'55 G80CS... Saga - Caburetter specs?!

Information relating to the Matchless G85 500cc Heavyweight, AJS 7R, Matchless G45 and Matchless G50
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'55 G80CS... Saga - Caburetter specs?!

Postby Matchymarty » Sat Sep 29, 2018 4:19 am

Hi all,

Thank you again so much for helping me through my ordeal.

I've sent my head off for new valve guides, valves and seats, and have the barrel at engineer getting a new +060" JP piston clearanced.

I've stripped the carb, and done some measuring.

1. It appears that it is a AMAL 389/203 for a T120 Triumph.
2. Bore is 1 1/8" - as opposed to the manual's 1 5/32" requirement. - Hence, undersized by around 0.031"
3. I've used anything from a 260 to the current 400 main jet.
4. I have now identified the slide as a #3 1/2. (the '.5' marking is very faint, but definitely a bigger cutout compared to a spare #3 I have.

My new piston, and most of the previous ones have been aparently, are 8.5:1 even though I was told they were 7.3:1.

Basically, I would have thought, that if the carb is too small, this would make velocity higher and also , make the jetting weaker? That said, with the bigger jet (400), one would think it wouldn't be too lean?

So, given all this information, Any ideas on how I should Jet my carb or, is it to small for my Matchy?

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