389 carb on standard G2

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
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Location: Dorset

Re: 389 carb on standard G2

Postby Richard Hyatt » Thu Jul 11, 2019 7:04 pm

hi again,
well , all jets and settings are back to book now - it is a 389 on a standard G2
therefore the original 376 was 1 and 1/16" bore and the 389 is 1 and 1/8" bore - so a shade bigger bore

but I still have this strange fast running on tickover where it races away despite happily sitting at tickover 5 minutes before.
I just cannot get the tickover to be stable and reliable with the right mixture strength.

the only way to slow it down by making it excessively rich (screwing the air screw in) - I can get it balanced between the stop screw and the air screw one minute and then all on its own it speeds up - crazy.

I have tried AR springs etc and cleaned and cleaned this carb and poked everything through.

I have done enough carbs over the years , but nothing has behaved like this all on its own - daft !!!

any thoughts gratefully received.

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Re: 389 carb on standard G2

Postby Paul R » Thu Jul 11, 2019 10:04 pm

If you had to swap out the emulsion tube from 2 to 4 stroke it may be worth checking the needle jet as I seem to remember that these also came in either 2 or 4 stroke versions . Amal site certainly lists some 2 stroke needle jets .

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Re: 389 carb on standard G2

Postby Richard Hyatt » Fri Jul 12, 2019 5:13 am

Hi there,
Yes good thought.
Fortunately , I live very near burlen in Salisbury who make amal. .
They have been supplying the new parts to convert the 389 that was on the bike to work on the G2.
Albeit slightly bigger bore.
I went in the other day, and the first thing he said was , did I realise that it had a 2 stroke spray tube ?
Luckily just a press fit.
Worth a try for sure

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Re: 389 carb on standard G2

Postby Richard Hyatt » Fri Jul 12, 2019 7:06 am

Hi again Paul R

You mentioned needle position .
Intriguing as this would normally only change mixture 1/4 to 3/4 , but you say this had an effect on the idle ?

I know everything comes up through the main then needle and needle jet , but I didn't think the taper started until well open

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Location: west yorks UK

Re: 389 carb on standard G2

Postby Paul R » Fri Jul 12, 2019 4:39 pm

I did move the needle a notch up however I did this when setting the carb up through the rev range using an old style gunsen colourtune , all I can honestly say is that it sorted itself out in the end . The only other thing that crosses my mind would be to check the free movement of the float and fuel cut off needle , any hesitation there would cause inconsistency in the fuel to air ratio and erratic performance .

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Posts: 113
Joined: Sun Jan 27, 2019 8:52 pm
Location: Dorset

Re: 389 carb on standard G2

Postby Richard Hyatt » Fri Jul 12, 2019 5:28 pm

Thank you for these thoughts,
I'm going through everything as you'd expect , just checked the float and it's needle , it might benefit from a viton tip needle valve but this one doesn't weep or flood . Something for me to consider for sure.
Even though the 389 on the csr should be a 20 pilot (which is in it now) I may try the 25 back in that i have spare.
Failing that I'll wait for a jumble and pick up a 376 !!!

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