Rocket scientist needed

[old_skool]

Why have vacuum advance ? Under part throttle, air intake into the intake manifold is restricted, so a vacuum develops in the manifold. Because of this vacuum less air/fuel mixture is drawn into the cylinder. This, in effect, lowers the engines compression and slows the burn rate of the fuel mix. To get the full power out of the engine under these conditions requires more Ignition advance. The Vacuum advance gives you this advance when there is vacuum in the intake manifold.

Ok, so I understand the the whole vacuum advance thing....no I don't..Why don't other bikes have this, did they use some other vacuum advance method to compensate, and an even bigger question, I read somewhere here(don't need the link) where some experimentation was being done with drilling other makes of carbs to add a vacuum advance port so they can be utilized, if its only a vacuum needed, why can't the ones on the intakes be used. So how is it I see so many signatures listing other 1100 and 1200 carb banks in use? What did you/these guys do?

[jetmechmarty]

I'm not going to answer your question entirely, but I can tell you that other bikes use an electronic advance rather than vacuum.

The advance can not use the tap on the intake because there is too much pulsing in the signal. The signal from the carb is "smoother".

[cywelchjr]

It's not the smoothness, as the exact same pulsing is on the port on the carb, and you can add restriction to the line to lower the pulsing, but the issue is that at idle, there is no vacuum applied to the advance unit from the port on the carb, where with the port on the boot you get high vacuum at that point, the exact opposite of what you want.

Vacuum advance generally works off the vacuum right behind the throttle plate of the carb, as for ignition purposes it's generally the most accurate for preventing pinging if you don't have computer controlled ignition with a knock sensor. Strangely, even though I've not seen another bike with a vacuum advance, either electronically or mechanical, many of the carb bodies actually have a spot on them in their casting prepped for where the vacuum port would be drilled if needed, so it was apparently anticipated in the design of the carbs.

[old_skool]

My carbs should be fine, once I get them back together and cleaned for the third time...lol, but I like the possibilities of other options should the need arise.

[DGXSER]

On the XJ1100 there is no vacuum advance. The internals of the engine are identical. The difference is, on the XJ, they added a program to the TCI to adjust for vacuum. They took the vacuum signal and converted it into a modulating electronic signal and change the engine timing to coincide. What you see on modern bikes and cars for that matter is a very similar thing. They all measure engine vacuum at some point or source, and then use that signal as input the onboard electronics so they can adjust the timing.

As to older bikes not having it, well, my theory is that the gas then was not as prone to the knocking and pinging and did not need the timing as fine tuned in a smaller engine. Just my theory.

[crazy steve]

The vacuum advance is on these bikes for one reason only; to improve fuel economy. Without it, your cruising mpg can be as much as 15 mpg less. It does nothing to help 'performance', as it only comes into play at part-throttle/light loads and you could disconnect it if you wanted to as long as you're willing to accept the poorer mileage.

Most newer bikes have something similar to what the XJ11 has; the XJ uses a vacuum operated switch built into the TCI to change the timing based on manifold vacuum. If vacuum is above a certain value (light engine load), the TCI applies more advance with a predetermined curve, but this is a on/off type of setup and isn't quite as efficient IMO. Harley uses the same method on their bikes.

[bikerphil]

This guy will explain it better...

http://www.youtube.com/watch?v=rLDgQg6bq7o

[jetmechmarty]

Thanks for the enlightenment.

Harley uses the on/off method?

[crazy steve]

Harley uses the on/off method?

Yes, and I haven't seen any other method for doing this electronically except for what the much newer vehicles use, which is a MAP sensor (manifold absolute pressure). These are pretty much only found on engines with EFI and considerably more sophisticated electronics than what you'll find on an older carburated motor.

I'd be willing to bet that the XJ uses the same method as Harley; you have two predetermined curves built into the ignition box. If manifold vacuum is below a certain level (on Harleys, about 4"hg), you get the 'mechanical' advance which is only dependant on RPM; so many revs, so much advance. But if manifold vacuum goes above 4"hg (lighter load, partial throttle opening), the switch closes and the box then switches to another curve with more advance, but again predetermined and dependant on rpm.

This actually works reasonably well, but if you make changes to the motor you can run into problems. Raise the compression, different cams, different exhaust, or alter the timing and you may find the 'factory' curve isn't so good. Nearly all the aftermarket ignitions feature multiple, switchable curves so you can select the one(s) that works best, some will even allow custom curves via a computer. I've looked at these as a possible conversion for a XS, but given the small gains to be had, it's still pretty expensive. If the supply of factory TCIs ever dries up, it'll be time to take another look...

The usual problem on Harleys is the 'factory set point' of the vacuum switch is too low for some motors. Particularly on high-compression motors, at 4"hg the 'vacuum advance' comes in too early and you'll get pinging when the ignition transitions between the two curves. By digging the silicon sealer out of the 'calibration' screw, you can change the set point to a higher value (usually about 6"hg) and fix the issue.

The 'mechanical' vacuum advance on the XS is a 'better' method IMO, as it will change the advance based only on manifold vacuum (i.e. load/throttle opening), theoretically giving the most advance (and best fuel mileage) at any given load/rpm. But it's more complicated and has many more 'wear parts' to it (as well as requiring a bulky vacuum can), so most went to an electronic version.

[John]

The vacuum advance works only under light load conditions, like cruising. Under heavy load, with the throttle opened, the vacuum in the manifold decreases, and the vacuum actually retards a little, allowing the spark to come in a little later to prevent pinging, if I remember correctly.

[XS1100_OEM4ME]

Thanks Phill, I had forgotten about the importance of the Lotious endellow plate in the function of nober trunkins reactions

This guy will explain it better...

http://www.youtube.com/watch?v=rLDgQg6bq7o

[bikerphil]

Sorry for the thread interuption, I thought of that clip when I read 'rocket scientist needed'. It's kinda like CrazySteve explaining it.

[XS1100_OEM4ME]

Cool

Hey, not all of that helps on ign. problems, but the use of the inverse reactive current for unilateral phase detractors expains alot, now old sparky fires right up

Sorry for the thread interuption, I thought of that clip when I read 'rocket scientist needed'. It's kinda like CrazySteve explaining it.

[cywelchjr]

I think someone checked the manual on the XJ and found that it's a 3 setting system with NO advance, partial and then full advance above a certain amount of vacuum, and it gets it's signal from the same place the XS does, from the number 2 carb. At least that's my recollection, but that does sound correct, it's basically selecting from like 3 advance curves depending on the amount of vacuum seen at the fitting on the number 2 carb. This could get interesting to tune around on the XJ with it's sensor system with it's proprietary chip in the TCI as well.

I believe most MAP systems work pretty much the same way as well, with the exception of the ones with knock sensors which add the ability to dynamically adjust timing based on detection of pinging. These are the engines that are supposed to be able to take advantage of higher octane fuel because they will advance the timing automatically till detonation is detected (and they can sense it long before the human ear can hear it) and then back off a little bit. It's a bit fun with my Camaro as it actually gets BETTER mileage with premium than with regular, as it will automatically adjust, and runs well on either. But I'm still not sure the price per mile isn't still better with regular than with premium even in that case.

[XS1100_OEM4ME]

Knock

My Jag and BMW use those sensors very well, they do increase millage and preformance. Don't know the $$ to milage thing, but with the extra additves to clean the motor and such a good buy These bikes were "bleeding edge" stuff. Ahead of it's time and we are the ginnie pigs When it all works, it works well, when not, makes you mad as H#ll

I think someone checked the manual on the XJ and found that it's a 3 setting system with NO advance, partial and then full advance above a certain amount of vacuum, and it gets it's signal from the same place the XS does, from the number 2 carb. At least that's my recollection, but that does sound correct, it's basically selecting from like 3 advance curves depending on the amount of vacuum seen at the fitting on the number 2 carb. This could get interesting to tune around on the XJ with it's sensor system with it's proprietary chip in the TCI as well.

I believe most MAP systems work pretty much the same way as well, with the exception of the ones with knock sensors which add the ability to dynamically adjust timing based on detection of pinging. These are the engines that are supposed to be able to take advantage of higher octane fuel because they will advance the timing automatically till detonation is detected (and they can sense it long before the human ear can hear it) and then back off a little bit. It's a bit fun with my Camaro as it actually gets BETTER mileage with premium than with regular, as it will automatically adjust, and runs well on either. But I'm still not sure the price per mile isn't still better with regular than with premium even in that case.