Roadholder 370 - October 2018

24 Alternative engines (Pt.2) Mark Woodward is 0.0985". That gives you a 'doughnut', or delta area, that is only 0.00325" wide or about the thickness of a human hair. To put this in perspective an engine that has the correct air/ fuel mixture with a 0.106" needle jet would be too lean with a 0.105" or too rich with a 0.107" needle jet.' I calculate the delta area referred to as 0.00375" but the argument still stands. Jim Bush's, 'Bushman’s Carb Tuning Secrets' - Nov. 2015 also has a useful section on needle jets. It's also worth checking that the needle is straight by rolling it on a piece of plate glass, bearing in mind that the needle is tapered. If you're having any work done on the carburettors, make sure that the needle jets are checked for wear. Patrick had been told by a Classic bike specialist that the 'carb adjustment had reached its limit' which is sort of true. No amount of changing the needle clip position or adjusting the pilot air screw would have compensated for such a badly worn jet. However, such a statement could lead to unnecessary expense rather than simply replacing a needle jet at a cost of around £6 each. In summary, if the bike is running rich, I recommend checking the following before moving on to other possibilities, e.g. fuel level in the float bowl: 1. Check choke operation and that the cable is not broken. 2. Remove the carburettor and check that the needle and clip are correctly located. 3. Check the state of wear of the needle jet by removing and comparing with a new one, or even better, measuring with a 2.7mm drill bit (for the standard 0.106" jet). ----------------------------------- Conclusion I've tried to provide a few pointers here for new owners and restorers - I appreciate that much of this will be 'old hat' to seasoned NOC members. All the references listed at the start of this document are well worth a read if you're planning to tune or recondition AMAL Mk.1 Concentrics. Appeal for information: If any of you have had the 'moving clip and needle problem', please get in touch with me via a NOC message email (Andrew_ Heathwood). Any photographs of the problem would be appreciated. I would like to get in touch with AMAL to suggest some improvements to the design to prevent this issue, but will only do so if this really is a widespread problem. Having only ever laid a spanner on two Concentric-equipped Norton Commandos, 'two out of two' with the problem suggests that this might be the case. Acknowledgements: The use of material from John Healy’s document and the Britbike Forum is acknowledged, as is the loan of the worn pilot jet by fellow NOC member, Patrick Bapty, who also allowed me to work on his bike. © Andy Heathwood July 2018 L ast time I looked at alternatives to the 'poppet' valves used in most modern internal combustion engines. Some of these alternatives were purported to produce more power than poppet-valved motors and a little better efficiency. One type of engine that was aimed purely at fuel-efficiency was first proposed by James Atkinson in 1882. The basic mode of operation was similar to the Otto four stroke cycle of 'suck - squeeze - bang - blow' however, by the clever design of the engine crank assembly, the engine had a different induction volume to combustion volume, i.e. it drew in a small volume of fuel mixture but retained the same power stroke of a 'normal' engine. Atkinson actually developed a number of differing designs. One idea combined all four cycles of the Otto design into one revolution of the engine crank, another operated in a two-stroke fashion but one common feature of all of the designs was the difference in length between the