Thank you so much...great to have you out there helping an old fart like me reliving his youth.  I bought my SC500 used around 1977.  I always feared starting that bike because of the notorious kick back. My dad once tried to start it...got kick back and said screw that thing!!  I even bought motocross boots just to protect my ankle. I eventually parked it. It's been sitting all these years until now and I want to get it going again.  It was well taken care of...never raced... so I didn't have the other issues with detonation/piston failure. I bought it to out-do my buddy who bought the Suzuki 400 enduro...another bike on the worst ever list.  Thank again.  Great forum!!!!  Proud to own a "worst bike ever made".

Maybe i'm having a bad day, but the condenser addition was something we never heard about down here that i recal. & reading that yamaha story it reads like it's only use is to advance it at 1000 instead of 2500 which aint helping kickback 'at all' [correct me if i'm wrong]. The two stage pulser sits on retard to start so just 'Shouldn't' kick back. It's pretty much a DT360A mag. So if genuinly kicking back scarily hard then "what's" going on. Not pulser wire crossed--i guess they had a double plug so not likely crossed. Flywheel key broken--first thought]. Bolt on a late DT400 or "79 DT250 cdi mag with better pulser & box. When we got them new they used to SNEEZE on kicking, not kick back. & the cure was to get the source coil rewound to a higher value to give a better spark at kick speed. So anyway--where was i--what day is it lol. & put me right if i'm thinking wrong.

Would an HPI ignition conversion work good? I see them for sale but I only see people mentioning the DT400 or MX360 conversions. I just acquired a 74 SC500 and I can only get a spit and kickback upon trying to start it so far. Enjoying reading info on this site. 

HPI or VAPE/Powerdynamo? 

HPI is used less than VAPE here and not sure HPI makes one that has enough lighting power for a DT400? 

Either should work...  I would lean towards the VAPE more for "flywheel mass"...  but actually I'd probably look for a Yamaha points magneto setup from a 74-on 250...  but those are getting expensive these days...  not likely to find one for $100 anymore. 

Or maybe a "79 DT250 or 400 CDI mag. First off with yours i'd check the crank key incase its let go for spitting & kickback. Truly shouldn't kick back like an old RT2 but spit or sneeze-yes. Pulser fires near tdc on retard. They had an underwhelming source coil from new & we got out shop bike rewound to a higher value to put out more kick speed volts. They're a brute to kick fast enough to get started even with decomp working.