57 500 CS Jetting

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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silverfoxracing
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57 500 CS Jetting

Post by silverfoxracing »

This is all new to me so be gentle!! When I bought the bike it was running on Methanol, but trying to put it back on petrol. Timing set at 11.2mm btdc. It has a Monobloc 389/12 carb on it. I put a 30 pilot, 260 main, clip in the middle 3.5 slide,106 needle jet. bike starts fine, so happy there, Pulls well of the bottom but then just revs and not goes anywhere, Did a plug check and its white!!!. I will lift the needle to the bottom clip, but wondering what size main to go to. I am reading some are running up to 400/440 mains. I am at close to sea level. just about to order main jets but don't want to order to many!!! alloy head with the short reach plug and I believe comp cams in it, but no high comp piston. Anybody able to give me a ball park main jet size please.
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Pharisee
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Re: 57 500 CS Jetting

Post by Pharisee »

My '56 G80CS is running a 1.3/16" Monobloc (389/12) with a 30 pilot jet, 440 main jet, a 106 needle jet and a No. 3 slide. Living in the Lincolnshire Fens, my house is actually a couple of metres below sea level.

Generally, engines running alcohol fuel have a much higher compression ratio... 13:1 not uncommon, and a main jet you can drive a bus through. Any idea what yours is?
John
I'm from the Fens.... Gimme six.
SPRIDDLER
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Re: 57 500 CS Jetting

Post by SPRIDDLER »

Pharisee wrote: Fri Jul 15, 2022 9:30 am My '56 G80CS is running a 1.3/16" Monobloc (389/12) with a 30 pilot jet, 440 main jet, a 106 needle jet and a No. 3 slide.
That's also the same set-up as shown in the Supplementary Info Manual for 1957 competition models (plus other helpful Comp specific info) in the link here:

http://archives.jampot.dk/book/Owners_m ... models.pdf

Here's an extract from the guide......

1957 CS Owners manual.JPG

The manual also mentions that is for an open zorst pipe. However, Pharisee's works, presumably with a silencer.

1957 CS jets. Open pipe.JPG
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dave16mct
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Re: 57 500 CS Jetting

Post by dave16mct »

That's how I jetted mine, seems OK. Funny though, I've a mate who has a '63 G80CS and he runs his with a much smaller main jet. I think he said 260. I can't understand why but it seems OK for him. Both ridden on the road with silencers.
Dave.
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Samuel
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Re: 57 500 CS Jetting

Post by Samuel »

dave16mct wrote: Fri Jul 15, 2022 11:55 am That's how I jetted mine, seems OK. Funny though, I've a mate who has a '63 G80CS and he runs his with a much smaller main jet. I think he said 260. I can't understand why but it seems OK for him. Both ridden on the road with silencers.
Dave.
Maybe he is running on a concentric? - they use lower jet sizes for similar engines.
Sam
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dave16mct
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Re: 57 500 CS Jetting

Post by dave16mct »

No it's a monobloc:
Image
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silverfoxracing
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Re: 57 500 CS Jetting

Post by silverfoxracing »

Pharisee wrote: Fri Jul 15, 2022 9:30 am My '56 G80CS is running a 1.3/16" Monobloc (389/12) with a 30 pilot jet, 440 main jet, a 106 needle jet and a No. 3 slide. Living in the Lincolnshire Fens, my house is actually a couple of metres below sea level.

Generally, engines running alcohol fuel have a much higher compression ratio... 13:1 not uncommon, and a main jet you can drive a bus through. Any idea what yours is?
John
Prev owner says it did not have a hi comp piston. Main Jet I pulled out was a 1200!!!!! yes 1200 for methanol. Will order from 400 up mains. Thanks gents
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dave16mct
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Re: 57 500 CS Jetting

Post by dave16mct »

This is mine.

Image

The frame is '58 and just over 200 numbers after yours. Engine is '57.
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silverfoxracing
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Re: 57 500 CS Jetting

Post by silverfoxracing »

That sir is mint!!!!
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