Poor tickover and low throttle performance
Posted: Thu Feb 16, 2017 8:43 pm
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
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