Join Date: Apr 2009
Location: Anoka, Minnesota, USA
Year: 1982 GL1100 Goldwing and 1982 V45 Magna
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.
Hondas are my favorite motorcycles.
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