The final hurdles were blocked jets and a bad earth on the horn switch. The bike had sat for a considerable time, years ,with just the occasional kick over and odd start up. During this time the airfilter had turned to fine powder and I'm guessing the last time I started it up this stuff made its way into the carby. I fitted a new filter and gave the carb a quick clean a few weeks ago but it would not start. Finally after finding a post on trials central about Bultaco Shepras that are hard to start I took a small syringe and filled it with petrol and took off the airfilter and squited petrol straight down the intake, it fired first kick but soon stopped. I eventually had it running on the main jet whick indicated that the problem was still in the carb. This time I pulled it apart completely and gave it a really thorough clean out, the bike started easily.the horn was my last hurdle, the wiring was correct by the diagrams I had found on the net for the 158 but the horn just wasn't working. I took the wire from the kill switch and used it as the horn button and presto the horn worked. So the problem was the switch it seemed. I pulled it apart and took the green wire (horn) out of the switch and touched it on the handlbars, no go, so I tried it on the clutch lever and bingo the horn gave a loud Bultaco squwark.
I didn't think that the anodising on the renthals would have stopped the earthing but it did. A few rubs with wet and dry and earth was reestablished and the horn works. I'd like to thank those people on the forum who took the time to reply to my posts asking for help, I am really grateful. With some luck I may yet get to a twin shock ride day to see just how out of practice I am.
