Vacuum Advance Modification

[IanDMacDonald]

So, I know the premise behind the vacuum.advance. I know the PO modified mine for pergormance readons "The distributor centrifigul advance curve reduced, allowing the initial to be increased, no vacuum advance."

I had a buddy admiring my bike when he saw that the advance had been modified, and he disliked that. From what I can tell, Scott stuck a bolt in the advance at whatever position it's in, and capped off the hose barb. The buddy said that it will negatively affect the performance of the bike, as well as fuel mileage. However, Scott owned the bike 21 years, and maintained it regularly, so I take his knowledge of his modification over the buddy.

The question I have is why I don't see anybody else on here with that mod?

[crazy steve]

"The distributor centrifigul advance curve reduced, allowing the initial to be increased, no vacuum advance."

A couple of things going on here.....

First, disconnecting the vacuum advance isn't a 'performance mod' per se. Under large throttle openings/high load/low manifold vacuum, it isn't doing anything anyway.

Disconnecting it is 'old school' thought. The XS11 was the first bike to feature a vacuum advance, and this was done to both improve mileage as well as to reduce emissions. There is no performance penalty for having one if it's working right. But it was perceived as a 'minus' as race motors didn't use them, but they also didn't need them, operating at WOT as they do. So when these appeared, many 'old school' tuners disconnected them to return to the familiar, 'high performance' timing of race motors.

Now, if you start playing with the ignition curves, it's possible to have issues with the vacuum advance. But any advance curve is a compromise of some sort, a vacuum advance is simply a way to adjust timing over a wider range of operating conditions that isn't available with a simple mechanical advance. For maximum efficiency, you want to run the most advance you can. Vacuum advance is a way to do this while taking load into account, something mechanical advance can't do.

The XS11 'vacuum-mechanical' system is arguably a better system than the later vacuum-switched electronic 'vacuum advances' found on newer bikes (such as the XJ) when it's working right. These offer only a choice of two curves, with the switching occurring at a specific point that offers the fewest issues but certainly not the best performance. It was 'better' for the manufacturers as it allowed them to use a less expensive and easier to make 'switch' rather than a vacuum can that needed to be calibrated across different vacuum signals. Fewer moving parts too, so from a warranty standpoint it's 'better' that way too.

So why don't you see more owners disconnecting the vacuum advance? Because it hurts mileage, with no corresponding increase in performance. If you've heavily-modded the motor, there may be a case for disconnecting it (high compression, blower/turbo, nitrous, etc) but on a more-or-less stock motor, it's better to leave it working.

[IanDMacDonald]

Wow, really interesting. So, not that I'd touch it, but I do know based off my current carb turn my fuel mileage is poor. I'm curious if I hook it back up, what might happen?

[bikerphil]

The vacuum advances the timing during very light throttle openings (crusing) and your fuel mileage should improve. It does not come into play at idle or heavy throttle openings. Make sure you hook it to the #2 carb body and not the carb boot.

[Courtney]

I'll tell you what will happen. Your bike will get better gas mileage and probably perform better all around. On a related issue, I couldn't figure out why my 67 bug was such a dog so I took it to a performance tuner. He took one look at it, and said "who took the vacuum line off the distributor?" I immediately blamed my dog because I didn't want to look like a newbie idiot. He put it back on, and said try it now. The difference in pickup and mileage was like night and day. you know what my bill was? He made me read the book, "How to HotRod VW's" by Bill Fisher. Great guy, and I learned a lot!

[IanDMacDonald]

Hmm. I'm not comfortable touching it till I talk to Scott and see why he did it, and the effects of it.

[crazy steve]

I'll make a few suppositions here....

The only major difference between the '78 and '79 bikes was the ignition curves. There may be two things going on here; one, he tried to mimic the '78 curve by limiting the advance and dialing in more initial advance, and two, he may have got a momentary knock when cracking the throttle when cruising. Disconnecting the vacuum advance will stop this, but this is fairly common on stock bikes; mine did it from new, with no harm that I could tell. Because of the 'altered' curve, it may have been worse on yours.

While changing the ignition curve like that can improve throttle response, it won't improve maximum power and will be a mileage-killer....

You might want to look at this.....
http://www.xs11.com/forum/showthread.php?t=37189

The vacuum cans had different curves for each year too ('matched' to the mechanical advance) in spite of what the FSM says.

[IanDMacDonald]

Yeah, that post is what provoked me to ask that question. Scott is not answering back on emails, but I really am not sure it's a good idea for me to touch it without asking him. The only thing I see that he did was put a bolt in the advance unit with blue thread locker.

[crazy steve]

If he put a bolt/screw in one of the advance slots, that's taking a lot of timing out of the bike. Note that the slot length difference between the '78 and '79 reluctors is only .05" (a 6/32 screw is twice that at least) and that's five degrees of timing. You're probably losing 10 or more degrees of timing, and that would certainly explain the poor mileage. You're likely giving up quite a bit of power on the top end too.

You really need to check it with a timing light and see what you have.

[Rasputin]

Honestly, I would wait a bit for Scott to answer. This is not something that "needs" fixing tonight. I would see what his reasoning is for it and then post his answers and ask if that makes sense.
Yes instant gratification is nice but there may be a reason it is done that way. I do not know why but with the other modifications he has done to it, well who knows. Relax and enjoy a beer and wait till you know why he done it. Could be he made a mistake or could be he changed parts around that you do not yet know about. Dunno but it could be the difference between a well running scoot and an oopsie!
For my part I have to agree with what Steve says but I am not the one who did it nor am I the one who has to live with it.

[IanDMacDonald]

For once, I am going to be patient and fight the urge to experiment and leave it alone. I'll wait for Scott to reply. If nothing, then I'll take some pics, check timing and update yall.

[shousey440]

The bolt is not placed in the advance slot, it's just in the hole in the cover where the vacuum hose used to be. The centrifigul advance slot was resricted by me to have more initial advance. The compression was up a bit also with 171# compression test average. With the vacuum attached there was too much advance. When I sold it it got 44 mpg without vac adv.

[shousey440]

Total timing was set.

[IanDMacDonald]

Again, as stated before, I believe my current carb tune is to be the cause of my poor mpg's. I will check the timing again to make sure it's not off when I took that plate off. Also, I will re-check the plug color again

[shousey440]

I no longer remember what the sweet spot initial timing ended up being after shortening the cent adv slots, but it is more advanced than stock. 34 degrees at 3000 RPM should be about right for total timing.