sound like the timing is out! its a chain on this model it needs to be replaced as it stretches over time
carl, January 2010
when a car backfires years ago this indicates the ign timing was firing late, this could rectified years ago by adjusting at the distributor and with the aid of a timing light(strobe)
wirecutterman, January 2010
service the thing, new plugs oil, filter etc, id guess that would stop it
