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Makotosun

1971 Yamaha AT 1 ...runs with pilot screw all the way in.

  • mpritch41
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So I finally have the 1971 AT 1 running.
cranks easy idles smooth. Revs quickly...but only with airscrew turned all the way in. If I adjust it out at idle even 1/2 turn it dies.
It will crank up but bogs at 3-4k.
​​​​​​so I'm thinking lean. Standard jet is #30 ( and I have tried that) now have #35 in.
main jet is#150( tried #160 and ran worse) and I have needle clip on lowest position after trial and error.
Plug looks rich.
Checked for airleaks around carb etc.
Carb is clean. Timing is correct.
​​​​​​compression feels adequate.
so with the screw all the way in...runs clean to 3k...sputters 4-6 k then revs on out. 
  Any other ideas?

 
21 Jun 2021 06:26 #1

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Did you replace the flywheel side crank seal in the end?
Might be worth doing a leak down test, there are a couple of current threads running about the process.
Experts will be along later to help you, I'm sure.
Yamaha CT1-B
Yamaha CT1-C
Yamaha AT1-C
Yamaha CT3
1978 Yamaha DT175MX
2020 Honda CB500X
21 Jun 2021 09:21 #2

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in my unqualified opinion I suspect either a plugged primary circuit or an air leak.
 
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21 Jun 2021 09:25 #3

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Yes...replaced all seals.
Thanks
21 Jun 2021 17:56 #4

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