Timing, SVDA and Advance Curves.

[Amskeptic]

With vac if you blip the rpms you can see 40 + degrees of advance briefly.

This is flat out wrong. When you get on the throttle the engine vacuum goes down not up. So the timing would not advance under these conditions.

If you were missing a spring the timing would come in sooner i.e. lower RPM but it would not go any higher than it otherwise could.

Actually, it is observed all across this land.

When we check the vacuum advance, we are NOT reading advance during the "acceleration" of the engine rpms, we are reading the advance curve on the "overrun" back down to idle. Imagine how a vacuum gauge needle responds to a throttle blip. The needle dives down to 5in. let's say, during the acceleration phase, but quickly ramps up to 20in. as the engine returns to idle. The vacuum advance gain we are reading on the scale of the Type 4 engines actually goes up when we float the rpms around 3,000 rpm and maxes when we let go of the throttle.

Let's chill with the "this is flat out wrong" until you have asked at least one clarifying question of the poster, for politeness' sake.
Colin

[chitwnvw]

Lol. As usual, I am half wrong, yet somehow I keep my fleet on the road.

[tvargs]

Update! Something I hadn't noticed before: the hose connecting the air cleaner to the vacuum advance and the throttle body (including the "T" connector) was missing! So when I was revving the engine and watching the timing, I thought that all the vacuum hoses were connected. In fact, it was just the throttle body connected to the vacuum advance, and the air cleaner diaphragm was unconnected to either (and unblocked, as well).

One thing that confuses me is why connecting the throttle body directly to the vacuum advance would cause the engine's timing to increase so much when revved, but when disconnected (i.e. so all three points are open and unconnected) the timing advances only to about 30 degrees. I keep on referring to this diagram, but wouldn't a single missing link cause the same issue as missing multiple links?

In other words, why would the advance be limited to 30 degrees when the throttle body, vacuum advance, and air cleaner are all disconnected, but when the throttle body is connected to the vacuum advance, the timing shot up to 40 degrees? I imagine it is because the vacuum advance is affecting the timing, but if the air cleaner isn't connected, how is it producing a vacuum at all?

[Randy in Maine]

Will the EEC valve in the air cleaner hold a vacuum?

[Amskeptic]

why connecting the throttle body directly to the vacuum advance would cause the engine's timing to increase so much when revved, but when disconnected (i.e. so all three points are open and unconnected) the timing advances only to about 30 degrees.
I imagine it is because the vacuum advance is affecting the timing, but if the air cleaner isn't connected, how is it producing a vacuum at all?

The air filter EEC valve diaphragm often has a leak that kills the vacuum advance. If the EEC valve is leaky, people will just run the hose from the throttle body to the advance unit. THEN you get to have your vacuum advance that let's you read 40*. With a leaky EEC valve in the circuit, you get no vacuum advance and you will only be reading the centrifugal advance of 28-30*.
Colin

[Gypsie]

So I just want to understand the graphs a bit. Looking at the SVDA (yellow line)

mechanical advance graph: AT 1000 rpms the advance curve starts at 0 (that does not mean zero on the scale but zero in relation to how much advance is being applied to the timing?....right...?)
By setting to 28* at 3200 RPM we are setting to an optimal point in firing for that RPM.

I read the mech. advance curve to indicate that the advance timing at 3200 would be approximately 18*-19*. Would this mean that, if set to the above optimal point (28* at 3200), at 1,000rpm the timing would be around 9-10*?

What am I missing here?

Sometimes ignorance is bliss cause it seems that the more I try to understand the less I do...

[Randy in Maine]

something to note....the SVDA (and according to ratwell where this came from this is the aircooled.net SVDA) has about 24º of centrifical advance when it is all in. If you were to set it to 28º when all of the centrifical advance it has to offer, the "intial advance" would be somewhere about 4º BTDC (28º-24º) at idle. If you set it to 32º when it is all in, that would put the initial advance at about 8º BTDC (32º-24º) at idle.

We always want to set the max centrifical advance to be 28-32º as our intent here is not to end up with too much centrifical advance and pre-ignition. Death to our engines.

Does that help?

[Gypsie]

Yes. It also leads me to think I should be looking to set more near the 4,000 rpm mark(or at least where I note that the
advance has stopped advancing...) as 3,200 isn't max advance.

I run at at least 4,000 on the freeway anyway.

My question was primarily to clarify the difference between the "advance curve" degrees and "set to" degrees on the scale.

[bajaman72]

I run at at least 4,000 on the freeway anyway.

That seems kinda high.But im runnin 30inch tires and dont know crap about busses yet. Still seems kinda high.

[Amskeptic]

Yes. It also leads me to think I should be looking to set more near the 4,000 rpm mark(or at least where I note that the
advance has stopped advancing...) as 3,200 isn't max advance.

I run at at least 4,000 on the freeway anyway.

My question was primarily to clarify the difference between the "advance curve" degrees and "set to" degrees on the scale.

Set at 3,200-3,600rpm as per your distributor . . . then check for additional advance past 3,200-3,600rpm. IF additional advance does occur, then yes, you must choke it back a bit so that you never exceed 28* centrifugal at any rpm.
Colin

[Bleyseng]

with a 091 tranny and 185x14 tires I run 4000rpms at 70mph in my westy...given that I don't know the speedo error so thats just indicated 70 mph.

With the timing setting, Colin has it right so set it at 3500rpm to 28 degrees BTDC with hoses off and you're good but check to see if the timing advances beyond 28 by reving it to 4000 rpms after you set it still with the hoses off.