How to Adjust the Ignition Timing of an Old 2-stroke Engine with the Height of the Piston?

0001b.jpg How to

The ignition timing is a very important element of the engine, as it allows the spark to fly to the spark plug. 4-stroke engines generally use the angle of the crankshaft to indicate the position of the piston before the top dead center, while 2-stroke engines use the height of the piston before the top dead center to determine the ignition timing. How can we know the correct piston position? Here are some tips on how to adjust the plugs to ignite at the proper timing.

Contact breakers only remain on some older motorcycle.

0002.jpg In the 1960s, the 90 to 125cc class was a popular class for young people, and each motorcycle manufacturer introduced a variety of models. Yamaha's HS1, launched in 1968, was a full-fledged sports model with a 90cc two-cylinder engine and a five-speed return transmission.

0003.jpg The Suzuki Wolf 90, which was introduced at the same time, had the same two-cylinder engine as the HS1 and featured a stylish design. Compared to the HS1, the HS1 had a more orthodox style that would never get old. The contact breaker was mounted on the left side of the engine.

It has been about 20 years since carburetors were abolished and fuel injection became the mainstream, but the contact breaker disappeared from the motorcycle world even before that. The contact breaker sends an electric spark to the spark plug at the timing that produces the most power when the mixture explodes and burns, and the electricity to produce the spark is cut off by opening and closing the points.

About 40 years ago when the electric technology developed at a dash, that is, after about 1980, the mechanical contact disappeared, and the non-contact type that decides the ignition timing electrically became mainstream. The maintenance item of the adjustment of the ignition timing has disappeared along with it, and the opportunity to do the point adjustment even for the professional mechanic has become very few.

At present, the point adjustment is required mainly for the out-of-print motorcycles from 1960's to 1970's, and there may be some owners of out-of-print motorcycles who are doing the adjustment without any choice in the popularity of out-of-print motorcycles. However, if you actually perform this point adjustment, you will be able to understand the basic structure of the engine and the mechanism of combustion, such as the fact that the spark to the spark plug is somewhat before the top dead center of compression, and that it is better to accelerate the ignition timing when the engine speed becomes high.

When tuning an injection vehicle by adjusting the fuel and ignition maps, the vehicle's computer is accessed from the outside to increase or decrease the injection time of the injectors and to input the desired ignition timing. The process of inputting the information by tapping the keyboard itself is digital, but the process of spark flying to the spark plug at the set ignition timing and the mixture spreading in the combustion chamber is the same for both contact breaker ignition and non-contact digital ignition.

In this respect, it is useful to know the analog type contact breaker, in which the moment when the point contact opens mechanically according to the angle of the crankshaft can be seen, in order to understand the contactless ignition that has followed since then.

  • Point 1: Only some vehicles such as out-of-print motorcycles and old motorcycles before 1970s ignite spark plugs by contact breaker.
  • Point 2: Ignition map tuning for fuel-injected vehicles is the same in theory as contact breaker position adjustment.

Ignition timing for 2-stroke engines is determined by the height of the piston before top dead center.

0004.jpg The ignition timing given in the maintenance manuals of the time is indicated by the distance from top dead center, not the angle of the crankshaft. 1 before top dead center. The ignition timing of around 8mm is a value that many models, regardless of displacement, have adopted close to this.

0005b.jpg The magnet base is stuck to the down tube of the frame, and the measuring element of the dial gauge is inserted from the plughole, and the ignition timing is adjusted. It is not necessary to measure all 43mm of the piston stroke, it is good if you can measure from 5mm before top dead center.

0006.jpg When the piston reaches top dead center, the dial gauge is set to zero, and since there is no cam chain like a 4-stroke, the 2-stroke can be rotated in either forward or reverse. After repeating the measurement several times, the reference zero point is determined.

0007.jpg As the piston is moved back before top dead center, the dial gauge scale turns counterclockwise. This gauge is 1mm per revolution, so when the piston is lowered to 180 on the small scale, it is 1.5 mm before top dead center..8 mm.

The ignition timing of 4 stroke is determined by the angle of crankshaft. In the case of Kawasaki Z1, the ignition timing at idling is 20 degrees before top dead center, and when the rpm rises to 2350rpm, it becomes 40 degrees before top dead center. the mechanism of the ignition timing advance from 20 degrees to 40 degrees is introduced in the article of spark advancer before.

In contrast, the ignition timing of a two-stroke engine is traditionally determined by the height of the piston. When the piston rises from bottom dead center to top dead center, the points are usually set to open when the piston is within a few millimeters of top dead center. If the position of the piston is known, it should be possible to replace it with the angle of the crankshaft, but somehow it is set by the height of the piston.

The way to know the piston position is very simple, you put the dial gauge on the top of the piston and rotate the crankshaft backwards. In the case of the 4-stroke, a full circle protractor is mounted on the end of the crankshaft, and the top dead center is determined by the scale of the protractor. However, if you use the 2-stroke system where the dial gauge is placed directly on the piston top, there is no way to make a mistake.

The reason why 2-stroke can determine ignition timing in this way is that there is no intake and exhaust valve on the piston like 4-stroke. If you remove it, it is obvious that the cylinder head of 2-stroke has such a simple structure that it can be said to be the lid of the piston, and the measuring instrument of the dial gauge can reach easily. In Yamaha, the dial gauge holder which is used by screwing in the plughole as a special tool has been set up for a long time, and it can be seen that the ignition timing adjustment in the contact breaker age was very natural to adjust the height.

The same work is possible if there is a general dial gauge and a magnetic stand though the ignition timing can be easily adjusted if this special tool is used. All you have to do is to stick the stand on the place where magnet can be attached such as frame or iron cylinder, insert the measuring element of dial gauge through the plughole and make contact with the piston top. When the piston reaches and passes the top dead center, the needle of the gauge will move in the opposite direction, so you can reverse the crankshaft to the top dead center just before it rises and falls.

In the case of the 90cc two-cylinder Yamaha HS1 engine from the 1960s used in the sample, the ignition timing is 1.5 seconds before top dead center..8±0.It is set at 15mm, so adjust the contact breaker so that it opens when the piston drops to that position.

  • Point 1: 4-stroke ignition timing is determined by the angle of the crankshaft, whereas 2-stroke ignition timing is determined by the distance from top dead center to the top of the piston.
  • Point 2: Insert the dial gauge through the plughole, and check the manual for the amount of drop from the top position to the front of the upper dead center, as it varies depending on the model.

Adjust the timing so that the points open the moment the ignition timing is reached.

0008.jpg When the lead wire of the tester is applied to the spring of the contact breaker and the crankcase, and the crankshaft is turned in the resistance measurement mode, the resistance value changes at the moment when the contacts are separated. This change is 1.5 times before piston top dead center..8±0.Adjust the mounting position of the contact breaker so that it occurs at 15mm.

0009.jpg There is a small knurled line on the washer under the bolt at the end of the crankshaft, and there is also a knurled line on the crankcase side, but the alignment position of the case side is screwed with a long hole, so it can be shifted anywhere. I don't know how to handle this alignment because it is not described in the manual at that time, but it seems to be good to match the timing of the point contact opening and to align the case side with the crankshaft tick line at that time.

The timing for the contact breaker points to open is determined by making fine adjustments to the position of the breaker body and the position of the piston. Specifically, the crankshaft should be rotated forward and backward at a point 1.8mm before top dead center, so as not to miss the moment when the closed point is about to open.

The plus/minus range of 0.15mm is severe, but after turning the crankshaft many times, you will be able to grasp the point opening and closing timing quite severely. However, it is important to make sure that there is no rattle or wear on the shaft of the contact breaker or the point heel in contact with the crankshaft so that the points can always be opened and closed under the same conditions.

If you are worried about visual inspection alone, you can also utilize a circuit tester to determine the ignition timing. The HS1 has two cylinders and two sets of contact breakers, so the contact points should open at 1.8±0.15mm before top dead center on both the left and right pistons. Since the HS1 has two cylinders and two sets of contact breakers, the contact should be adjusted so that both pistons open at 1.8±0.15mm before top dead center.

After adjustment, start the engine and use the strobe light from the timing light to check if the ignition timing is correct. However, the HS1 has a long hole plate on the engine side that corresponds to the alignment mark on the end of the crankshaft, and this plate can move at will, making the timing light unreliable. So I used a tester to set the timing for the contact to open at 1.8±0.15mm before top dead center. Then, you can set a reference by aligning the crankshaft alignment mark with the tick line on the engine side plate.

Among the two-stroke motorcycles that have become a minority now, the point ignition motorcycle is a rare character with even fewer numbers, but the understanding and interest in the engine and ignition system should be deepened further by knowing how the ignition timing to the mixture is decided.

  • Point 1: The moment when the contact breaker opens is found by turning the crankshaft around the ignition timing.
  • Point 2: Make sure that there is no rattle or uneven wear on the shaft or point heel of the contact breaker so that the contacts always open and close at the same time.
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