Just read your post again Ken and instrument lights going out (maybe just dimming?) indicates not enough voltage to them with jump leads making no difference. Suggest therefore checking battery leads and connections, particularly the earth strap from chassis to engine block as the effect of any resistance in the leads will be greatly increased by the high current demand from starter, reducing volts to same and elsewhere, similar to DaveW's experience above.


Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone