Vacuum Advance

[D. P. Larmee]

I had a few symptoms on my standard that had been bugging me for a few months (loss of power when accelerating from a steady cruise on the highway, a lot of popping on deceleration, and she just seemed to be running hotter). I finally got around to checking the ignition timing and it turns out the vacuum advance had a hole in the diaphram and wasn't holding any vacuum. I swapped out the vacuum canister from my special and now she runs like new again. Now, I seem to remember reading on here that people have dropped their vacuum advance without much performance loss. I just don't see how that is possible; on my bike, it seemed like night and day. Is this just in my head?

Also, It doesn't look like the vacuum canister is still available from Yamaha. Has anyone found an automotive can that is readily available that will swap out? I was hoping that there would be some numbers on the can that could be referenced at the parts store, but alas, nothing. I've got a friendly parts place down the road that should let me dig through their stock to try and find a match, but I figured if someone else has already done the research, they might be able to point me in the right direction.

Thanks all,

Dan

[bikerphil]

Vacuum Diaphram

I don't think they are available new anymore, just used. I've carefully pried one apart before and repaired it with RTV. This was about 6 mo. ago and still holding up fine. I've also adapted a vacuum break off mid 80's Cadillac carburetor to fit. Wasn't easy! Easiest way would be to find a good used one from maybe Andreas or eBay. Yeah, the bike will lose power if adv. isn't working.

[bikerphil]

Vac. break mod.

Ok, found it. Here's a pic of the adapted 85? Caddy vacuum break adapted to fit an XS11. The rod of the OEM break was cut off and flat rivited to the Cad. unit. With an extra bracket to achieve the proper angle, it can be mounted to the bike. Ain't pretty, but I used this particular set-up for years, and it operated the advance curve the same as the OEM.

[D. P. Larmee]

I appreciate the help. That's pretty clever with the caddy canister. I might end up doing something similar. Luckily, the special is put up for the winter already, so I have plenty of time to get things sorted out. I think I will attempt to take apart and repair the original canister first and then look at other options if that doesn't work.

[boyat68]

D. P. Larmee:

I too have read on this site that the vacuum advance only improves fuel economy, but am inclined to believe that it does more than that, how ever small the improvement is. I would think that if you disconnected the advance, then the very least you would have to do is re-synch the carbs.

[TopCatGr58]

Hey Dan,

Matching up an auto one to work like ours was one of the many projects I had planned to do also, but just got busy with other stuff. The manual specs are max pull should be at 150mmHg/5.9inHg, but it doesn't say how FAR that pull is. Measuring the total travel that is required for it, and then trying to find one with similar pull range, and vac. range should be possible!? If you have a vacuum gauge/pump you could take that with you to be able to put them onto the vacuum line and test both operation/range of pull at the specified vacuum level! Like BikerPhil did, should be able to then mount the vacuum pot onto the XS11, some may bolt on almost directly, I've seen them with offset and angled mounts, but if not, then should be able to put the vacuum pot alone onto the xs11 hardware/mount.

With the loss/removal of the vacuum advance/retard unit, I would think a modest increase in the static advance would be in order to help compensate for the need for it in the lower rpm/low load performance range. Max cent. adv. is attained at ~5k rpm, and under WOT/FULL throttle, the vac. adv. isn't providing anything, but under low loads/modest throttle input, even at 5krpm, it's bound to provide some additional adv., hence the bike mags quote of a "possible" max adv. of 52 degrees! However, the lesser adv. of 36 degrees was reported as optimal for WOT/FULL throttle response.

So...might also depend on how OFTEN you would use WOT/FULL throttle, and I would wager that most of us do NOT use that very often or for very long, so increasing the static adv. about 10 degree MORE than stock, would give you max of 46 degrees adv at 5k rpm. Might be a good compromise point for folks without the vac. adv.!? Sounds like some fun experimenting could be done on this, unless someone has already done it, but I don't remember reading anything like this in the last 6 years on channel 11!?
T.C.

[bikerphil]

Measuring the total travel that is required
T.C.

Should be 8mm. I've experimented with more travel and the engine would "ping".

[TADracer]

Here is how I figure it all works

Vacuum advance mechanism helps drivability in the lower RPM ranges and may not necessarily provide 'advance' as the name suggest.

The simple function is that the vacuum is a quick acting timing adjustment while the crank speed is a much slower acting adjustment. So the combination of the two means that when you open the throttle the vacuum will drop giving a quick timing retard to better light off the resulting rich mixture. Let off the throttle the vacuum rises in an instant so the vacuum unit makes an instant timing advance to better burn the resulting lean mixture (which was causing your popping on deceleration since your vacuum unit was malfunctioning). Crank speed just will not react that fast.

Here is how I figure it all works. Static timing, centrifugal and vacuum systems all work in concert and are sometimes layered on to give total timing. So if you remove the vacuum part then you will have to make a change some other place to compensate. Static and centrifugal advance only should give you 36 degrees (31 degrees for special) but when the vacuum advance is added in under, part throttle cruising where the vacuum is high you should have a total advance of around 52 + degrees. Now, dump the throttle open and vacuum drops giving you an instant 20 or so degrees of retard to burn the rich mixture until you close the throttle again and the mixture leans out and the vacuum rises. If your centrifugal advance is already at max when you do this (and you have no vacuum advance) then you would have no timing adjustment at all to compensate for the richer mixture. Thats why (so I am told) some of the later XS'es dont have a vacuum advance but use a different TCI box which makes some adjustment there.

Vacuum advance will give a really smooth operation to a carburated motor. Modern fuel injected motors do the same thing but with the engine control module which constantly monitors the RPM and throttle position. Do the same throttle open/close routine as described above then the computer instantly changes the fuel map and timing curve electronically, achieving the same result we get with the vacuum advance.

[Bigleo]

vacuum advance

Hi, I just went thru this when I switched to aftermarket carbs without the vacuum port. I plugged the vaccuum line to the canister. My bike being a Special had the decidedly non performance 31 degrees max advance, (static and centrufugal combined). For a performance motor you should have about 36 to 38 degrees. TC is right, going to 41 degrees is a good compromise if you are not using the vaccuum advance. You will notice throttle response is better than with the vacuum advance connected. The down side is: if you really crank the throttle open at low RPM ,you don't get the instant timing retard that TAD racer is talking about. This can, or will, cause pinging due to excessive ignition advance. To cure this, you can either run premium gas, which will get rid of most of the pinging, or just back off on the throttle a bit till the RPM picks up. Once above about 4000 RPM there is no pinging in any case.
Leo