What happened to your start relay? It should be rated for something pretty hefty. It's DC, remember. Judging by my guestimate of what size the cable is for my 650, should be somewhere around 8 AWG, which in the code book I have is rated for 70 amps free air rating. Most starters are about the same. I can throw the amp clamp on my 650 this weekend while I turn it over and get you a definite measurement. Then you just over rate it a bit and you've got what you'll need. DC is fairly hard on contacts, so you have to make sure whatever switch you get is rated for DC, as the AC rating will be different (higher). Hope that answers your questions.
Rudy
