Jaguar, HD, Rocket CDI's OHHHH MY
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FurryOnTheInside
In Memoriam
I think the technical terminology needs explaining for this thread to make proper sense.
Unfortunately I am a bit beyond sense right now.
Retardation is the change in spark timing (in degrees Before Top Dead Centre (BTDC) ) as RPM increases.
Advance is the change in the opposite direction as RPM increases, which is often said to be beneficial at some point during lower RPM.. depending on where the spark timing was at the start, of course.
Base timing is just the starting point from which spark timing is timed by the CDI itself. We can change the base timing for free, by altering the woodruff key beneath the magnet.
The CDI is a capacitor which discharges after some time.
The time it takes for the spark to ignite the entire charge in the combustion chamber is affected by many variables, including the engine speed, but also the scavenging actions of tuned pipes and our port work. Of course the time it takes for the crank to move around to the ideal position is decreasing as RPM increases!
When we talk about the retarding of spark timing it is generally meaning the spark occurring later in terms of degrees, as we increase the RPM of the engine, as we accelerate the bike.
We really want the spark timing to vary in just the right way that it occurs at whatever moment needed to give the correct amount of time (which is varying as we progress through the RPM range!) for the entire charge inside the combustion chamber to ignite, when it will time the Peak Combustion Pressure (PCP) to occur at the best place in the crank rotation to push the piston and rod and turn the crank to best effect.
Earlier PCP gives more degrees for the pressure (we've put so much effort into creating!) to push down on the piston/crank, obviously.
However we must be careful, as the vertical movement of the piston is slowed when it is very close to Top Dead Centre (TDC) so a too early PCP will be stifled and trapped, and very high pressure. With nowhere to go, a too early PCP will waste energy as heat into the piston and head. It may damage the piston or blow out the head gasket, some pressure will escape by blow-by through the ring gaps too.
A terribly early spark causing a terribly early combustion will be a "backfire".
Later PCP gives more angle at the crank for applying torque, as well as allowing greater piston speed so the pressure is able to move the piston rather than being boxed in momentarily and going sky high, which gives us less wasted energy (as heat or blow-by) at the start of the power stroke, but of course a later PCP also limits the degrees remaining that the piston is able to be pushed down before the exhaust port opens up.
That part, the moment when Peak Combustion Pressure occurs, we cannot see happening, cannot test with a spark timing light, but we can test the outcome by riding the bike, seeing how fast it accelerates, whether the head/ gasket holds, and how the piston looks after some use.
Personally. My base timing is later than stock (i will not say retarded here, this is a static situation) because my generic old 66cc often called a "happy time" engine has a stock CDI and was often said to be jittery and self-limiting by vibration and short lived at higher RPM; but being in Europe and poor while I was building my bike, I never got around to ordering a Jaguar CDI or any of the CDIs available from the USA.
I changed the base timing before ever attempting to run the engine. The best advice at the time suggested that to run "stock" was to run a seriously malfunctioning system and cause unnecessary damage.
I had only raised the exhaust port half a mm or so (sorry for not being able to say in degrees but that is irrelevant to your YD100), so having a later base timing (with a CDI that is said to advance too much and continue advancing when it should be retarding as RPM increases) appeared to be a good investment (it's free!).
Actually it could have been free but I spent £2.50 on an offset key for a different machine and made it fit, just because I wanted perfect.
I wanted the key to get sheared in a situation where it is relied on to release, and not bent over until it snaps off and (I imagine) ends up twisted into the keyway, not that there is really much of any force from the magnet in use. To tighten the magnet keyway against a narrowed or notched stock key requires the builder hold things together while tightening, depending which side of the key you're trying to tighten the keyway against.
My full width key doesn't require this so is a tiny bit easier to fit.
A narrowed or notched stock key can be removed and reversed to quickly go back to stock base timing, to compare the performance, but my offset key can only give later or earlier than stock timing by being the intended or reversed position. I carry stock keys as emergency replacement parts but have never used them and never tried the stock base timing.
I made my engine and made my own judgement as to where "stock" or my starting point should be. I took the best advice as I believed it to be, and decided to not repeat everyone else's hard work and experimentation, but just take their advice from their experience they had paid the price for.
My engine feels pretty smooth at mid to highish RPM compared to how other motoredbikes hobbyist bike tuners at the time were describing theirs with entirely stock ignition systems. I can only go by the descriptions of the vibrations and seat of pants feeling of acceleration, but I think I did the right thing in making the base timing later than stock on my generic 66cc.
Unfortunately I am a bit beyond sense right now.
Retardation is the change in spark timing (in degrees Before Top Dead Centre (BTDC) ) as RPM increases.
Advance is the change in the opposite direction as RPM increases, which is often said to be beneficial at some point during lower RPM.. depending on where the spark timing was at the start, of course.
Base timing is just the starting point from which spark timing is timed by the CDI itself. We can change the base timing for free, by altering the woodruff key beneath the magnet.
The CDI is a capacitor which discharges after some time.
The time it takes for the spark to ignite the entire charge in the combustion chamber is affected by many variables, including the engine speed, but also the scavenging actions of tuned pipes and our port work. Of course the time it takes for the crank to move around to the ideal position is decreasing as RPM increases!
When we talk about the retarding of spark timing it is generally meaning the spark occurring later in terms of degrees, as we increase the RPM of the engine, as we accelerate the bike.
We really want the spark timing to vary in just the right way that it occurs at whatever moment needed to give the correct amount of time (which is varying as we progress through the RPM range!) for the entire charge inside the combustion chamber to ignite, when it will time the Peak Combustion Pressure (PCP) to occur at the best place in the crank rotation to push the piston and rod and turn the crank to best effect.
Earlier PCP gives more degrees for the pressure (we've put so much effort into creating!) to push down on the piston/crank, obviously.
However we must be careful, as the vertical movement of the piston is slowed when it is very close to Top Dead Centre (TDC) so a too early PCP will be stifled and trapped, and very high pressure. With nowhere to go, a too early PCP will waste energy as heat into the piston and head. It may damage the piston or blow out the head gasket, some pressure will escape by blow-by through the ring gaps too.
A terribly early spark causing a terribly early combustion will be a "backfire".
Later PCP gives more angle at the crank for applying torque, as well as allowing greater piston speed so the pressure is able to move the piston rather than being boxed in momentarily and going sky high, which gives us less wasted energy (as heat or blow-by) at the start of the power stroke, but of course a later PCP also limits the degrees remaining that the piston is able to be pushed down before the exhaust port opens up.
That part, the moment when Peak Combustion Pressure occurs, we cannot see happening, cannot test with a spark timing light, but we can test the outcome by riding the bike, seeing how fast it accelerates, whether the head/ gasket holds, and how the piston looks after some use.
Personally. My base timing is later than stock (i will not say retarded here, this is a static situation) because my generic old 66cc often called a "happy time" engine has a stock CDI and was often said to be jittery and self-limiting by vibration and short lived at higher RPM; but being in Europe and poor while I was building my bike, I never got around to ordering a Jaguar CDI or any of the CDIs available from the USA.
I changed the base timing before ever attempting to run the engine. The best advice at the time suggested that to run "stock" was to run a seriously malfunctioning system and cause unnecessary damage.
I had only raised the exhaust port half a mm or so (sorry for not being able to say in degrees but that is irrelevant to your YD100), so having a later base timing (with a CDI that is said to advance too much and continue advancing when it should be retarding as RPM increases) appeared to be a good investment (it's free!).
Actually it could have been free but I spent £2.50 on an offset key for a different machine and made it fit, just because I wanted perfect.
My full width key doesn't require this so is a tiny bit easier to fit.
I made my engine and made my own judgement as to where "stock" or my starting point should be. I took the best advice as I believed it to be, and decided to not repeat everyone else's hard work and experimentation, but just take their advice from their experience they had paid the price for.
My engine feels pretty smooth at mid to highish RPM compared to how other motoredbikes hobbyist bike tuners at the time were describing theirs with entirely stock ignition systems. I can only go by the descriptions of the vibrations and seat of pants feeling of acceleration, but I think I did the right thing in making the base timing later than stock on my generic 66cc.
Last edited:
FurryOnTheInside
In Memoriam
There is so much we have to infer. Its mysterious and exciting!
We can only measure cranking pressure and possibly the spark timing ° (mostly we believe what were told rather than getting/ making a timing light) but even if we know when the tuned pipe comes on we don't know what amount of pressure it adds and we don't know how much that reduces the time it takes to ignite the whole or majority of the charge. We feel and see the results and keep on tuning. Which is fun and comparatively inexpensive (to similar vehicle tuning hobbies and motorsports).
We can say one CDI is ahead (further °BTDC ) or behind (later BTDC) of another and usually use the terms more advanced or more retarded, but specifically when and by how much each CDI advances and retards its timing at what specific RPM is the real indicator or differentiator and we can only guess or experiment with the available CDI options before we know what suits our own bike build.
There is personal preference too, of course, and I had a preference in high torque-producing late PCP, with a fairly low exhaust port to leave plenty of ° power stroke, and low RPM use 6000-7000 cruise and 8000 max ever (due to fears of shoddy bearings), and partially gutted, extended stock muffler for stealth.
My compression isn't high, because my cranking pressure is not high and there is no tuned pipe effect. Last I measured cranking pressure was the first time I measured before break-in so I don't even want to say now. Started at 120psi or something iirc which I might not. ☺
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