G9 Matchless 1958

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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Sams Dad
Posts: 21
Joined: Mon Apr 19, 2021 6:49 pm
Location: Lancs UK

G9 Matchless 1958

Post by Sams Dad »

Hi all,
I have lifted the heads on my G9. All looks ok - maybe a bit black/oily, but otherwise looks ok. 2 issues I am worried about
1. Does the head need to be modified to take unleaded fuel ? Is it possible to tell if this has been done already?

2. How to check the head is flat ? Do I need to get an engineering shop to do it ? I am not sure the left pot was too hot but the right one had the downpipe glowing red within minutes.
:o
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GSAX1
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Joined: Wed Sep 14, 2005 1:00 am
Location: NORWAY

Re: G9 Matchless 1958

Post by GSAX1 »

Unleaded fuel is no problem. My '55 M20 has been running on it since I bought it some 16 years ago. It still has the original valves and seats from 1955. No problem with valve recession.
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clive
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Location: LONDON UK

Re: G9 Matchless 1958

Post by clive »

Sams Dad wrote: Mon Jan 17, 2022 2:07 am Hi all,
I have lifted the heads on my G9. All looks ok - maybe a bit black/oily, but otherwise looks ok. 2 issues I am worried about
1. Does the head need to be modified to take unleaded fuel ? Is it possible to tell if this has been done already?

2. How to check the head is flat ? Do I need to get an engineering shop to do it ? I am not sure the left pot was too hot but the right one had the downpipe glowing red within minutes.
:o
Sounds as though the timing is retarded if the pipe was hot enough to glow, because the petrol is still burning when the exhaust valve opens. I assume the bike has a magneto. The issue may be that the points cam in the magneto is no longer 180 degrees between the two cylinders. If the timing is set by one cylinder the other may be retarded. The compromise is to check both cylinders and set the timing between the two so the advanced one a little retarded and the retarded one a bit more advanced.
clive
if it ain't broke don't fix
Sams Dad
Posts: 21
Joined: Mon Apr 19, 2021 6:49 pm
Location: Lancs UK

Re: G9 Matchless 1958

Post by Sams Dad »

Thanks everyone for your help. Sorry it's taken so long to reply. Spent yesterday taking the rough bits of the meeting surfaces on both heads 240, 320, 400 and p putt heads back on today. Came to fit the carb manifold and..... One nut kept turning... the stud was snapped. Took off the right hand head again ( luckily copper gasket ) and got out my little left hand drill bit. Go slow it's the trick, but that meant it took longer to drill right through it. Hmmm torch bit tapped gently but like tiki mean it until the drilled hole. It slipped so next torch up and 1/4 drive ratchet and end and voila the attic came out in one! Oh and a bit of penetrating fluid and heat in between. Hope to get it back on tonight, but it's quite cold again now. I think that stuff had been botched up before and that may be why right pot was drawing air to over heat it. I did not put any excess force on it today.
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Duncan
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Location: HAMPSHIRE UK

Re: G9 Matchless 1958

Post by Duncan »

If you weren't already aware the carburettor manifold should be lightly attached when fitting the cylinder heads to ensure they are all square to each other and that there are no gaps where air could entre.
Sams Dad
Posts: 21
Joined: Mon Apr 19, 2021 6:49 pm
Location: Lancs UK

Re: G9 Matchless 1958

Post by Sams Dad »

Hi Duncan,
Yep, that is how little pressure I put on to 'snap' this stud. Looks like it was just caught on a couple of threads! The nut was a bit tight going on then very loose as if the thread had gone, but in fact the stud was spinning. The thread that is left looks battered as well so just been on Amoc spares and ordered a new one. So much for firing up today. Never mind, best to do everything right where I can. Probably be next weekend now.
BruceB
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Location: INVERNESS-SHIRE UK

Re: G9 Matchless 1958

Post by BruceB »

I had a carb mounting stud strip on a G12 after owning the bike and running it for two years without touching it. The bike started running badly out on a run and the stripped stud was what was found when I returned.
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