Video: Manifold and Advance Vacuum

[3Phase]

This is my first-ever YouTube video. It's a 17 second clip of two vacuum gauges and the mechanical ignition advance while the bike is at rest in the garage.

The large gauge is connected to a tee in the vacuum advance line, the smaller gauge is manifold vacuum from the #2 intake boot. The line to manifold gauge was slightly restricted to eliminate most of the needle flutter. Nothing was done to the vacuum advance line except the tee.



I haven't worked out how, exactly, to mount the camera to catch both gauges and the advancer plate movement while I ride the bike to put it under load.

Holding the camera in one hand down by my leg and pointing it in the general direction of the gauges and pickup coils without using the viewfinder while I try to work the throttle with the other hand is a no-no from the get-go.

If I can get it to work tomorrow: Cellphone! On a stick!

[petejw]

theres a big difference to how the needles
react, if you wanted to take the pulsing out of the
manifold connect the four carb nipples with hoses and
t pieces, tho itl still give you the same readings with
a restrictor.
so what are your thoughts on this after viewing the guages?

[3Phase]

so what are your thoughts on this after viewing the guages?

It works just about how I thought it would work. Advance vacuum rises ahead of manifold vacuum on acceleration to steady throttle; slight lag then no advance vacuum while manifold vacuum rises on deceleration and steadies at idle.

Tomorrow I'll re-verify the gauges to make sure they're both reading the same because the numbers looked a little different than I thought they should look, then I'll see how it works under load.

[tadtl1]

load

Perhaps a less dangerous way to do this would be to put it on the center stand and use the brake and run it thru the gears to load the engine. Although I have never tried this either and perhaps it would be more trouble than anything. Just a thought. I salute you for trying to get to the bottom of this issue with the vac advance.

[motoman]

Take no risk there Scott!..........can guarentee timing vacuum advance IS ported vacuum. Not only on these scoots but on ALL older Detroit iron also. Think your more interested in the challange than the result. Careful out there.

[3Phase]

Thanks, Tad!

I had ... an incident years ago with my KZ when I was using the centerstand and brakes to emulate a running load that I'd rather not go through again in this lifetime.

If I can't get the camera and gauges mounted to make a decent riding video I'll just note the needle movements and the high readings and call it a day!

[tadtl1]

I can just imagine,,,,,,

Somehow I am imagining that kaw rear wheel spinning, you needing about three legs and seven hands to accomplish what you needed and the whole works vibrating off the centerstand and the tire burning a hole thru your forehead in the end, yikes.

[3Phase]

Somehow I am imagining that kaw rear wheel spinning ... yikes.

Pretty much, and the KZ had no tipover switch so with the throttle locked at RPM and no brakes it ran like the Energizer Bunny. It kept going, and going, and going, and going, and ... until it ran out of me to run over and bounced off into the side of the house then took off doing flip-flops and donuts and all kinds of mean, nasty, horrible things.

Never. Again.

[3Phase]

Ride Video Not Dialup Friendly

I managed to get a video without breaking anything. I couldn't make a camera mount that wouldn't vibrate and ruin the video. I don't have a million dollar SteadyCam mount like they use on helicopters so I had to go to plan B: Hold the camera and point it at the gauges.

Zip-ties under the gas cap and a shop rag hold the vacuum gauges to the gas tank.



The large gauge is vacuum advance and has the active range of the advance pot marked in masking tape between 38 mm HG (1.5 in HG) and 111 mm HG (4.4 in HG).

Vacuum advance begins at 38 mm HG (1.5 in HG) and full advance is at 111 mm HG (4.4 in HG).





The linked ride video is ~14M. Vacuum higher than 111 mm HG (4.4 in HG) vacuum adds nothing to the total advance.

http://www.youtube.com/watch?v=mVrJ97B95iY

The vacuum does about what I thought it would do under load and that's quite enough for now!

[3Phase]

I had the timing curves for the the 1980 XS1100s but I couldn't find them so I did a quick scan and conversion. As you can see the curves are not really marked.

Based on the actual vacuum readings I made I believe the bottom line is the one for the 80G, the one on top is for the SG. Of course it's possible the original owner had an 80SG advance put on his 80G but there's no part number on it so who knows?

Vacuum advance curves for the 1980 XS1100G and SG


I haven't checked the mechanical advance yet but the two curves seem to be pretty close to one another. It won't be difficult to check because one curve peaks at 3,900 RPM and 30 degrees and the other curve keeps going.

Mechanical advance curves for the 1980 XS1100G and SG

[TopCatGr58]

Hey Scott,

I watched the video, and it shows exactly how it's supposed to behave.

When you first start out you are giving it modest throttle, and near the end of that run up the gears you back off and hold for just a second, and you can see the adv. vac rise considerably...wish you had held it there at a cruising rpm for a little longer.

Then the second run up thru the gears you were getting on the throttle much stronger, and the adv. vac. stayed very close to zero which keeps the timing at the better (less than 30 or so degrees of adv) power curve amount for strong throttle response, but again when you back off the throttle, it then rises to adv. the timing more for leaner/cruising type performance!
T.C.

[3Phase]

...wish you had held it there at a cruising rpm for a little longer.

Me too, T.C., but I ran out of clear skies (Simi PD), clear road and clear traffic. Holding a camera in one hand while driving down the street and staring at the gas tank through the cellphone screen is not the way to have a accident/police-free afternoon ride.

The resolution sucks because of the movement and because it's a cellphone.
I'm going to take some time and find a better video camera and a mounting system, then try again.

... the second run up thru the gears you were getting on the throttle much stronger, and the adv. vac. stayed very close to zero which keeps the timing at the better (less than 30 or so degrees of adv) power curve amount for strong throttle response, but again when you back off the throttle, it then rises to adv. the timing more for leaner/cruising type performance!
T.C.

Yes, the advance vacuum and manifold vacuum was low at WOT. It's not lean so it doesn't need help to burn the gas.

I should probably have put some colored tape on the other gauge to show the 1.5 - 4.4 range. The first mark on the small gauge is only 5 inches of vacuum and the advance and manifold vacuum pretty much tracked one another on acceleration at WOT until I slowed a little.

What surprised me was just how little vacuum the '80 models need for full advance and how little advance they add compared to the '78/'79. That was an eye-opener.

[crazy steve]

So there you have it; vacuum advance and manifold vacuum are more-or-less the same. The vacuum advance lags what the manifold vacuum does, but that's due to the restrictor in the carb connection; it takes time for the vacuum/pressure to get through that little hole.

But that begs the question; on transition from mostly closed to open throttle, the fact that the vacuum advance falls out slower could cause knocking or pinging under the right conditions. With the design, that may be tough to tune out.

And Pete's dissected vacuum advance (in the original thread) shows how the factory calibrated the advance; the screw head is for limiting total advance, and the outer shell adjusts the spring rate, or speed of how fast the advance comes it.

'78E original owner

[3Phase]

So there you have it; vacuum advance and manifold vacuum are more-or-less the same. The vacuum advance lags what the manifold vacuum does, but that's due to the restrictor in the carb connection; it takes time for the vacuum/pressure to get through that little hole.

Sort of. I'll have to take another ride with a better setup so I don't have to make like a freakin' chameleon trying to watch the cops, the camera and where I'm going all at the same time. I have a tripod and a camera to fit it so I'll figure out something.

But that begs the question; on transition from mostly closed to open throttle, the fact that the vacuum advance falls out slower could cause knocking or pinging under the right conditions. With the design, that may be tough to tune out.

Tuning would have to vary by the year/model of the bike. Ivan already tried changing the total advance with the set-screw and it wasn't enough.

I don't have the curves for the '81 H, SH/LH.

The '78/'79 look like they have the same vacuum advance curves according to the book but that may not actually be true across both years and three models.

There are at least three pots for the 1980 models: the factory G, SG/LG pots and the 'no-knock' replacement pot.


None of the pots are marked. There may be different colors of epoxy covering the adjustment screw or a paint dot somewhere for year/model/pot/setting but even if there is a part number stamped on it there's really no reliable way to tell them apart. They'd have to be verified and a baseline established with a vacuum pump, an accurate gauge and a set of calipers before attempting any tuning.

And Pete's dissected vacuum advance (in the original thread) shows how the factory calibrated the advance; the screw head is for limiting total advance, and the outer shell adjusts the spring rate, or speed of how fast the advance comes it.

That's solid! Okay, it's not so solid now that he took it apart but you know what I mean!

[crazy steve]

...Tuning would have to vary by the year/model of the bike. Ivan already tried changing the total advance with the set-screw and it wasn't enough.

I don't have the curves for the '81 H, SH/LH. The '78/'79 look like they have the same vacuum advance curves according to the book but that may not actually be true across both years and three models. There are at least three pots for the 1980 models: the factory G, SG/LG pots and the 'no-knock' replacement pot.

None of the pots are marked. There may be different colors of epoxy covering the adjustment screw or a paint dot somewhere for year/model/pot/setting but even if there is a part number stamped on it there's really no reliable way to tell them apart. They'd have to be verified and a baseline established with a vacuum pump, an accurate gauge and a set of calipers before attempting any tuning....

All these differences in advance curves and vacuum units leads me to another thought; all the parts swapping that sometimes goes on could cause subtle changes in timing that may be some of the tuning issues that crop up. This makes me even wonder about the carbs; with the restriction in the vacuum feed to the advance (shown by the lag between the gauge readings in your test), who's to say that the restrictor is the same for each year/model? The G/SG vacuum curves you show are markedly different from my E; the one curve is all-in at 75 mmHG, while the E needs 150 mmHg for full advance. That's a pretty big difference...

But like you said, determining all this would be an involved process and the likely only 'easy' fix would be to try to stick with 'matching' parts if possible. Keeping the mechanical/vacuum advances and the carbs as 'sets' would seem to be the best choice.

But with the 'frankenbike' history that many of these have, that will be tough to do...

'78E original owner