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1971 AT-1B No longer starts.

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1971 AT-1B No longer starts. was created by caelenhaxton

I bought a 1971 Yamaha AT1B several days ago (not running and sitting for god knows how long), cleaned the tank and carburetor then I used the battery out of my XS400 and it started, but wouldn’t idle below 2000rpm. It stalled about minute later and now WILL NOT restart. It won’t even bark off on starting fluid. It has gas as the plug(s) are wet, and spark. Compression was about 98psi. When I pulled the plugs a lot of whiteish gray smoke poured of the cylinder. Any help would be greatly appreciated.
16 Jan 2022 18:31 #1

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Replied by adguy2112 on topic 1971 AT-1B No longer starts.

Might have popped a crankshaft seal. White smoke often indicates trans/clutch oil is getting past the seal and getting into the combustion chamber. It could also be the reason the bike won’t start…among other things. It’s not uncommon for this to occur when a bike has been sitting. Perhaps the seal rubber adheres to the crankshaft and spins out on first start. 
The following user(s) Liked this Post: RT325, Tinkicker
16 Jan 2022 21:01 #2

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Replied by RT325 on topic 1971 AT-1B No longer starts.

Yep i second 'that'--RH crank seal popped out.
17 Jan 2022 16:03 #3

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Replied by caelenhaxton on topic 1971 AT-1B No longer starts.

Is it possible to replace it while the engine is mounted? Everywhere I’ve looked involves removing it.
18 Jan 2022 09:45 #4

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Replied by Pete-RT1 on topic 1971 AT-1B No longer starts.

"Is it possible to replace it while the engine is mounted?"
Yes, no need to remove the engine.
Same goes for the left side seal.
Yamaha RT-1
Yamaha CT1-B
Yamaha CT1-C
Yamaha AT1-C
Yamaha CT3
18 Jan 2022 11:54 #5

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Replied by shyted on topic 1971 AT-1B No longer starts.

And part of the sealing system is an O ring on the end of the crank under the primary drive gear spacer collar so don't just replace the main seal, do both. The part is readily available.
Shyted
Last edit: 18 Jan 2022 15:06 by shyted.
18 Jan 2022 15:04 #6

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Replied by RT325 on topic 1971 AT-1B No longer starts.

Pull the cover to have a look. Will be obvious if hanging out. If not that then refit cover & start again--keep looking--but order a seal anyway--& Oring as mentioned that i'd not thought off.
18 Jan 2022 15:21 #7

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