Can some one help I have recently bought two MZ TS 250/1’s One as a bit of a project.(new owner and MZ rider.)

Both bikes start ok but will only run for  about 10 to 20 miles.Plugs fowl up wet and sooty,they only start agian when I replace the plugs,with a new one. Timing on both are to spec and plug gaps, carbs etc., hope its not as I suspect by what I have learnt and by my knowledge ,that the oilseal’s on both bikes need replacing.