generator/welder

I have a generator/welder. It must be an older one as the leads for the welder and generator sides are seperate and there is a switch to change from one to the other. The welder side works perfectly. On the generator side when I tie the throttle down till it stalls almost I still have 150 volts. If I let it run normal it blows out two diodes (and a hole in the base of a lightbulb I was testing with. Any ideas would be greatly appreciated. Yosef Guttman