the incorrect or spasmodic ignition causing the backfire can be from dirty points , incorrect point gap, loose condenser,faulty condenser,broken earth wire for the plate in the distributor
if it has a CDI( capacity discharge ignition or transistorized ignition instead of points ,the problem may be in that unit
check for vacuum leaks around the carby/s and any hoses that should be connected
further into the ignition system check for problems in the distributor drive worn , cracks in the cap, moisture or dirt/carbon , worn carbon button on the rotor button, loose rotor button
All of things should have been check out by the shop so as to provide service and make the customer happy with the bike and that is why I suggested a second opinion from another bike shop-not necessarily a yama shop--but a shop that repairs bikes in general
I hope that I have given you food for thought and now have some idea so that you can talk with the mechanic and ask the right questions thank you for more information. Now I can suggest close answers As I stated the back fire is from combustion fire getting back through an open inlet valve ( back fire is when the explosion is out of the air inlet ---fire out through he carby/s as to an explosion in the muffler which is not back fire--just so that I am talking at the right end) as I stated it could be from incorrect valve clearance should be around .012" hot for the inlet and .0104" for the exhaust hot have a compression test done to see if there is indications of a loose valve seat insert which will give the same result next the ignition module is that item which produces a current at the right timing that flows through the coil/s to produce the spark for the plugs I am not totally familiar with the bike as it may have points and not electronic (CDI) ignition Assuming that it has points , the incorrect or spasmodic ignition causing the backfire can be from dirty points , incorrect point gap, loose condenser,faulty condenser,broken earth wire for the plate in the distributor if it has a CDI( capacity discharge ignition or transistorized ignition instead of points ,the problem may be in that unit check for vacuum leaks around the carby/s and any hoses that should be connected further into the ignition system check for problems in the distributor drive worn , cracks in the cap, moisture or dirt/carbon , worn carbon button on the rotor button, loose rotor button All of things should have been check out by the shop so as to provide service and make the customer happy with the bike and that is why I suggested a second opinion from another bike shop-not necessarily a yama shop--but a shop that repairs bikes in general I hope that I have given you food for thought and now have some idea so that you can talk with the mechanic and ask the right questions

Thank you so much Bill!! your the first person to give us actual pointers. We actually went to see a mechanic today and looks like were putting a whole new motor in it now, because for some reason now the bike is sputting out alot of smoke all of a sudden so the mechanic said were gonna blow a rod or piston or something lol i stopped listening after he said we need a new motor lol, thank you very much for all your help!! Thank you so much Bill!! your the first person to give us actual pointers. We actually went to see a mechanic today and looks like were putting a whole new motor in it now, because for some reason now the bike is sputting out alot of smoke all of a sudden so the mechanic said were gonna blow a rod or piston or something lol i stopped listening after he said we need a new motor lol, thank you very much for all your help!!