I have been running my 1929 2 speed Scott for a couple of monthes now and have noticed it starts first kick every time when cold.Even when the engine is stopped it will also start first kick, but leave the bike for ten minutes or so, nothing.
Ive noticed some other members have had similar problems, but mine only gives trouble after a period of time.
The magneto was build last year and produces a healthy spark under compression
Has anyone got any ideas
Carburettors make quite good refigerators, not too amazing if you stop to think about it, they both work the same way, both turn a liquid into as gas. In the case of a carburettor it is a matter of more or less which might be the root of the problem.
As you turn a liquid into a gas you are faced with Boyle’s Law, (P1 . T1 . V1 = P2 . T2 . V2), (at the risk of teaching a lot of Grannies to suck on eggs). It simply says that if you have a quantity (V) of a fluid at a certain temperature (T) and under a particular pressure (P) and you change one then the others have to change too. Simply put both sides of the equation have to remain equal.
Where does this get us? Well, as the engine sucks petrol and the carburettor vapourizes it the volume increases so the temperature of the petrol vapour must drop, despite global warming, you are not going to have too much effect on atmospheric pressure so temperature is the only game in town! In fact a good deal of the petrol that gets into an engine gets there by running in rivulets along the walls of the induction system and not as a vapour at all. It improves as the enging reaches full working temperature but does not entirely stop. Don’t beleive me? There is plenty of research and film to back me up. It also goes some way to explain why fuel injection is worth the effort.
So what has this to do with Arron’s engine? Well to get the brute to fire at all the mixture has to be right. With a cold engine a bit of tickling and choke compensates for all the liquid fuel that is not going to burn. (Liquid petrol does not burn, it is the vapour that is inflamable. Sounds odd but true.) When the hot engine is stopped and then started right away the excess drops of fuel that is hanging around in odd corners has not had time to evaporate and richen the mixture beyond the useable stoichiometric ratio. If however the enging is left for a little time so that all the liquid in all the nooks and crannies has time to evaporate then you will have an over rich mixture and no start.
Is there a cure? Short of a re-design of the engine not a lot! If there is room then a thick fibre spacer between the carburettor and the engine it would reduce heat conduction from the engine to the carburettor which in turn will reduce vapourization in the carburettor. There is after all quite a bit of liquid petrol in a carburettor and even hand temperature will markedly increase the rate of evaporation let alone a hot engine. It is worth noting that the carburettor, at least on my lump, is moderately downdraught, petrol vapour is heavier than air and there is nothing to stop vapour coming up the jet tube into the carburettor choke.
The down side of a fibre spacer is that it might result in carburettor freezing but the experiment might be worthwhile as any change in the engine’s manners will tell a story.
It would be wise to check the fuel height and the effiency of the float needle.
Also turning off the fuel well before intending to stop will reduce the fuel height in the carburettor which should help.
Try draining out the crankcase oil wells with a hot engine, a petrol/oil emulsion would be an ideal reservoir.
I can’t help but think this is not the end of the story though…
When I park my Scott for anything up to a couple of hours or so, I always rev the engine whilst closing the air lever and hit the mag cut-out button.
Nice rich mixture in the crankcase. Starts first kick every time (well nearly always!).
My 1929 2 Speeder needs a flood from cold and or 7 kicks to start. This is typically after it has been standing for 3 weeks or so. When warm or hot it always starts 1st kick without any need for tickling. I wonder if your mixture is on the rich side? What carburettor do you have? I use an AMAL type 6 with the same settings as a 3 speed Flying Squirrel. I do not use any special stopping procedure. I just close the throttle and use the ingnition cut out to stop the engine.
The float chamber level is always worth checking. Does your engine idle smoothly when warm?
Having seen this post about engine starting, or the lack of it, and near burst me lungs pushing at times myself, I thought the following might amuze, if you can’t be bothered go to the end… This is item is of no help at all, not ment to be, but then wattaya expect?
The carburation of petrol is really a very simple matter. Petrol is a trade name for a highly volatile spirit or distillation of petroleum, and differs only in degree from benzene or benzole, being recognized by chemists as of the “Methane” series; but by whatever name you call it, you may be pretty sure you have the right thing if the specific gravity does not exceed 0.68. The density, or S.G of the spirit, is easily tested with a densimeter and as petrol of 0.68 S.G. vaporizes very quickly at all ordinary temperatures down to 59 degs. F., (which is the temperature at which the density of the petrol should read 0.68). The reading or the density varies 0.45 of a degree for every degree of temperature. Thus, in very cold weather the density would read heavier than on a warm day.
There is this peculiarity of petrol when vaporized in a surface carburettor or otherwise exposed to air, that the lighter and more volatile constituents of the spirit are used up first, so that the S.G. of the residue is constantly increasing. It will be found that any petrol remaining in a vessel, which is not absolutely air-tight, will, alter some twenty-four hours, read about 0.70, instead of 0.68. So for this reason it is well to fill the carburettor only sufficiently to accomplish the journey in hand the quantity required being soon found by experience, for when the S.G. drops to 0.70 it will be with difficulty that the motor can be made to start, and this only in warm weather, or by previously heating the petrol tank. Old petrol will not mix with new—i.e. if the density is found too great, the addition of fresh petrol does not remedy matters, the engine simply uses up the fresh petrol, which floats on the top, and then stops. The vapor from the petrol is in itself not explosive, it must be mixed with a certain proportion of air. The exact proportion varying with the temperature of the atmosphere, but a mean quantity would be about 8 of air to l of vapour, the training of the ear to the sound of the explosions in the engine being the surest guide as to when the mixture is correct. With fresh petrol, which is very volatile, a much larger proportion of air will give effective explosions, but as the more volatile spirit is used up and the S.G. increases and at the same time the air space in the carburettor is increased, it becomes necessary to alter the adjustment valve and admit less air to the mixing chamber.
Why should this be of some interest to all “puffers and panters”? Because there is nothing new under the sun, this little gem was extracted from a series of articles about home building a motorcycle. And I do mean building, engine and frame, all of it! So you have joined a long and noble list! Why am I sniggering? Because my pile of bits is a long way from the starting stage, but maybe not so far from the kicking…
Have a go at guessing the date this was published. I’ll make a nice cherry & walnut cake as a prize for the closest guess.