hi there what year is the engine , is it a kickstater only / or a kickstarter /electric starter or just electric starter. if it is a kickstarter only there is a bit of a knack to starting them , if it is make sure you have the decompresser cable fitted or you WILL brake the kickstarter . to set the tappets on all the engines is the same , on the casing in front of the gear lever there are 2 screw in plugs 1 in the middle 1 at the top. undo these and with a socket on the nut in the middle (crank nut) turn the engine counterclockwise ( its easyer if you take out the sparkplug ) there are marks on the rotor you are turning seen though the top small screw hole, align the (T) mark with the mark on the casing the T may be on its side. take off the tappet covers both sets of tappets should wiggle up and down , if not turn the engine one full turn and try again . now with the right feeler gauge for your engine check the gap the inlet and ex will be different you are after the feeler gauge sliding in the gap with a slight drag on the gauge if you need to set them undo the nut and turn the sq bit to adjust then tighten the nut its a bit fiddly at first . the popping may be the exhaust if its a quad it probably is not a standard silanser , or it may be the coasting enricher diaphram in the carb is stuck if it is it will pop and bang very LOUDLY like a shotgun going off how bad is the popping?. i would give the engine a full service new plug airfilter or clean re oil etc to see if this helps the starting also drain the carb by the screw at the bottom of the carb to get rid of any crap in there also set the mixture screw as per your model of engine it could be set very weak , basicly you need a service sheet / manual for your year of engine . good luck zigzag