i have 185 3 wheeler older model1981 to 1984.Ive bought new carb I thought was the problem.I have spark but could it be the thing on the left side on the near the top side of the motor might be called the spark advancer it spins has like a clutch looking thing with springs?I have spark though I thought if ya had spark and good plug only thing could be the carb?its getting gas,somebody please help me out with this,could it be points or what ever can i have spark and still something like that mess me up from starting,it will only pop once in awhile.just got the bike..

 

The advance drives by a small pin on the camshaft ... And the advance could have be taken apart and assembled 180 off , this would cause the engine to fire at BDC instead of TDC ..

Here is some advance info ..
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did this just start happening? could also be low compression, i would say do a compression test....

did you just buy the bike? or was it sitting over the winter (or longer?)