-
Enjoy XS650.com? Consider making a donation to help support the site.
XS650.com receives a small share of sales from some links on this page, but direct donations have a much greater impact on keeping this site going.
XS650 Ignition timing revisited - Are we too advanced?
- Thread starter TwoManyXS1Bs
- Start date
-
- Tags
- electrical engine maintenance
2M I think you a pretty much nailed it with your first post.
Craig Weeks sent out an update/addendum to his XS 650/750 performance guide a while ago.
It was based on back to back dyno testing of a race motor the upshot was that there was indeed an ideal advance setting for the motor being tested but as long it was set above 30 degrees the power difference (loss) was minimal.
He went on to suggest that for a stock or mildly tuned road bike 36 degrees would be the minimum.
I have my road bike set at 35.
Craig Weeks sent out an update/addendum to his XS 650/750 performance guide a while ago.
It was based on back to back dyno testing of a race motor the upshot was that there was indeed an ideal advance setting for the motor being tested but as long it was set above 30 degrees the power difference (loss) was minimal.
He went on to suggest that for a stock or mildly tuned road bike 36 degrees would be the minimum.
I have my road bike set at 35.
dps650rider
XS650 Addict
Resurrecting this old thread. I have been battling problems with part throttle detonation even thought the timing was set to 35 or even a bit less. At that timing setting if I set the midrange a bit rich it is not bad only showing up riding double on long hard climbs when the humidity is low. If I set the mid so it burns clean the detonation is a problem.
So today I leaned out the midrange a bit and backed the timing off to about 30 for the next test.
I am running Iridium plugs and it made be wonder, if these plugs do indeed do a better job of lighting the mix would they not also demand less advance? It would seem to make sense. BTW, my bike is 700cc with JE pistons milled off flat at the top to reduce the CR to about 9:1.
So today I leaned out the midrange a bit and backed the timing off to about 30 for the next test.
I am running Iridium plugs and it made be wonder, if these plugs do indeed do a better job of lighting the mix would they not also demand less advance? It would seem to make sense. BTW, my bike is 700cc with JE pistons milled off flat at the top to reduce the CR to about 9:1.
Last edited:
I also use JE 77.5 mm. pistons with .020" base gasket, .040" copper head gasket, NGK BPR7EIX iridium plugs, Probe ignition, and Accel 4 ohm coil. Static CR is around 8.8:1. 38* BTDC at full advance gives good results, no pinging at any rpm, load, or throttle position with 91 to 92 octane fuel. More retard probably wouldn't harm performance noticeably, but if it ain't broke....What determines optimal advance setting in a given motor is speed of flame travel, and that's conditioned mostly by fuel and CR.
Last edited:
dps650rider
XS650 Addict
Interesting, even at about 35 degrees my plugs look like there is too much advance as per the Gordon Jennings article. Maybe it is due to the combustion chamber shape I have due to milling the pistons. Griz, do you ride anywhere there are long hard 5th gear climbs? That is when the detonation shows up if running around 3K RPM. If I am going faster say 3.5K or higher no problems.
Long hard 5th gear climbs? Sure. I live in hill country, the flatlands are to the north. I'm scratching my head, but I doubt that altering combustion chamber shape by milling pistons would cause a problem. Another 700 cc. top end I built used 11:1 Venolia race pistons, and I had .030" milled off the tops to get static CR down to around 10:1. Timed the ignition to ~38* on that motor too, and had no grief. Switched to lower CR setup because that motor needed premium pump gas, and I got tired of having to guess dosages of questionable octane booster when I wound up in a spot where the only thing available was 87 octane. When I took down that top end the pistons were in perfect condition, no signs of detonation at all.
But I have to ask--since your motor isn't making much power at 3K, why would you ask it to haul you up a hill in 5th from that rpm? To be honest I don't think I've ever tried that. My motor might ping under those conditions too, and I wouldn't blame it a bit for bitching at me!
But I have to ask--since your motor isn't making much power at 3K, why would you ask it to haul you up a hill in 5th from that rpm? To be honest I don't think I've ever tried that. My motor might ping under those conditions too, and I wouldn't blame it a bit for bitching at me!
dps650rider
XS650 Addict
It could also be that my CR is higher than I think. The pistons were supposed to be 10:1. I made the assumption that was using stock gaskets and calculated how much material needed to be removed to get to 9:1. Would have been better to measure it but I didn't do that.
At 3K RPM it has gobs of torque and can easily pull most hills around here even riding double in 5th. If it pings I either open the throttle more, I assume the richer mixture shuts it up, or I downshift.
BTW, took a test ride at 30 degrees and the seat of the pants says it runs great. Need to run it some more and check the plugs.
At 3K RPM it has gobs of torque and can easily pull most hills around here even riding double in 5th. If it pings I either open the throttle more, I assume the richer mixture shuts it up, or I downshift.
BTW, took a test ride at 30 degrees and the seat of the pants says it runs great. Need to run it some more and check the plugs.
Looks like I misunderstood you. I would have expected detonation to occur as you were rolling on throttle from low rpm to climb a really serious grade. Do you hear pinging, or are you going by the appearance of the plugs? Conventional plugs might be easier to read than iridiums.
dps650rider
XS650 Addict
I hear the pinging and never let it continue.
I was thinking the same thing about the plugs, may swap them back to stock to make it easier. That center electrode in the iridiums may be fooling me into thinking it is too advanced.
I was thinking the same thing about the plugs, may swap them back to stock to make it easier. That center electrode in the iridiums may be fooling me into thinking it is too advanced.
Hey, dps650rider!
Ruh-roh.
That particular rpm raises the hackles on the back of my neck.
Old Delco distributor timing machine manuals had a warning about over-advance in what they called the "Detonation Prone Zone", the 2,700-3,000 rpm zone. No explanation, just something that mysteriously happens.
A mystery that I just had to explore.
Years later, playing with my engine simulator,
http://www.xs650.com/media/albums/dream.1680/
I found an interesting phenomenon occurring at around 3.000 rpms.
Warning, highly debatable stuff follows. I've never been able to get this idea across.
Difficult to explain, but it appears that, within that rpm range, the decelerative force of the piston as it nears TDC is equaled, then exceeded, by the combustion pressures (depending on throttle position), and the piston side-wall thrust (based on rod angle) reverses rapidly. And, this side-wall thrust does another rapid reversal just after TDC. These rapidly reversing side-wall forces create a kind of piston rattle, what folks call "ping of detonation", but is really a unique kind of piston slap, what I like to call "piston flutter". These side forces stress the piston at the ring grooves, potentially causing the top of the piston to shear away. My simulator shows this as a normal phenomenon, that "detonation" (as accepted by the masses) doesn't exist, and is easily avoided by just a little ignition retardation at that unique zone.
Honda mitigated this issue by having a 1mm offset in their pistons' wrist pin holes.
I incorporated the same in the Dragon.
What this boils down to is, check the amount of spark advance at that particular rpm. You may be hitting full advance too early. Remember the distributor spring kits of yesteryear?
In the worst case scenario, worn rod ends will produce serious rod knock at those rpms. Usually detectable by varying throttle, acceleration and deceleration, causing a change in the sound of rod knock.
Ok, now I'm going to flee before things get thrown at me...
Ruh-roh.
That particular rpm raises the hackles on the back of my neck.
Old Delco distributor timing machine manuals had a warning about over-advance in what they called the "Detonation Prone Zone", the 2,700-3,000 rpm zone. No explanation, just something that mysteriously happens.
A mystery that I just had to explore.
Years later, playing with my engine simulator,
http://www.xs650.com/media/albums/dream.1680/
I found an interesting phenomenon occurring at around 3.000 rpms.
Warning, highly debatable stuff follows. I've never been able to get this idea across.
Difficult to explain, but it appears that, within that rpm range, the decelerative force of the piston as it nears TDC is equaled, then exceeded, by the combustion pressures (depending on throttle position), and the piston side-wall thrust (based on rod angle) reverses rapidly. And, this side-wall thrust does another rapid reversal just after TDC. These rapidly reversing side-wall forces create a kind of piston rattle, what folks call "ping of detonation", but is really a unique kind of piston slap, what I like to call "piston flutter". These side forces stress the piston at the ring grooves, potentially causing the top of the piston to shear away. My simulator shows this as a normal phenomenon, that "detonation" (as accepted by the masses) doesn't exist, and is easily avoided by just a little ignition retardation at that unique zone.
Honda mitigated this issue by having a 1mm offset in their pistons' wrist pin holes.
I incorporated the same in the Dragon.
What this boils down to is, check the amount of spark advance at that particular rpm. You may be hitting full advance too early. Remember the distributor spring kits of yesteryear?
In the worst case scenario, worn rod ends will produce serious rod knock at those rpms. Usually detectable by varying throttle, acceleration and deceleration, causing a change in the sound of rod knock.
Ok, now I'm going to flee before things get thrown at me...
Last edited:
Hmm....Interesting thoughts re. detonation, 2M, but it may not be an either/or kinda question. Bits of aluminum on sparkplug enamel is typical pre-failure evidence of detonation, and it's hard to see how shearing forces could put 'em there. Also, all of the XS650 piston holes I've seen were small (dime size or less) and looked to have been eroded rather than sheared--no sharp edges in evidence.
bikefreakjon
XS650 Addict
I would like to check my timing and maybe even slot the trigger coil mounting in order to play with it, but a previous owner lost the timing mark that is ordinarily affixed to the bottom of the stator housing!
I suppose I can either extrapolate from top dead center with a degree wheel or maybe try to find another stator housing.
That said it doesn't knock or ping at any RPM and it isn't set up excessively rich, so I feel like I'm safe to keep running it the way it is.
It would just be interesting to know for sure...
I suppose I can either extrapolate from top dead center with a degree wheel or maybe try to find another stator housing.
That said it doesn't knock or ping at any RPM and it isn't set up excessively rich, so I feel like I'm safe to keep running it the way it is.
It would just be interesting to know for sure...
Hey, Griz!
Let's just say that the piston deals with 2 realms.
Above the crown.
Tremendous heat and pressures of combustion, rapid cooling from induction.
Below the crown.
Tremendous forces up/down thru the wristpin, from pressures above, and rapid acceleration/deceleration. Sidewall thrusts, from conrod angularity. Friction scuffing from the sidewall thrusts. Impact loads from whatever clearances exist. Thermal effects, hot-to-warm, running vertically from top to bottom.
In the classic perspective of the engine cycles, we can visualize the conrod in compression during the compression and power strokes. A natural reaction to the pressures above the piston. But, once certain rpms occur, usually around 3,000 rpms at WOT, or below at less throttle, the conrod enters the reverse world of being in constant tension from about 80° BTDC, thru TDC, to about 80° ATDC. Even *with* the massive force of combustion trying to push the piston down. Just from the decelerative and accelerative forces. Getting greater with rpms. Racing conrods are specifically spec'd for tensile strength for the intended rpms.
In that transition zone, Delco's "Detonation Prone Zone", the decelerative/tensile forces below and the combustion pressures above, tend to somewhat balance, but not exactly, since one is sinusoidal, the other adiabatic. Hence the resulting "flutter" forces thru the sidewall thrust.
Poor ol' piston has to deal with all that.
Amazing that they survive at all...
Last edited:
dps650rider
XS650 Addict
Honestly, any time I work on an engine and think about what happens in there it amazes me that they stay together at all!
Update, seems like the biggest problem I had was carbon buildup caused by bad valve seals. As I mentioned in another thread I had an inner valve spring fail so I had to tear it down. The valve seals were all shot (1 from the broken spring...) and there was a lot of carbon. Got it back together yesterday and it runs great and no detonation even as I have been leaning out the mid range. Currently at about 35 degrees and am going to try bumping it up a bit more just because. Freaking thing runs STRONG now, I bet my oil consumption drops to almost nothing too.
Update, seems like the biggest problem I had was carbon buildup caused by bad valve seals. As I mentioned in another thread I had an inner valve spring fail so I had to tear it down. The valve seals were all shot (1 from the broken spring...) and there was a lot of carbon. Got it back together yesterday and it runs great and no detonation even as I have been leaning out the mid range. Currently at about 35 degrees and am going to try bumping it up a bit more just because. Freaking thing runs STRONG now, I bet my oil consumption drops to almost nothing too.
Several years ago, I set my XS1B ignition timing a bit retarded to improve my idle stability, reduce vibrations, and give a smoother and more docile power output to match my sedate 'old man' riding style. Then, after building my LED Ignition Timing Light, I reset my retarded ignition timing back to the factory spec of 40° BTDC full advance.
This new performance of the engine was awful. The idle became unstable again, but more importantly, the highway performance was alarming. To maintain 60-70 mph required much more throttle, power was down, vibration was way up, and my typical hot oil temps rose from 230°F to around 250°F.
Whut's goin' on?
Warning: *Ramble mode engaged*
The XS1 was developed in the late '60s, and held a respectable position in the early '70s. Fuels, oils, and sparkplugs of that time have changed, and we've had to adapt. I'm still catching up on this new world of low-octane gasohol, and its effects on vintage engines. Back in the late '60s - early '70s, gasoline was easily available in 98-108 octane, enhanced with tetra-ethyl-lead. My primitive 'oldschool' understanding of octane ratings was that the higher octanes had a higher flash temperature and a slower, controlled burn, a requirement of the high-compression muscle cars of that time. Folks thought that higher octane gave you more power, but the reverse was true. To get more power, you increased the compression ratio (C/R), which then required the higher octane rating. On a lower C/R engine, more power was realized on the lower octane (faster burning) fuels.
Fast forward to our modern gasohols, with octane ratings of 87-91. Which makes me think that these modern fuels burn faster, and need LESS spark advance.
A Google search of "ignition timing curve" and "spark advance curve" shows numerous charts where the vast majority of modern engines limit max spark advance to around 30° BTDC, with very few going to 35° BTDC, and rarely to 40° BTDC (for vintage engines).
Thanks to advancements in engine design technology, and associated analysis tools, a new perspective exists that may be applied to our vintage engine. For example, this pictorial of combustion chamber designs shows associated max spark advance values. Note the vintage side-plug hemi-chamber value of 40°-42°, versus the newer pentroof at 30° and less. Our combustion chamber is somewhat between the two, with its higher mounted plug position. This would seem to support reducing our full advance timing.
I'm not having much luck finding good comparisons of fuel burn rates, fuels of 45 years ago versus modern gasohols.
http://performancetrends.com/Definitions/Burn-Rate.htm
But, this somewhat summarizes my thinking of what's going on:
Energy density of gasohol is about 3% less than gasoline, requiring more throttle.
More throttle requires less spark advance.
Faster burn rate of lower octane rated fuels requires less spark advance.
A search thru this forum found recommendations from XSJohn (rip) of retarding timing about 5°, from this thread:
http://www.xs650.com/forum/showthread.php?t=2426
View attachment 2243
And, I found a post from grizld1 of his running a max of 38° BTDC. There may be more, like the recommended ignition timing setups for the track, but those would probably apply to the old days and vintage fuels.
For now, until I can do more exhaustive testing, these are the ignition timing positions I'm running:
View attachment 62032 View attachment 62033
This represents my modified current ignition timing curve:
View attachment 62034
So, I'm thinking that if you're experiencing poor highway performance, excessive vibration, hot engine, poor gas mileage, an ignition timing 'retard' of about 5° may help your condition.
Comments, guys...?
I did that on my old 'hemi' Jaguar strait 6 engine....added a Pertronix pick up; MSD box; had orig. dizzy rebuilt and curved so I could adjust idle to +15 deg. and not run over 28-29 deg over 3000 rpm. Helped smooth over hi-lift cam idle and more / smooth power up to thru 6000. running 93 unleaded on 10.3:1 CR. I was spoiled.. a friend had a software program that allowed me to just input a certain piston lift measured volume and what my deck height variable is to out put the CR. That said, every engine tuned to it's own wants...
Last edited:
This has been a really interesting thread!
If I could jump in here... my issue is that if I retard anywhere below max advance, idle becomes very difficult. I've tried bumping up idle speed but it hasn't helped. It frequently and intermittently dies at idle.
Any thoughts?
If I could jump in here... my issue is that if I retard anywhere below max advance, idle becomes very difficult. I've tried bumping up idle speed but it hasn't helped. It frequently and intermittently dies at idle.
Any thoughts?
what ignition are you running? other details (a pic or 2) of the bike?
The stock mechanical advance is known to wear after a lot of miles causing a larger advance and less reliable timing.
A sticking advance rod is one easily fixed timing problem.
Sounds like your state of tune isn't really good yet, most likely the carbs. These bikes should idle reliably and without stalling with retarded timing set in the 10° to 12° BTDC range. Granted, the idle will be a bit "lumpy", but the bike shouldn't stall. Both of mine idle fine with the more retarded timing setting.
Similar threads
