1974 Yamaha MX360

12/8/2017 1:23pm Edited Date/Time 12/9/2017 3:51pm
High all -- we are a small shop in Austin TX, West Coast Racing, specializing in Custom builds - on Vintage bikes - both street & competition. We have 2 1974 MX360's in right now, for race prep. First one, the barrel was badly scored, so it is on its way to LA Sleeve for a new sleeve and boring for a + 0.25 RT2 piston (it was at 1mm over), that came with the bike (only recently bought by customer) we sort of assumed that piston was correct -- but, can any confirm that please?
It also needs crank seals, and on pulling the clutch side case off, we find a LOT of slack in the crank balancer chain -- is this normal? Don't have a manual for the bikes - so, just going by 'feel'!! Will also need to be knowing what the piston ring gap should be, when we get the barrel back.
PS -- Facebook page is www.facebook.com/westcoastracingvintage

12/8/2017 3:39pm
Also found this piece of hard plastic, wedged behind the balance shaft sprocket -- any ideas where it came from?

12/8/2017 10:00pm
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.
12/9/2017 11:47am
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)
12/9/2017 3:36pm
Looks too loose.

The Shop

12/9/2017 3:36pm Edited Date/Time 12/9/2017 3:51pm
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.

12/9/2017 4:05pm
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.

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