Want to set the points dwell on you bike but don't want to fork out for an antique dwell meter? Well read on...
First; what is dwell? Dwell is a measure of the amount of time the ignition coil is being charged. It's not measured as a function of time, because that varies wildly between idle and redline. It's measured as a function of degrees of crankshaft rotation, which is why it's also referred to as dwell angle. For instance, if the book calls for 90° of dwell and the fire mark is at 15° BTDC, then the points will close at 105° BTDC, charging the coil, and open at 15° BTDC, at which point the coil will discharge and fire the plug.
Notice I used points as an example. All ignitions systems have a dwell setting, from the Fred Flintstones points system to the modern computerized ignition, all have dwell. The reason I picked on points is because they're the only system that allows you, the mechanic to adjust the dwell. I'll add the disclaimer that some modern electronic systems allow dwell adjustment as well, but that's done on a computer keyboard and is beyond the scope of this discussion.
Why is dwell important? Excellent question. When dwell is set correctly, you'll get a solid spark from idle to redline. If there's too much dwell, the coil is being energized longer than it should be. That overworks the coil. Being energized too long can cause overheating of the coil and a reduced life span. If the coil isn't energized long enough, there won't be enough time at high revs for the coil to fully charge. That means misfiring and a loss of power at high revs. So you definitely want to shoot for the sweet spot of "correct" dwell.
How do I adjust dwell without a dwell meter? Simple, remember dwell is a function of degrees of crankshaft rotation. We can put a socket (17mm) and ratchet on the crank's rotor nut. If we hold the ratchet straight up then spin it CCW until it's straight up again, that's 360° of crank rotation. If we hold it straight up and rotate it until the ratchet points forward, that's 90° of crank rotation. Got it?
Our XS650's use 93° of dwell, ± 5° ... so anywhere between 88 to 98° of crank rotation. So 90° is a good round number to shoot for, eh... The setup for checking/adjusting dwell is exactly the same as used to set the timing (static). You connect a timing/troubleshooting light to the points you want to check. Connect it to the wire terminal on the points and ground the other end. With the key on, the light will come on when the points open and go out when the points close. So rotate the crank until the light goes out. This means the points have closed and the coil is charging.... the starting point for checking dwell. Rotate the ratchet backwards until it's pointing straight up. Now turn the crank CCW (normal rotation) until the light comes back on and stop. If your dwell is set correctly, the ratchet should point straight forward, 90° from the vertical you started at. If it's not, adjust the point gap. Just remember, if you adjust the dwell, it'll knock the timing off. You'll need to go back and readjust the timing.
If you don't have enough dwell, it means the gap is to large, close it slightly and check again. If you have too much dwell, the gap is too small. Open it up and recheck.
If you want to get all OCD, put a degree wheel on the crank and you can dial it in as close as you want, but the good ol' Mark I eyeball and a ratchet will get you close enough.
Cheers.
First; what is dwell? Dwell is a measure of the amount of time the ignition coil is being charged. It's not measured as a function of time, because that varies wildly between idle and redline. It's measured as a function of degrees of crankshaft rotation, which is why it's also referred to as dwell angle. For instance, if the book calls for 90° of dwell and the fire mark is at 15° BTDC, then the points will close at 105° BTDC, charging the coil, and open at 15° BTDC, at which point the coil will discharge and fire the plug.
Notice I used points as an example. All ignitions systems have a dwell setting, from the Fred Flintstones points system to the modern computerized ignition, all have dwell. The reason I picked on points is because they're the only system that allows you, the mechanic to adjust the dwell. I'll add the disclaimer that some modern electronic systems allow dwell adjustment as well, but that's done on a computer keyboard and is beyond the scope of this discussion.
Why is dwell important? Excellent question. When dwell is set correctly, you'll get a solid spark from idle to redline. If there's too much dwell, the coil is being energized longer than it should be. That overworks the coil. Being energized too long can cause overheating of the coil and a reduced life span. If the coil isn't energized long enough, there won't be enough time at high revs for the coil to fully charge. That means misfiring and a loss of power at high revs. So you definitely want to shoot for the sweet spot of "correct" dwell.
How do I adjust dwell without a dwell meter? Simple, remember dwell is a function of degrees of crankshaft rotation. We can put a socket (17mm) and ratchet on the crank's rotor nut. If we hold the ratchet straight up then spin it CCW until it's straight up again, that's 360° of crank rotation. If we hold it straight up and rotate it until the ratchet points forward, that's 90° of crank rotation. Got it?
Our XS650's use 93° of dwell, ± 5° ... so anywhere between 88 to 98° of crank rotation. So 90° is a good round number to shoot for, eh... The setup for checking/adjusting dwell is exactly the same as used to set the timing (static). You connect a timing/troubleshooting light to the points you want to check. Connect it to the wire terminal on the points and ground the other end. With the key on, the light will come on when the points open and go out when the points close. So rotate the crank until the light goes out. This means the points have closed and the coil is charging.... the starting point for checking dwell. Rotate the ratchet backwards until it's pointing straight up. Now turn the crank CCW (normal rotation) until the light comes back on and stop. If your dwell is set correctly, the ratchet should point straight forward, 90° from the vertical you started at. If it's not, adjust the point gap. Just remember, if you adjust the dwell, it'll knock the timing off. You'll need to go back and readjust the timing.
If you don't have enough dwell, it means the gap is to large, close it slightly and check again. If you have too much dwell, the gap is too small. Open it up and recheck.
If you want to get all OCD, put a degree wheel on the crank and you can dial it in as close as you want, but the good ol' Mark I eyeball and a ratchet will get you close enough.
Cheers.
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