those old mercs have a diaphram fuel pump that when air gets in it will not function properly. If the fuel is not the problem, inkdabber is probably right. If you have fuel and loose power it is most likely from loss of fire. take a plug out and connect it too the plug wire and ground the metal end of the plug to the motor where there is no paint. You should see it spark as you turn it over and try each plug. You might want to use a rubber handled pliars for this. If you have fire to each plug then you still have a fuel problem in the carbs, but I think it is loss of fire or compression. A broken reed valve would do ti also..