389 carb on standard G2

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
Richard Hyatt
Posts: 113
Joined: Sun Jan 27, 2019 8:52 pm
Location: Dorset

389 carb on standard G2

Post by Richard Hyatt »

Hi all,
Has anyone had this combination please?
Our bike has this and I'm slowly going through jets and needles etc.
Has anyone had this on their bike.
Paul R
Posts: 43
Joined: Sat Sep 09, 2017 6:04 pm
Location: west yorks UK

Re: 389 carb on standard G2

Post by Paul R »

Yes . I'm running a 389 on my standard model 14 . Jetted according to the factory manual for a CSR and works fine . 200 main jet no3 slide .106 needle 20 pilot jet . CSR is slightly higher compression ratio I think 8 to 1 up from 7.8 to 1. .
Richard Hyatt
Posts: 113
Joined: Sun Jan 27, 2019 8:52 pm
Location: Dorset

Re: 389 carb on standard G2

Post by Richard Hyatt »

Ah , thank you.
I just wondered how they went.
Ours is to book spec except 180 main (what the normal 376 had).
I also was careful about blending the tufnol spacer from 1 1/16" to 1 1/8".
The reason I ask is because we have a wavering tickover that is not stable at all despite no air leaks , good fitting slide etc,
Just trying to think of other reasons
User avatar
dave16mct
Member
Posts: 3234
Joined: Mon Jan 01, 1990 12:00 am
Location: LANCASHIRE UK

Re: 389 carb on standard G2

Post by dave16mct »

It might be weak springs on your auto advance mechanism.
Dave.
Richard Hyatt
Posts: 113
Joined: Sun Jan 27, 2019 8:52 pm
Location: Dorset

Re: 389 carb on standard G2

Post by Richard Hyatt »

Ah right,
I'll check them out thank you
Paul R
Posts: 43
Joined: Sat Sep 09, 2017 6:04 pm
Location: west yorks UK

Re: 389 carb on standard G2

Post by Paul R »

I had some problems with erratic tickover despite fitting new A/R springs . Revs slow to die back down . Seem to remember this is a symptom caused by too lean a mixture at the lower end of the rev range . Bit of fiddling about with the pilot mixture and needle position cured it .
Richard Hyatt
Posts: 113
Joined: Sun Jan 27, 2019 8:52 pm
Location: Dorset

Re: 389 carb on standard G2

Post by Richard Hyatt »

Ah , slow to die back down is exactly what we've got.
You can start it (amazingly easily) and runs well , with the 389 set up , did a plug check last night , lovely colour.
I'm allowing for rings and piston to bed in but colour generally fine.
Pulls cleanly and well, no flat spots
But when you slow down at lights it runs fast and trying to snap the throttle won't slow it down , it might slowly slowly come down.
But at this moment , it's not even sitting on the throttle stop screw and still running.
This would indicate an air leak , but I've faced off the flanges and gaskets good , even tried a new throttle spring to try and push it down more.
I've got springs for the AR so will fit them and probably dismantle the carb again.
The slide is a good clean mechanically snug fit in the body , no fore and aft rock . I've polished it with solvol to ensure smooth movement in the body.
See how we go.

The only variation , is that i have a 180 main and you have 200 the original size.
Paul R
Posts: 43
Joined: Sat Sep 09, 2017 6:04 pm
Location: west yorks UK

Re: 389 carb on standard G2

Post by Paul R »

Took me a while but found this snippet on t'web .

Posted by Matthew on October 29, 2012 in Fuel & Air
How-To: Size Your Pilot Jet, Pilot Screw Explained


The Goal
What you are after when adjusting this screw, is too reach peak idle RPM and smoothness. This is the engine telling you it is happy, and it runs best at this mixture.

How do you tell if your pilot jet is too lean, rich, or just right?










What are some indications of a lean pilot jet and/or screw setting?
If your air screw is in further than 1 turn, you need a richer pilot.
If your fuel screw is out further than 3 turns, you need a richer pilot.
Off idle hesitation or bog.
Revving the engine in neutral will result in hanging RPM's, or RPM's will slowly drop back to idle RPM.
Richard Hyatt
Posts: 113
Joined: Sun Jan 27, 2019 8:52 pm
Location: Dorset

Re: 389 carb on standard G2

Post by Richard Hyatt »

Many thanks for digging that out.
We've done the springs on the AR and retimed it nicely.
I have the correct 389 jets (20 pilot) and I can get it to sensibly idle albeit a shade rich, which I can live with.
The pick up is clean and tidy , so I'll let it settle at that. I'll keep my eye out for a 376 at a jumble sometime.
Theoretically the larger bore 389 will have a different venturi air speed than the smaller bore 376. So this may upset things ,
I may even try a little k and n filter to see if that changes anything.

But I also created a baffle for the very noisy exhaust , so this will change things , the sun is shining , so out for a ride and checks .
Richard Hyatt
Posts: 113
Joined: Sun Jan 27, 2019 8:52 pm
Location: Dorset

Re: 389 carb on standard G2

Post by Richard Hyatt »

hi all,
done the AR springs and dismantled the carb to find a 2 stroke emulsion tube (slash cut) in there.
bought a new straight cut tube and punched it in, plus fitted a new slide as the other one was rattling a bit.
seems better in the garage , but time will tell for sure.
Locked