The backfire can occur also due to blocked fuel line or vacuum in the gas tank,as your friend said.
Resulting in a fuel starvation and a blowback from exhaust, but usually not with a black smoke.
If a fuel starvation is the issue, it normally happens after riding a few Km in high rev / highway,
And the fuel supply to the carb cannot keep up with the the demand.
Also, if there is a small "Cardboard" type fuel filter, it is best to replace it with a mesh none,
Or to install a big (Car) inline filter, with a bigger cardboard surface, in order to help the flow run.
The reason is that these type of filters (Cardboard) are designed to operate with a fuel pump,
And we have a gravity fuel system, so there is not always enough pressure to flow through the filter
To supply the demand.
Points where checked again and new sparkplug was fitted.
Until this morning every thing is fine, he reported (drove to work with her this morning).
Thank you all for your time.
