Just bought an ´80 XS650 Special and about to get all the kinks out of it. Have one problem I can´t figure out. It has a pecking sound in the area of the valves that gets worse under load. I´ve adjusted the valves twice, but it doesn´t help. Could it be that this bike was designed to run on leaded gas? If I added lead substitute to 93 octane gas, do you think this would cure the knock?