Poor tickover and low throttle performance

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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sjr
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Poor tickover and low throttle performance

Post by sjr »

I recently acquired a nice 1960 31DL through Kettering which I am thoroughly enjoying. Gradually getting it right for me, but one thing I have not yet been able to sort is the lumpy tickover. The bike had a number of peculiarities which lead me to think that it is a bitsa, (CSR footrests which interfered with the touring pipes and defo not in a CSR frame, strange not-in-series engine number, non-QD rear wheel but with QD type chainguard, Matchless steering damper, and so on). I wonder whether the (undoubtedly rebuilt) engine has got a lumpy camshaft in it. Or whether, as some of the tech notes in the Danish archive suggest, the poor tickover may be down to obstruction in the inlet tract disturbing the fuel atomisation.

One of the symptoms given in the tech notes seems to fit: if there is an obstruction in the inlet manifold, particularly the lower half of the tract where the slow running mixture flows past after it comes out of the pilot orifice in the carb choke, it upsets the atomisation so much that sensitivity to setting the slow running adjustment screw is much reduced. Mine is so insensitive that I can screw it right in and the tickover is unchanged, so long as I raise the throttle stop enough of course. This obstruction can be as minor as the frayed inner edge of a fibre gasket, and the tech notes also say that a smooth join between carb choke and manifold inlet is not necessary so long as the floor of the tract steps down rather than up.

I already looked at the inside of the manifold and it is rough. So I'm playing with a spare, filing out the rough cast finish inside the tract. Should I try to make it a perfect match with the carb choke, what do you think? Or should it be enough just to make sure there is no obstruction in the sense of an upward step?

Thanks for your thoughts
Steve
TrevorCSR
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Re: Poor tickover and low throttle performance

Post by TrevorCSR »

Is the carb a Monobloc or Concentric? Either one I would start by looking very carefully at it. If there is no difference when you adjust the pilot screw it points to a blockage in the pilot jet or internal drillings of the carb body. Does it start reasonable well? Is the slide worn? Check the slide bore as well. New carburettors are not a lot of money.
Last edited by TrevorCSR on Thu Feb 16, 2017 10:55 pm, edited 1 time in total.
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sjr
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Re: Poor tickover and low throttle performance

Post by sjr »

Thanks Trevor, its a monobloc. I replaced the pilot jet (and all the other jets) with new and blew out all the passages with compressed air while the carb was completely dismantled
TrevorCSR
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Re: Poor tickover and low throttle performance

Post by TrevorCSR »

Did you get air flow through the very small hole on the inlet side of the slide? At the bottom of the inlet just after the slide.
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clanger9
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Re: Poor tickover and low throttle performance

Post by clanger9 »

Sounds like a worn carb slide. Any wear here will cause poor idle and difficulty in setting the mixture.
If it's worn, buy a new carb (or get Martyn Bratby to overhaul it).
1989 Moto Morini Dart 350
1993 Ducati 750SS
raffles
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Re: Poor tickover and low throttle performance

Post by raffles »

Try looking at the paper washer under the jet block. Fit a new one and ensure that the jet block is correctly fitted.
Screw the pilot jet adjuster screw all the way in , then out three half turns. then give it kick
Tony
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Duncan
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Re: Poor tickover and low throttle performance

Post by Duncan »

Steve

Do you have a magneto or distributor, have you discounted the advance / retard?
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Re: Poor tickover and low throttle performance

Post by raffles »

Always try the simple things first
Tony
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