Carb on a 1952 18S

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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moley
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Location: EAST SUSSEX UK

Carb on a 1952 18S

Post by moley »

Hello, i have restored a 1952 AJS 500 single which i am overwhelmed with the handling and how well the bike goes.

I have only one problem however, i have put a 389 monobloc on it as th original side float is knackered.
i am not sure what size main jet to put in it currently it has a 270 . The bike tickes over fine and rides well but if you go to give a load of throttle it all cuts out and splutters..but gently on the throttle its fine.... i think its the main jet.

i would appreciate any comments.

magic sight


Andy
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clive
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Joined: Mon Jan 01, 1990 12:00 am
Location: LONDON UK

Carb on a 1952 18S

Post by clive »

According to the Roy Bacon bible the 389 carb for standard 18s had the following. Bore 1and5/32" main jet 260, pilot 30, slide 3and1/2, needle position 3 and needle jet .106. The comp model is shown to have jet size 250. Assuming you do not have a filter these should be ok. If it is spluttering on opening up I would look at the slide to make sure it has the correct cut-away ie that it is marked 3 and 1/2. Other possibility of course is a bit of crap below the main jet. Big handle full of throttle and it gets lifted and blocks jet, gradual increase in throttle leaves it at the bottom. Worth checking the trap below the jet and to take the jet out at the same time to check it is properly clear. I have found a piece of very fine wire in mine once. No idea how it got there but performance much improved when removed!
clive
if it ain't broke don't fix
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silverarrow
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Joined: Mon Jan 01, 1990 12:00 am
Location: LEICESTERSHIRE UK

Carb on a 1952 18S

Post by silverarrow »

This carb was std on the 55 on 500 and for that had a 260 main 30 pilot jet & needle position 3.

It sunds lkely that the carb passageways are blocked. Strip it completely Then claen everything and everywhere with proprietry carb cleaner which you can get from auto accessory shops. It worked on my 18s from those symptoms.

If you have an air cleaner fitted you should come down a couple of sizes in main jet - otherwise your set up sounds as if it should be near enough!

Les Smith
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