GL1100 spark timing question - Page 3 - Steve Saunders Goldwing Forums

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post #21 of 27 (permalink) Old 03-28-2019, 06:22 PM
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Way over my head, but 53 is in danger territory from every Thai earth read. I wonder if it isn’t 13 degrees BTDC up to 38.5 degrees with the mechanical advance, which is 25.5 degrees advance on the mechanical advance. That is how the 1000 mechanical advancers work. 75-77 years start at 5 BTDC up to 37.5 BTDC fully advanced for 32.5 degrees total advance (37.5-5) for the mechanical portion, while 78-79 years went for 10 BTDC to 37.5 BTDC fully advanced for 27.5 degrees advance for their mechanical portion.
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post #22 of 27 (permalink) Old 03-28-2019, 07:49 PM
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I'm going to try this one more time. Vacuum advance cannot add to full centrifugal advance. If the vacuum source is ported, the vacuum advance does not work at idle because the throttle plates are closed. It also does not at full throttle, because there is very little ported vacuum. It's a love/hate relationship as they work together, then ignore each other. As stated before, 35-38 degrees is total advance. The only time you will ever see 53 degrees advanced, is just before the engine scatters.

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post #23 of 27 (permalink) Old 03-30-2019, 05:40 PM Thread Starter
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What happens with the vacuum from the carburetor nipple on the road is different from what you see in the shop. In the shop I couldn't get any sustained vacuum from the carburetor nipple. On the road, I saw about 2" Hg during gradual acceleration, 4" cruising at 60, and 5" cruising at 70. Since the rpm is over 3000, there is 38.5° of advance from the centrifugal mechanism and my vacuum advance is fully actuated with 3 1/2" Hg, which adds another 15°.

With a small throttle opening at a steady speed, the air rushing past the throttle butterfly makes a vacuum over the same .030" air hole that relieves the vacuum with the throttle closed. This venturi vacuum adds to the manifold vacuum from the .015" hole. A little throttle will reduce vacuum and retard the timing. A lot of throttle will take vacuum down to zero. And, it also goes to zero with the throttle shut. This is the way it works.

The service manual doesn't tell us everything we want to know now, 39 years later. The purpose of the manual was for service, not for evaluating the design features and specs. It tells how to check the vacuum advance with vacuum, but not how to check it's effect on timing, so why should the manual specify something that is never to be checked. It didn't.
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post #24 of 27 (permalink) Old 03-30-2019, 10:27 PM
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Maybe this will help. As the engine speeds up, the centrifugal weights move the rotor opposite of engine speed increasing advance of spark. When the vacuum pulls, it doesn’t pull the mechanical advance, it pulls the plate holding the pulsers which are triggered by the rotor connected to the advance. Since this reduces the distance to the rotor, the advance is retarded, not advanced.
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post #25 of 27 (permalink) Old 03-31-2019, 05:48 AM
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Originally Posted by Ekvh1 View Post
Maybe this will help. As the engine speeds up, the centrifugal weights move the rotor opposite of engine speed increasing advance of spark. When the vacuum pulls, it doesn’t pull the mechanical advance, it pulls the plate holding the pulsers which are triggered by the rotor connected to the advance. Since this reduces the distance to the rotor, the advance is retarded, not advanced.
No, it moves the plate in the opposite direction of engine rotation which advances the timing.

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post #26 of 27 (permalink) Old 04-01-2019, 05:03 AM
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I just took my 1980 GL1100 for a test ride with a vacuum gauge taped-on so I could monitor the vacuum affecting the vacuum advance. I was pleased to see 4" Hg was fairly easy to get at steady speed on the highway. I did not think the .030" air hole in the little vacuum chamber that works with the throttle butterfly would allow this much, but it does if the diaphragm does not leak. I did not see this 2 years ago when I tested an 82 the same way, but that diaphragm leaked.

The vacuum port in the carburetor limits the flow of air with a .015" orifice, so a leaky diaphragm allows little or no vacuum to develop, depending on how badly it leaks. It is a good thing that the orifice is there, because it keeps that cylinder operating normally no matter how badly the diaphragm leaks. 4"Hg moves the vacuum advance as far as it will go, which is about 15°.

I have answers for all my questions now. Please forgive my previous criticism of the stock GL1100 setup. It is a great setup if the components are in good repair. The centrifugal advance is the most important, because it keeps working long after the vacuum diaphragm gives out. What I did not understand was how the vacuum advance fits into the picture, since the centrifugal mechanism makes the specified timing all by itself. I could not believe the timing normally advances 15° beyond 38.5°, but it does. "Full advance" is the specification for the centrifugal advance mechanism, not for the whole system. That absolute maximum advance number is not specified anywhere, but I know it to be around 53°. It has to be, because 38+15=53. In order to check it in the shop, you have to apply 4" Hg to the vacuum advance. This test is simply not in the manual. The high advance works because it only happens under light load and it retards if more throttle is applied or if the throttle is closed.
Almost all cars (at least "normal" European cars) worked this way some 30 or 40 years ago .
As you explained , one should be aware , that it is the possition of the butterfly valve that creates the "vacuum" and this has nothing to do with the vacuum in the manifold !
This "butterfly vacuum" only is present under wery ligth load on the engine , which also means that the centrifugal adwance isnt adwanced fully ( because of low revs on the engine (say around 2000 - 2500 rpm).
Yes you should have a total of 53 degree but that never happens in practice because the vacuum advange only works under ligth load .(when you press the accelerator to go uphill the vacuum for the advance unit instantly "disappears" and the unit "retards" the advance to avoid pinging from engine .
The meaning with this vacuum advance was to (sligtly) improve fuel economy and to improve comfort/noise/wibration when "cruising".
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post #27 of 27 (permalink) Old 04-01-2019, 03:45 PM
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No, No, No! 38+15=53. It says so in my math book!

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