Just came in from the actual cold outside....hey its where my bike is sitting. Anyhow...may have found my problem with my Dyna. The Dyna instructions say to fully advance the rotor to the 1&4 cylinder advance mark to make the test light (static timing) come on, while rotating the engine in a forward direction. Then if 1&4 are out, rotate the base plate. If everything ok, proceed to cylinders 2&3. The baseplate slots are not long enough to make the light come on. Now....I know what I did. I installed mine like a set of points.......I brought the advance rotor button all the way up to the F mark for cylinders 1&4 with 1 at top dead center. When I did this, I made sure the baseplate slots were centered on the screws. I made corresponding marks for the magnet and triggers so I could see them, then I moved the left module for 1&4 so the marks lined up, did the same for 2&3. The bike has run flawlessly for the better part of 3 years, now I have this problem with 2&3 cutting out.
Hondaman...here is where I am lost.......when I set mine up originally.....did I not set it up so that it had no advance at high RPM? If so, why did it run so damned well? We're talking 7500 shifts with no bog......80mph no problem....up to 100/115 with little problem. Every so often felt like running out of fuel, but not all the time. Your thoughts please.
Well, first thing: the advance comes from the spark advancer itself. So, don't confuse yourself about the advance.
This can be easily checked if you have a timing light, too. The Hall Effect sensors that Dyna uses are distance-sensitive, like most of them are. The little magnet in the advancer has only so much flux, so the pickups have to be positioned so that their distance from the magnet is close enough to trigger at full strength. This is the Achilles Heel of the system, and is often not explained (or understood) by Dyna to its customers. So, maybe a bit of the theory will help:
I think the max distance Dyna specs for the trigger-to-magnet is .060" (users, jump in here...), so for the sake of discussion, let's use that number...
If the trigger is set .060" away from the magnet, there is just enough magnetic flux to switch OFF the trigger when the magnet appears. Making it closer will trigger it sooner in crankshaft degrees: this is easy to see with a timing light, too. This is because the magnet's flux pattern actually resembles a flared mushroom at the little flat spot, so pushing the trigger closer lets it find the magnet a bit sooner it the rotation. Moving it back makes it later, until you get so far that it will not trigger at higher RPM, because the trigger does have a bit of a lag to it: the magnet comes and goes by too fast if the trigger is too far back, at speed. Just to confuse things, it still works OK at slow speeds...
So...ideally, you would want to set the triggers to be about .030" to .040" away (1/2 to 3/4 of the active distance) from the magnet. This should give nice, crisp switching action. Setting both triggers this way can be tricky, because the circuit board that Dyna uses is not all that accurate, nor is it temperature stable, dimensionally speaking (the middle hole gets bigger with temperature, about +.008" at 250 degrees). Dyna has had their instructions rewritten many times, confusing this issue: I would always start with equal distances on the pickups, then twist the baseplate until 1-4 lined up at idle under a timing light, then see how far I had to adjust the 2-3 set to get its timing right. (Many times I was found widening the slots with a rat-tailed file.) If they would not all line up, I'd "split the difference" in the distances, and try again. It was not unusual to spend several hours trying to get one set up just right. On the plus side, once they are set up, they generally last about 2-3 years of miles before the trigger wear makes you have to move them closer to the magnet again to get the same performance back. After that, they generally last until they die, if they do die. Some seem to last forever.
When the RPM goes up, the distance between magnet and trigger becomes critical. As the engine heats up, the triggers are also less able to quickly switch OFF to make the spark, and the timing starts to lag late. The trigger that is farthest from the magnet will be affected more than the other, and the engine feels weak and unbalanced due to unequal sparks. Then, when it cools off, or gets below that "magic" temperature, it runs okay again. If the triggers are close enough to the magnet to ensure full switchoff, this problem rarely appears unless the triggers are suffering from some damage (or low bike voltage).
The underlying "error", as I call it in this system, is the very short OFF dwell duration of these triggers. Honda's coils are designed for a full discharge time of a little over 1.7 milliseconds (that's how long their spark lasts). But, at engine speeds over 4500 RPM, presuming perfect magnet-trigger distances, the magnet dwell is already less than this required discharge time, so the coils begin to display a patially-charged magnetic field which resists their full charge for the next spark cycle. In other words, spark droop is more severe than with (good) points. This spark duration is important to the swirl-charge burn time in these shrouded-head cylinders, because the swirling charge is expecting a 1.7mS burn time to ignite everything properly. A shorter spark duration results in lost power, period. Higher spark voltage will not make this up, so just switching to high-voltage coils, while still using the triggers, does not help. Example: Dyna's 3-ohm coils discharge in 1.2mS, unless there are some other electronics to damp the spark a little for a longer burn (that's my ignition unit, but that's a different story, and a secret...
).
That said, burnt points also display this same 'lost high-RPM spark' scenario, typically after about 8,000 to 10,000 miles. But, before that mileage, points actually give a BETTER spark than a Dyna S will. That's why Dyna has always been careful to claim that the 'S' will "give about the same spark voltage as points".
That "running out of gas" feeling you describe was a common complaint in the late 1980s with the "S", due to this varying spark strength business, and riders often would buy the Dyna III box to add to it to help things out. This worked, but this was because the triggers no longer had to switch the full coil current: they just triggered the box to do it instead, and the box had a bit of a pulse stretcher in it to help the duration, too. Then they would fiddle around with the trigger distances until they got the timing just right, and ride on until the box failed, like my Dyna III did. Then I got mad and threw it away, and sent the triggers to Terry in AUS. Many riders carried along the old points plate, just in case.
Then I designed my box instead, so I could quit fiddling around with it and ride more.
The whole of this explanation is actually quite a bit longer, but I'm afraid of portraying that famous quotation, "Don't ever ask an Engineer what makes it work, he's just liable to explain it all night"...
Oh, but one more thing...if you have retarded ignition timing on a 750, but run low octane gas (85 or so), it will actually generate MORE midrange torque, but just a little less top-end horsepower. This can be useful, on the street.
