Hope you can help. I have brought a 1969 500cc Dommie, I took it out last week and it ran fine for 20 minutes, then cut out. It was very hard to restart. Before I start doing anything I did find the battery was 3 single cell torch like batteries they looked very old!!!, I will have to fit a new battery.
Hope you could point me in the right direction
Is that the correct year for your 500 Dommie, I thought they finished the 500 models in the mid 1960's?
If it is a magneto then you'll have to test if there is a spark when it cuts out, if not then I would suspect the magneto needs overhauling.
Is the battery a Cyclon cell type? They are costly and allegedly have a very long life. Google for more information. And test with a cheap meter before throwing it away. How wel is it charging?
Unfortunately, there are many possible causes of the symptoms you describe. When the engine cuts out, is it suffering from a loss of compression, and overheating? Does it start easily when cold? If so, it is possible that the wrong pushrods have been fitted. Mine were expanding, when hot, preventing the valves closing fully. A temporary solution is to increase valve clearance, when cold (try 6 thou. inlet & 8 exhaust). The motor may rattle a little, when cold, but quieten at working temperature. Weak mixture, and, or, retarded ignition timing, will also cause overheating.