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‘73 SC timing

01 Apr 2020 05:38
MEAW
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‘73 SC timing #1
Just purchased a ‘73 SC and having all sorts of problems getting it to run. Have given it to a good VMX mechanic to have a play with. He is playing with the timing but we cannot find the actual degrees BTDC as just about every manual differs. This model has the CDI and no points. Just need the correct timing as a good starting point. Thanks

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01 Apr 2020 06:31
RT325
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‘73 SC timing #2
With the timing from just a getting it running point of view it'll run anywhere within the slots on the back plate although about a central point is a good start & it'd be a brave man that turned the plate right back in the slots advancing it right up. By 'back' i mean towards the back wheel in the slots. Ok if it won't run & it's probably a spark issues it's time to check the source coil output & trigger coil resistance. Trigger will be a double wire unit white/green & red/white by memory. [nothing wrong with longterm but now i've forgotten the question]. The one i used to ride new at work was a bugger [& they all were] & would sneeze if you didn't get a good swing on the kick start but er got our source coil rewound to a higher value & fixed it. Talking the trigger, my DT360A had a crook trigger [same trigger] on the retard for starting side so wired it so it was advance right from the kick & had to kick it positively or it would bite back. Ran like that for years though & they normally flick to advance right after starting revs, just had to be careful & not toy with the kickstart. Make sure the flywheel key is in one piece as needs to be very tight on the flywheel nut. If the crank key was broken you'd still get some sort of life out of it even if it fired near BDC & blew the crank seals out. Hope you get some joy out of it. when going right they're a bit of a bear to ride hard. I [stupidly] took the ballancer chain out of ours [bosses bike i borrowed] & drove 6 hours to a christmas mx only to damn near have my eyeballs bleed with vibration--Bad Move That lol. Cheers.
ps, if ya stuck then a later CDI mag of a late DT400 would fix it but would need the cdi box too for that mag. Could use a points mag from a DT250 except you're stuck on whatever advance setting you set it at so would kickback but kick it hard from the top & follow it right through. They have the same stroke [70mm] as a 360 yam so way over square at 95x70. Who know what they were thinking when they came up with that configuration.
www.partzilla.com/catalog/yamaha/motorcy...sc500a/c-d-i-magneto

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01 Apr 2020 08:36
MarkT
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‘73 SC timing #3
Yamaha didn't list timing in "degrees" if that's what you are looking for.

The Yamaha Service Manual (available in Tech Library) and the Yamaha Service Bulletin showing the service data for the 73 SC500 say the timing is 2.7mm BTDC.

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

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