The bike could be running a bit rich, coked up a bit or the exhaust valve needs a little attention (hopefully just lapped in a bit).
Take the head off, give it and the piston crown a good de-coke. Inspect the condition of the valve guides (particularly the exhaust valve guide) and lap in the valves. Put it all back together and check the tappet clearances. Make sure that the de-compressor works properly.
If you have a solid copper head gasket, make sure you anneal it first before re-fitting it. Tighten it down, warm the engine up, let it cool down, then re-tighten. Check the clearances again.
Have you already checked the easiest things first;
another spark plug.
or the little 2 brushes in your ignition ? (One on the cable and one earth brush)
and check the ignition timing.
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.
