I'm also wondering how the advance curve is figured, since you no longer have a mechanical/vacuum advance mechanism, nor are there any other sensors involved, such as baro/map, knock, or throttle input(tps)
To understand timing, first you must understand how all aspects of an engine work so you can see why the timing is placed where it is optimal for performance, without hurting the engine.
I have previously explained what a timing curve is. Now let's start with the question of mechanical/vacuum advancers and intake manifold vacuum.
When your engine is at idle or under a light load (flat surface or a down hill) with the throttle minimally open, the manifold pressure will be high.
As the throttle is opened the velocity of the incoming air increases. The engine rpm then increases, and the mass airflow increases. Manifold vacuum however, actually DECREASES. Simply put, when you open the carburetor butterfly or slide you are opening a large hole. This allows more air in which fills the void, so vacuum decreases.
“How does that affect my Wing?”
The Gl1100 timing (depending on year) at idle is 10-13 degrees BTDC (before Top Dead Center).
Full advance is 38.5 degrees. Since the 1100 has flyweights, timing advance is added once the rpm exceeds 950 (when it was new anyway). So part of your 38.5 advance comes from the weights, and part comes from the advancer mechanism. Got it?
NOTE: When your engine first starts, you have NO vacuum. Once the engine is running your vacuum climbs to the highest reading. This causes your advancer diaphram to pull in the rod, and moves it to the MOST advanced position.
Don't get confused. Low rpm/low load creates high vacuum, which pulls your mechanical advancer to the MOST advanced position. (Let's say the 10-13 degrees of advance at idle is probably provided by your vacuum advancer...does anyone know? Please share if you do! ).
So your bike is idling, and the vacuum advancer is fully advanced. As you increase rpm the fly weights start to provide the rest of the advance. Now lets assume you are at full advance and you roll on the throttle to speed up. The load on your engine goes UP. That causes your vacuum to go DOWN, and causes the advancer to back off, which RETARDS the timing of your engine.
If for example, your advancer is responsible for 10 degrees of the total 38.5 degrees of advance, you can begin to see why a properly working advancer would be important. A change in load could retard your timing by up to 10 degrees.
Many forums have posts recommending plugging the VA hose and forgetting it. This would be a poor recommendation as this would reduce your timing and could affect engine operation.
Things to be noted:
Engine vacuum falls as the throttle is opened and is used to sense load
Engine Timing is reduced (retarded) under loaded conditions
