1974 Yamaha MX360

Also found this piece of hard plastic, wedged behind the balance shaft sprocket -- any ideas where it came from?

I'm guessing the block of plastic is a chain tensioner that fell out of place and that is why the chain is so loose.

FGR01 wrote:

I'm guessing the block of plastic is a chain tensioner that fell out of place and that is why the chain is so loose.

No tensioner on it FGR01! The block is just a guide - and the 'stray' piece of plastic did NOT come off it.
Anyway -- not that bothered really -- BUT, we now get to the crank seals - and spring orientation on them? The existing ones have the spring facing out - both sides (but are hardly likely to be OEM ones)

Looks too loose.

I removed the counter balance garbage on my 74 MX360 Champion. As one of the heaviest two stroke engines Yamaha ever produced, it won't miss the weight. I did have Falicon rebuild and rebalance the crank, though.Kind of a turd engine ,IMO.

Springs facing out is correct on the seals. The spring creates a bit of tension to seal around the crank ends. Also, if a spring were to pop off or break, it does not get sucked into the main bearings, chomped to bits, and leaving it's autograph your nice new cylinder sleeve.

Nice thing about the champion frame is - you can put a lot of different engines in there!!
Yup - we removed the counter balance mech, but on the springs -- clutch side is spring out, but mag side - spring in. We'll take our chances on that one - hehe! I once owned a Rotax 600 single (& single cam) RR bike, with a Terry Knight frame. Few folks know that, although Terry was well known for his DT frames, he did a short run of RR ones. It handled like a dream! Thanks for the advice -- looks like we're 90% with you.