Electronic distributor vacuum advance adjustment question (1 Viewer)

[fyton2v]

Anyone know the correct setting is for this spring in the picture below? It looks like those two nuts can retard or advance the base timing. Where is the starting point? Does it matter? I'm trying to get more familiar with the effect of vacuum advance on different driving situations. This dizzy is one of those advance and retard electronic units that started showing up I believe in '78. The next thing I'm trying to figure out is my total advance. If anyone knows what that should be, approximately, shout it out.

Thanks.

 

[firemanj92]

ask John (@4Cruisers)

 

[4Cruisers]

I'm pretty sure the spring is just there to dampen the return to base timing coming off vacuum advance. Later 2F distributors ('81 through '87) don't have a spring. The two nuts can be adjusted to set the upper limit of vacuum advance by limiting the actuator rod travel. I set the maximum travel at ~3.8mm, which corresponds to 10 degrees distributor advance. That's pretty much the factory setting.

 

[fjc-man]

Plus the centrifugal advance for a total advance of approximately 32 degrees?

 

[fyton2v]

I'm pretty sure the spring is just there to dampen the return to base timing coming off vacuum advance. Later 2F distributors ('81 through '87) don't have a spring. The two nuts can be adjusted to set the upper limit of vacuum advance by limiting the actuator rod travel. I set the maximum travel at ~3.8mm, which corresponds to 10 degrees distributor advance. That's pretty much the factory setting.

Thanks John. Just to be sure, you're limiting to 3.8mm of rod travel, not plate movement, correct?

And what's the ballpark of total advance? I'll be playing some games with my timing gun this evening. The thing that set me down this path was a notion that I can get my truck to not ping under load with ethanol laced California regular grade gas. 91 octane is still over 5 bucks where I'm at. Running 91, it's really rare that I'll hear pining with timing set at 7 degrees BTDC. And that's only under high speed hill climbs in 4th with heavy throttle. You can hear the 5 dollar gas being sucked away. I like the slightly more peppiness of 10 degree btdc base timing but it pings too often. I'm at sea level-ish.

 

[fyton2v]

Hey, so this is interesting. I think I've had my advance and retard vacuum lines backwards this whole time. I'm not 100% certain, but what was a pretty peppy engine at stock timing isn't as peppy now. Before, at 10 degrees advanced in base timing, I could pretty easily create some ping in 3rd gear under load. Can't seem to do that now with the vacuum lines hooked up correctly. More testing needed, but it's pretty clear the retard function in my dizzy is working just fine. What I'm now unsure about is if the advance function is working in the right conditions, which according to my emissions manual, is just 4th gear. I have to test to see if that switch is working and the VSV is doing its thing accordingly.

I still get pinging at high speeds when I roll on the throttle. We're talking 65mph+. Anyone know what RPMs we're at on 33s at a real (tire size adjusted) 65mph? I think my total advance may be too high. I'm unsure, in that RPM territory, if vacuum advance is doing anything. At someone else advice, I've replaced the bushing in the dizzy that stops the weights from advancing further. If that helped, it was only a bit. Perhaps it's carbon buildup ... so hard to know what's really going on at freeway speeds.

 

[brian]

Hey, so this is interesting. I think I've had my advance and retard vacuum lines backwards this whole time. I'm not 100% certain, but what was a pretty peppy engine at stock timing isn't as peppy now. Before, at 10 degrees advanced in base timing, I could pretty easily create some ping in 3rd gear under load. Can't seem to do that now with the vacuum lines hooked up correctly. More testing needed, but it's pretty clear the retard function in my dizzy is working just fine. What I'm now unsure about is if the advance function is working in the right conditions, which according to my emissions manual, is just 4th gear. I have to test to see if that switch is working and the VSV is doing its thing accordingly.

I still get pinging at high speeds when I roll on the throttle. We're talking 65mph+. Anyone know what RPMs we're at on 33s at a real (tire size adjusted) 65mph? I think my total advance may be too high. I'm unsure, in that RPM territory, if vacuum advance is doing anything. At someone else advice, I've replaced the bushing in the dizzy that stops the weights from advancing further. If that helped, it was only a bit. Perhaps it's carbon buildup ... so hard to know what's really going on at freeway speeds.

been fighting this as well.
runs great around town, but as soon as it gets up around 65-70, i get pinging, with lite throttle input...advance is only around 7-10 btdc.

 

[fjc-man]

Is your vacuum actuated carb secondary working? Have you tried the paper clip trick to check its travel?

 

[fyton2v]

On my truck, even a degree of timing, or premium vs regular fuel, is the difference between pinging or not it freeway speeds. You sure it's pinging if your timing is set at 7* before TDC?

 

[brian]

Turns out i was misinterpreting the values on the timing light. What i thought was 7 to 10, was actually 14-17.
With it actually now at 10 btdc, everything seems happy

 

[brian]

and in an interesting twist, it turns out i forgot to reconnect the vacuum advance pot....its been running just on the mechanical advance the last few days...and quite well at that. i noticed that while checking the oil level this evening.
i had already played with the vac pot travel limiter adjustment....so now do i take more travel out of it, or just let well enough alone.....i know that should just let it alone....but knowing that something isnt working the way it should will consume my soul.

 

[4Cruisers]

i had already played with the vac pot travel limiter adjustment....so now do i take more travel out of it, or just let well enough alone.....i know that should just let it alone....but knowing that something isnt working the way it should will consume my soul.

I've set all the distributors I've refurbished so there's about 3.8mm of travel - that corresponds to roughly 10 degrees vacuum advance.

 

[brian]

I've set all the distributors I've refurbished so there's about 3.8mm of travel - that corresponds to roughly 10 degrees vacuum advance.

yeap saw that post, and that is what i had initially gone with.....but was still getting pinging on the highway above 60-65mph with lite throttle input or inclines. my impromptu test with the vac disconnected said that the vac advance was putting too much advance into it at higher rpm, as it ran 130 miles today with about 60 on the highway.....and had it spun to about 75 for a spell.
i did go and fiddle with it again this evening, because i really cant leave well enough alone....reduced the travel to about 2.5mm...a 5 mile test and tune at 70mph says good things.....i have a 55 mile run in the morning, mostly highway, that will tell.

 

[fyton2v]

One of thoughts I've had on this problem is that the springs controlling the mechanical advance are somewhat weaker after about 50 years. That could be causing the dizzy to over-advance the spark at lower RPMs, relative to what we would have seen in the 70s, or with a new dizzy, today. It's made a little bit more noticeable by lower octane fuel. Getting the dizzy recurved by someone with a machine and actual knowhow is probably the real solution. I'm too stubborn for that. I think I've found a good balance in my situation and I'm pretty satisfied with how my motor is running.