OK - Easy steps then: 1. use the multimeter from the horn contacts (on the horn) to a good earth on the frame - Should read around 12v on one pin when the horn switch is pushed (ignition on) and zero on the other pin. a) If 12v on both, then the fault is on the earth at the battery (check connection to frame). b) If voltage is low or falls low when button pressed, the fault is in the Battery Live connections. 2. If 'a', then check resistance between battery earths side and the frame. Should be near enough zero ohms. Check both battery terminal clamp and the frame earth bolt for tightness and/or corrosion. 3. If 'b', then check battery live clamp, fuse connections, and all cable plug/sockets. All the above assumes you have checked the voltage across the battery when the horn switch is pressed (ignition on) and it does not fall below 12v. 4. If 2/3 are all OK - Check your ignition circuit - The igbnition swich may have a relay to provide power to the horn or, it may be directly switched. It will most certainly have a relay for the electric start. Just by using the horn circuit, you should be able to establish where the fault lies - once found: Fix it!