TIMING TALES
Mark Haycock with part two.
Last time we looked at what ignition timing is about, and why it is necessary to arrange for spark advance, and now I am moving on to look at how to set it up on a two-stroke engine. Before I start I should point out that this engine is perhaps a little removed from the type we would normally deal with, but it is the only two-stroke I currently have and we are just looking at basic principles so here goes! The quantity of spark advance can be expressed in two ways. Normally, it is shown as a number of degrees of crankshaft rotation before top dead centre (BTDC) but often on two-strokes it is instead as a linear measurement of piston position BTDC. This sounds easier to measure than degrees but that is not so I am afraid, and unfortunately many high performance two-strokes are particularly sensitive to the timing so it does have to be exactly right. Worse, the manufacturers’ timing marks found on some old engines are not very reliable. My engine is not very sensitive! In fact, the official instruction is simply to set the contact breaker points gap with a feeler gauge, set the timing in the middle of the range available… and that is it. Yes, very simple but on my engine it did not work. I could tell this because the engine was trying to force the kickstart up as I was starting (well attempting to start) the engine. This is a sign that the timing is too advanced. The actual setting for the timing is 1.6mm BTDC, and a little thought will enable you to come to the conclusion that this is not going to be measured with a steel rule. I suppose you might be able to use the depth gauge on the end of a Vernier (or digital) caliper after removing the spark plug but there is a more accurate way. We need to use a clock gauge – properly called a dial test indicator (Photo 1) which has a spring loaded plunger operating a needle which rotates round a dial somewhat like a clock. This will allow you to measure down to 0.01mm with reasonable accuracy. These used to be quite expensive precision instruments but I have seen that now they can be found for next to nothing – less than a fiver, making it a bit of a bargain. You will need to hold the gauge vertically in the cylinder, in line with the piston’s movement and fortunately most two-strokes have the plug in the middle of the cylinder head, pointing vertically down. If you feel like a little challenge, you can make something to hold the gauge in the correct place from an old spark plug. You grind off the side electrode and bash out the ceramic insulator (with eye protection I should add), then drill out the plug body such that the gauge is held by it when you screw the assembly in place. If you don’t feel like playing around like that, you can buy a ready-made tool instead for less than twenty quid. Unfortunately, my test engine is not fitted with a vertically-mounted spark plug
so I had to remove the cylinder head (Photo 2). This was not too arduous: just four nuts – and no, that stud was not meant to unscrew instead of the nut! I have used a flexible arm mounted to a magnetic base here to hold the gauge, pointing directly down in line with the cylinder (Photo 3). All you need to do is to find the point at which the piston is (for me) 1.6mm before TDC and the simplest way is to turn the crank until the needle on the gauge stops (that is TDC) then turn it back until the needle moves through 1.6mm. Each turn of the needle is 1mm so it is not too hard to work out. If you are fussy like me, you will ensure that you always turn the crank forwards to that point as then you do not introduce inaccuracies caused by bearing clearances, but that is harder. The check, then, is to ensure that the contact breaker points separate at precisely the point we have measured. It is not accurate enough to just make a visual check. In the past it could be found by attempting to pull out a cigarette (i.e. very thin) paper from between the points: the paper is released just as the points separate. A more modern approach is to use a multimeter, set to measure ohms. We just need to connect the leads to each side of the points, but not necessarily directly on the points themselves. So
I have connected one to the points terminal (Photo 4) and the other to an earth (Photo 5). The separation is indicated by the reading changing from practically zero (Photo 6) – it is actually showing the resistance of the leads here – to infinity which my instrument displays as O.L MΩ (Photo 7). You will probably find that the points separate before or after the measurement so you will need to adjust the timing
slightly and try, try again until it is right. You can, as an alternative, use a different approach to the measurement of piston position by converting the linear measurement into degrees before TDC. However, you will need to know one particular measurement which you will not find in the workshop manual. It is the effective length of the con rod: that is the distance between the centres of the small end and big end bearings. There is a rather complex algebraic formula to convert the distance BTDC to degrees but you will find it easier to use a ready-made tool such as that found at www.scooterhelp.com/tips/timing/timing. calc.html which allows you to convert either way. Alternatively, you could create a scale drawing and measure the angle with a protractor. You will need a degree disk to measure the angle you have found but instead of buying one you can make your own by visiting www.stonefisk.com/ doc-arc/engine_timing_disk.pdf (Photo 8). You can find the position of TDC with your dial gauge and fixing up a pointer to align with the zero on the disk. More convenient would be to make your own permanent timing mark on the rotor for future use.
Next month I shall look at the process on a typical Japanese four-stroke.