Carb jetting?

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Chazzyb
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Carb jetting?

Post by Chazzyb »

My bike needs a reset. I have a new Wassell Evolution 930 carb (AMAL Concentric clone) (thanks Spridddler) to replace the rather worn old Concentric my 18S has been fitted with forever.

What sort of jetting do people use with Concentrics and modern fuel on their 500 singles?

I would also ask what oil I should use, but that's another matter. :lol:
Charles
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clive
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Re: Carb jetting?

Post by clive »

SAE 50-20-50 of course Charles, although its difficult to find.
clive
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MikeM.
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Re: Carb jetting?

Post by MikeM. »

Crikey Charles, you mean that after all the years you've had a Model 18 you don't know what oil to use. Have you never done an ol change? MikeM..
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Re: Carb jetting?

Post by SPRIDDLER »

clive wrote: Fri Aug 12, 2022 11:01 pm SAE 50-20-50 of course Charles, although its difficult to find.
Sprid's tip of the day..........

If you buy multigrade 20/50 just pour about the top two thirds of the can into your oil tank so that you leave the heavier 50 grade behind.
It's my belief that because the heavier 50 grade always settles out to the lowest point it's the main cause of wet sumping in an engine left standing for a while.
The remaining 50 grade isn't wasted as you can use it in the gearbox.
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Chazzyb
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Re: Carb jetting?

Post by Chazzyb »

MikeM. wrote: Sat Aug 13, 2022 12:09 am Crikey Charles, you mean that after all the years you've had a Model 18 you don't know what oil to use. Have you never done an ol change? MikeM..
No need to change the oil Mike. I just top it up and it comes out the bottom somewhere (everywhere). :rofl:

To be fair, The only leaks currently are from the gearbox inner cover and swinging arm.
Charles
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Chazzyb
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Re: Carb jetting?

Post by Chazzyb »

SPRIDDLER wrote: Sat Aug 13, 2022 12:10 pm
clive wrote: Fri Aug 12, 2022 11:01 pm SAE 50-20-50 of course Charles, although its difficult to find.
Sprid's tip of the day..........

If you buy multigrade 20/50 just pour about the top two thirds of the can into your oil tank so that you leave the heavier 50 grade behind.
It's my belief that because the heavier 50 grade always settles out to the lowest point it's the main cause of wet sumping in an engine left standing for a while.
The remaining 50 grade isn't wasted as you can use it in the gearbox.
Now then, I think you have it wrong there. The 50 grade oil is actually less dense than the 20W bit, so it floats to the top! It's true; a bloke down the pub told me.
Charles
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Re: Carb jetting?

Post by Andy G »

Unless you're in the Southern hemisphere of course
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Re: Carb jetting?

Post by ajs-matchless »

Hi, coming back to the carburation, my Amal MK I concentric 928/300 carb refuses to go leaner in the pick up from idle section. Reduced to 100 main jet and the needle down, when accelerating it feels as if I have to pull the engine out of a hole and little smoke occurs in the process. Any advise on this would be appreciated. Thanks
AJSingleDave18
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Re: Carb jetting?

Post by AJSingleDave18 »

Hi Charles.

Sorry if my response is too late but I’ve only just found your request for jet sizes.

CARB JETTING
I replaced a very worn out AMAL Monobloc with a WASSAL Premium Concentric MK1. But as I couldn’t find any data on the correct jetting I had to determine them myself by trial and error. Initially this was very confusing as the float was too light, floating high above the fuel and marginally catching the main body above thus giving random fuel levels. However once I’d found this out and sorted the problem the jet development was fairly straightforward.

The jetting I established is as follows. NOTE this for my 1960 AJS 18 (500 single) with an aircleaner fitted, but otherwise standard.

These values may not be quite ideal for earlier and later models with less and more power.

Main Jet 210
Slide Cutaway 4
Needle Jet 105
Needle Clip in the centre grove.

I have done about 5000 miles with these settings and during this time I have experimented with alternative jets to double check my results and conceded the above setting are correct. I get 70 to 85mpg, with raise gearing I get easy cruising at 60mph and max cruise speed of 70mph.

OIL GRADE
SAE 20/50 has the viscosity of straight SAE 20 oil when cold and the viscosity of SAE 50 oil when hot. This means the oil is far more runny when cold so far more likely to cause wet sumping. I use SAE 50 in my bike and it is often left standing for weeks without any problems. SAE 50 is what the bike was designed for.

Regards David
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Re: Carb jetting?

Post by AJSingleDave18 »

Charles,

Further to my previous jet size information I forgot to mention that with increasing the ethanol content of petrol from 5% to 10% caused a loss of power at ¾ throttle on steep climbs. This was corrected by increasing the main jet from 200 to 210.

David.
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