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AMC 25T - Carb jetting

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AMC 25T - Carb jetting

Post  BarnettBilly on Sun Nov 20, 2016 5:21 pm

Hello all,

I am just enquiring what experience other AMC25T users have had with jetting their carb (Amal 389).
I know for the 1957 season a main jet of 420 was used and then the following years a 320 was used.
I currently have an Amal 389 fitted with a 320 main jet. My carb is new so not worn at all.
The reason I ask is because i'm convinced that mine isn't set up quite right. I cruise up to 50mph with relative ease and everything is as smooth as I could ever expect it to be, but once I get to 50mph I seem to run out of throttle. (I have recently posted about air leak problems, but these have now been sorted). I remember reading one fellow Cruiser 80 owner stated that they cruised at 55-60mph all day long. I know some of you will say to just live with 50mph, but I use my bike as a daily ride and have been on some very long rides, and I think the extra 5-10mph is a bit of a game changer.
Any sharing of experience will be greatly received,
Regards
Christian
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BarnettBilly

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carb data

Post  piston 197 on Sun Nov 20, 2016 8:32 pm

Christian it will depend which carb you have
The early carb was a 389/17, this had mixing chamber 389/002, 420 main jet and a No3 air slide

the post 1957 carb was a 389/27 , this had a mixing chamber 389/017, 320 main jet and No 3 1/2 air slide

but it would suggest if you are not getting past 50MPH then either your main jet is too small or the throttle needle is set too low ??

I have had problems with aftermarket ( new reproduction) jets having the right number on the side, but having the wrong size hole in them, notably with a B31 that would not run properly off choke, I replaced the main and needle jet with the same numbered second hand original jets and the problem was solved, the biggest culprit was a .106 needle jet which in practice had a smaller hole in it than a genuine .105 original Amal one.
It took me 3 Hours to find the problem was the newly rebuilt/ refurbished carburettor fitted with the right numbered jets, but with wrong hole sizes !
JH

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Re: AMC 25T - Carb jetting

Post  BarnettBilly on Sun Nov 20, 2016 8:58 pm

Hi John,

Thanks for your reply. I have suffered on a few occasions with new replacement parts which have been found to be faulty, condenser, switches etc. One problem I do have with the carb is the tickler. When I turn the fuel tap on and tickle the carb, the fuel does not overflow from the tickler, but weeps out of the bellmouth instead, which I have never had before. I believe that the fuel should always overflow from the tickler? I was wondering if this is part of my problem. I have tried various floats and needle valves to no avail.
I will dig out some old jets and see if this helps increase the speed.
Thanks always,

Christian
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