"How to" video for adjusting the points on a vintage 2-stroke Yamaha motorcycle using a dial gauge.
This motorcycle is a 1967 Yamaha YCS1c, with a 180cc motor.
Factory points timing is 1.8mm (0.0709") Before Top Dead Center (BTDC), but I am setting mine slightly more advanced at 2.0mm (0.0789") to accommodate a few other performance modifications I have made to the engine.
I believe that the procedure is more or less the same for all of these old bikes.
Here is the short version of how to do it:
1. Find TDC, and make that "0" on your dial gauge.
2. Use the gauge to precisely measure 0.0789" of piston travel before TDC.
(The engine rotates counter-clockwise.)
3. Make your points open at exactly that moment.
I was asked a good question about the extraneous rotations of the needle in the gauge.
For my answer I will assume you have a SAE dial gauge in inches, as opposed to a metric one in millimeters.
First of all lets discuss the spinning full revolutions of the needle. We'll assume the dial gauge is secured in the spark plug adaptor, and the adaptor is securely threaded into the spark plug hole so there is no wiggling, etc. Any wiggle movement of these pieces will prevent accurate measurements. On a standard non-metric "0-100" dial gauge, like the one I used in my video, each full rotation of the needle represents 1/10th of an inch of the gauge's plunger's vertical travel. If the dial gauge is sitting more deeply in the adaptor, then the gauge's plunger will come in contact with the piston earlier, causing more needle travel. If the gauge is sitting more shallowly in the adaptor, then the plunger will encounter the piston later and the needle will have less travel. The depth which the gauge sits in the adaptor can be adjusted to reduce needle rotations -- exactly 1/10" of depth for each rotation of the needle. The number of needle rotations is ultimately irrelevant to the measurement of the piston once the tool is calibrated for the job with the gauge set to zero at TDC. However I see how the extraneous rotations are definitely distracting and confusing.
An additional critical detail is that it is important to remember these old Yamaha's engines rotate COUNTER CLOCKWISE while running. The dial gauge does not indicate whether you are Before or After TDC -- it only shows the vertical distance that piston has traveled up or down from TDC -- it is important to measure to the point that is BEFORE top dead center, as the piston is traveling up and the engine is rotating counter clockwise. The gauge can be potentially very confusing because it's needle rotates as then engine is rotating, but the gauge does not indicate direction of engine rotation! The gauge only shows the vertical travel of the piston.
...Assuming I want my timing set to 2mm/ 0.0789" and I have the dial gauge correctly installed and set so TDC is indicated at zero. I want the points to be closed as I manually rotate the engine counterclockwise, the piston moving up toward top dead center, and then for the points to open exactly at 2mm/ 0.0789" before top dead center. Again, the engine will be rotating counterclockwise, but my gauge will be rotating clockwise as it approaches 0.0789", 'counting up' toward my indicated zero.
If the gauge's needle has to make several rotations as it approaches TDC, make mental note of how many rotations it makes. Or one can adjust the height of the dial gauge in the adaptor so that there is a total of less than 1/10 of an inch of plunger travel so there are no extraneous rotations. Be sure to re-zero the gauge after adjusting it in the adaptor.
Hope this helps.
-Steve
30 сен 2024
