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Post by Burns on Oct 5, 2012 11:59:05 GMT -5
It is such a rare opportunity to have MrRiggs and Pamco Pete (two of 5 or so heads on the XS650 Mount Rushmore) at the same table, that I must raise this issue that has been on mh mind for some time.
Advance Curve
Back when hot rods had points there was a cottage industry for springs that varied the advance curves in those distributors.
But I have never heard the topic addressed in relation to the XS650.
The TCI bikes don't even have a "curve" just two fixed settings.
The ATU bikes get full advance by 3500 and I can't imagine that is optimal from there to redline. And folks mess with the tension on those springs all the time just to get them to "come home" from full advance, so I'm sure that in practice those "curves" are pretty random.
PAMCO is ATU advanced, how about the HEI?
Has anyone determined what an optimal curve is for various applications?
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Post by mrriggs on Oct 5, 2012 20:27:04 GMT -5
Think of the ATU as an automatic timing retard and not as an advance. You set the total advance for best performance and the ATU retards it at low speed so the motor is easier to start. If you are a big guy with tall gearing then the retard can also prevent pinging if you lug it off the line.
I got rid of my ATU and run with the timing "locked out" at full advance. It idles smoother and quieter, and launches harder. I have never experienced any pinging but my bike is under 500 pounds with me on it and has lower gearing. Starting is not an issue either since I'm not running a battery.
Normally the biggest problem with having the timing locked out is starting the engine. If the engine isn't spinning fast enough to overcome the burning fuel mixture then you will experience a kick back when kicking. With electric start it simple won't turn over. To overcome this you either retard the ignition, which the ATU does automatically, or you prevent the ignition from kicking in until after the engine is spinning, which the batteryless system does automatically. You have to spin the PMA fast enough to get power for the ignition so it's impossible to get a spark before the engine is spinning.
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Post by Burns on Oct 6, 2012 2:05:49 GMT -5
Thank you for your reply. Since you are setting the spark at some fixed point I suppose that point would be at the most advanced spot that did not produce detonation, where ever that might be for rider/state of tune for a particular motorcycle.
I must say that iit s counter-intuitive to me to have no variability in ignition lead time relative to piston speed.
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Post by kopcicle on Oct 6, 2012 4:13:26 GMT -5
~snip Normally the biggest problem with having the timing locked out is starting the engine. If the engine isn't spinning fast enough to overcome the burning fuel mixture then you will experience a kick back when kicking. With electric start it simple won't turn over. To overcome this you either retard the ignition, which the ATU does automatically, or you prevent the ignition from kicking in until after the engine is spinning, which the batteryless system does automatically. You have to spin the PMA fast enough to get power for the ignition so it's impossible to get a spark before the engine is spinning. Obscure but interesting anecdote... I once had to contrive from previously butchered parts an ignition retard for a '34 BSA WT . In an odd arrangement the points were inside the point cam and the cam was movable via cable or lever depending on the year . Even for a flat head the consequences of starting the bike fully advanced were dire and usually painful . Giving this and the replacement of the key in the lawnmower some thought ( You know , the pull the recoil cord and it tries to separate your fingers from your hand thing ) I looked at the original alcoholic , 743cc , 12:1 , Shell#2 , 3mm oversize valve , otherwise nutcase bike that had sent many a " if you can start it you can ride it " hopefuls away with more than their pride wounded and thought .. (yeah , scary I know ) I disabled the ATU and instead of fixing the point plate to the head I stood the screws off with short spacers so that the plate could move freely between two stops , full advance and somewhat less . I thought I had been brilliant until arcing between the ferrous plate and the aluminum head stuffed up the works . So with instantaneous voltage at the points being a ragged square wave with 200+ volt peaks and the chassis not having a true earth ground imagine what it was like to have parts of your body touch the bike at any distance apart greater than a foot . With the experience of attempted electrocution behind me I attacked the problem with a few less beers in me and sanitized the whole thing in an afternoon . I was fairly proud of my ingenuity and accomplishment and cleaned up preparing for a celebratory ride to the local pub . I returned to the bike and verified it was off , opened up the fuel tap , opened the throttle , kicked it through twice , closed the throttle ,, found TDC , switched on and ... ...neglected to make use of or in fact pay any attention to the specific item of my labors from the previous four hours . I attempted (attempted is a good word here ) to start the the bike fully advanced . Ever had that bone jarring sensation that is immediately interpreted as shock . Yes that kind of instantaneous pain that leaves you gasping for breath . I had just tried to start a 12:1 alcoholic with 42 degrees of ignition lead in it . Adding insult to injury the little girl from two doors down asks "are you all right mister?" I weakly responded " no , not at all " as she is riding away she says " my mom says motorcycles are dangerous " automatic ignition retard indeed .... ~kop |
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Post by Burns on Oct 6, 2012 12:27:29 GMT -5
Great story, Kop.
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Post by mrriggs on Oct 6, 2012 13:14:33 GMT -5
Thank you for your reply. Since you are setting the spark at some fixed point I suppose that point would be at the most advanced spot that did not produce detonation, where ever that might be for rider/state of tune for a particular motorcycle. The whole "advance it until it pings" thing is a popular misconception. The further you advance the timing, the longer the piston is pushing against a burning fuel mixture which wastes power. Ideally, you should be looking to set the total advance as low as possible to get the most power. Unfortunately, with poor chamber design and low octane fuel, detonation will stick it's ugly little head in there and mess up the works. So the goal is not to advance it until it pings, but advance it for best performance and IF it pings before you reach that point then pull it back a few degrees. The two main factors of spark advance is speed and load. Speed is easy to understand, the faster you spin it, the more you advance the timing. Load is less intuitive, more load requires less advance. The traditional method for adjusting the advance for load is with a vacuum advance. Under normal conditions, at low speed, the mechanical advance retards the timing then the vacuum advance un-retards [advances] the timing so you are pretty much back at full advance. At high speeds, the mechanical advance does not retard the timing and the vacuum advance does nothing so you are at full timing. Looking at it this way, it's easier to see why a motor performs well with the timing locked out. The only situation that the locked out timing does not cover is when the motor is "lugged". With high load at low rpm you want retarded timing. The solution is very simple, though. DON'T LUG THE MOTOR. Lugging the motor is bad practice even if you have a functioning retard mechanism. |
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Post by mrriggs on Oct 6, 2012 13:19:44 GMT -5
Ditto! "my mom says motorcycles are dangerous" Laughed my a$$ off. |
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Post by Burns on Oct 6, 2012 16:50:17 GMT -5
The whole "advance it until it pings" thing is a popular misconception.
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well, it is "advance it till it pings then back it off till it doesn't" actually, but isn't that what automobile computerized ignitions with "knock sensors" do?
and, since engine speed and load are the variables - and vary they do - how can any one fixed setting be correct for all rpms and loads? And why has pretty much every gas motor in history had some sort of spark varying device on it?
like I said, it is pretty counter-intuitive.
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Post by mrriggs on Oct 6, 2012 21:29:02 GMT -5
And why has pretty much every gas motor in history had some sort of spark varying device on it? Because they all need to start. It's not perfect but close enough for the rpms and loads that a motorcycle will see. The factory "curve" really isn't much different. It's retarded at start and idle, then full advance at pretty much any rpm you will see while riding. The power to weight ratio of cars is much worse than bikes so an actual advance curve is often needed. Computer controlled cars have elaborate preconfigured maps to set ignition timing based on rpm, throttle position, air pressure, temperature and whatever else. Even then, the setup won't always be perfect. In the event that it is wrong and the engine detonates, the knock sensor will pull some timing out of it to keep the motor in one piece. It is. Just forget about everything you have ever read or been told, including everything I've said here. Try it for yourself and make up your own mind about what works and what doesn't. |
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Post by Burns on Oct 7, 2012 0:02:41 GMT -5
The TCI bikes have "start/idle" and a "run" with nothing in between and the ATU bikes in practice aren't much different, as we have both observed in different words.
I've always wondered if that "curve" was optimium, and I suppose the answer that I am getting here is that it is "close enough" i.e. on the wrong side of the cost:benefit ratio to persue, and that a single fixed setting someplace in between is quite functional.
I'd need a dyno to find the exact Goldilocks zone, but you seem comfortable with the one that you have found by trial and error.
XSJohn has long advocated less full advance than the Yamaha standard setting.
I'm reaching the conclusion that the timing is just not all that critical. If the bike starts easy and doesn't ping under load you can leave the timing light in the box.
Thanks for the discussion.
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Brian
New Member
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Post by Brian on Oct 7, 2012 11:41:06 GMT -5
I'd be interested in your analysis on this ignition unit; www.probe-eng.com/servlet/the-7/XS650-dsh-90L/DetailI bought one several years ago for my 270 cranked XS650SE and although it was more expensive, it eliminates the centrifugal advance and allows for adjustable all in advance between 3-4K among other features. Once installed, which was real easy, set the timing with a light and never looked back. Seems to make for an easy starting bike. Your thoughts. Attachments:
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Post by tomterrific on Oct 8, 2012 10:14:19 GMT -5
I believe the TCI has an advance curve Burns. It is analogue and quite difficult for me to figure out as I am not trained in electronics, not even self trained as a hobby. This is the way I think it works. The pickup at the rotor is in two parts, one an idle pickup and the other high speed. In the TCI is a 'ladder' of resistors (for lack of a better term). As the engine spins faster and faster the pickups make more and more voltage. The increasing voltage overcomes the resistors one at a time advancing the timing like climbing each step of a ladder. Lame, I know, but that is the only way I can explain it.
If you think this is hard, try an SR500 ignition!
Speaking of big singles and ignition retard, my 450 Ducati kicked back so hard that it drove my knee up and bloodied my nose! I learned how to use a compression release quickly after that. The same bike broke the foot of a member of a bike gang after he insulted my little bike. That bike was a demon to start!
Tom
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Post by Burns on Oct 8, 2012 15:53:35 GMT -5
That's the first I've heard of a series of resistors advancing the spark. I'd always understood that ther were two pick-ups and spark was triggered by one or the other.
I suppose you could put a light on it and see if there is any RPM that produces a spark between the 'start' and 'full advace' marks.
I remember the old thumpers. I weighed under a 100 pounds in those days and would start my buddy's BSA by putting it in gear and rolling it backwards till the compression stopped the bike. That put the piston on the power stroke (over TDC) and made for a soft kick. It took several attempts as I recall and you had to have the tickle/choke/throttle position drill down right. Ah yes motorcycling was an art in those days.
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Post by antonio on Apr 14, 2018 21:09:50 GMT -5
I wanted a simple, reliable and inexpensive system for XS650 riders, and I think that I have achieved that. No arguments here. Before the Pamco ignition, the only choices for the XS650 masses were to either live with the points, convert to TCI, or shell out the bucks for a Boyer. Your ignition filled a valuable market niche. |
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