carburation - off/on

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
StephenG80
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Location: ARGYLL UK

carburation - off/on

Post by StephenG80 »

Hi - having only ever ridden my own g80s ( 1954 model), I am not convinced the carburation is set up quite right but have nothing to compare it with apart from my RE '52 Model G from a good few years ago. She is running a new monobloc from spares (jetted for a '54 G80). She starts, accelerates, if you are cruising at 60 mph and grab a serious handful she'll sweep pass 70mph very quickly :rofl: , spark plug is good colour, no popping when closing throttle off and very strong engine breaking BUT, if trying to cruise in 3rd or top at say 30mph through to 55mph - you can't feather the throttle at all without the engine "stuttering" and not running smoothly - its fine if opening or closing the throttle. So I have to run the throttle either on or off which is a bit tiring. I am delighted with the overall performance but would like to iron out this last annoying characteristic :headbang: . Is this normal? - she was running a little lean at higher speed so needle is on the highest setting. Maybe I need to go up a main jet size? :?:
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1608
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Re: carburation - off/on

Post by 1608 »

If trying to cruise btn 30 and 55 I doubt the main jet is being used. I'd suggest that the mixture is a bit too rich, despite the plug colour. Try lowering the needle a notch.
StephenG80
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Re: carburation - off/on

Post by StephenG80 »

Thanks for that suggestion 1608 - I am not dishing your advice but curious to know why the symptom I described leads you to think it is down to too rich a mixture - ie what is going on? - I just like to fully understand these things, so any further detail as to why you have drawn that conclusion would be very welcome.
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ajscomboman
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Re: carburation - off/on

Post by ajscomboman »

Basically there are 4 phases, pilot jet which controls idle mixture, the slide which comes into effect from idle to 1/4 throttle, the needle and needle jet which controls mixture from 1/4 to 3/4 throttle and then finally the main jet which controls mixture from 3/4 to full.
In 3rd gear at speeds of 30-50mph you'll be in the range of going from slide onto needle/needle jet hence 1608's suggestion of lowering the needle.
StephenG80
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Location: ARGYLL UK

Re: carburation - off/on

Post by StephenG80 »

I know lowering the needle is a solution because the mixture is too rich at present BUT why does a rich mixture cause the stuttering at feathering - my brain needs to know why :headbang: - not disputing the advice just like to understand these things :rofl: - anyone?
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1608
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Re: carburation - off/on

Post by 1608 »

Very simply, the road speed does not correspond with the amount of fuel being delivered.
StephenG80
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Re: carburation - off/on

Post by StephenG80 »

well evidently - but why does too much fuel give this characteristic, what is actually happening in the combustion chamber when too much fuel is present and why does it make the engine splutter when its being feathered?
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ajscomboman
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Re: carburation - off/on

Post by ajscomboman »

Why does it matter? Just adjust the carb and get on with riding. If you're that keen to find out about the principles of combustion just google it, it's all there.
Mick D
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Re: carburation - off/on

Post by Mick D »

Hi

Are you saying that the engine will not accelerate smoothly when the throttle is opened slowly? - if this is the case it could also be a timing issue.

Do you have auto advance? in which case I would suggest the mechanism is sticking slightly.

If your bike has a manual advance I'd check that it can achieve the full advance position, (the points not the lever ;) .

Regards Mick
Trefor
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Re: carburation - off/on

Post by Trefor »

I agree with Mick, check out the ignition system. I recently cured what I thought was a carb problem with an improved earth. They say 90% of carb problems are ignition. Cheers.
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