Read my spark plug...

[theholycow]

Originally Posted by theholycow in the vacuum gauge thread

This is where it's currently hooked up. This thing has 3 nipples and a rubber boot that fits all 3, combines the outer two into one, and sends the middle one to the distributor (formerly, to the thingy that went to the distributor). Doesn't look like all the other vacuum check valves but could be.

I was thinking about my spark knock/vacuum advance situation, how I can't make it knock when it's cold but can when it's hot and how my fuel economy has improved this way. I had an idea to use a thermal vacuum switch to change the vacuum source so I can have the current configuration when cold but another configuration when hot. I googled to see if anyone has done it and I found this:

http://firstgenfirebird.org/FAQ/engine/carburetor.html

Here is the text of an article that AME's FAX'd me. Looks like it was from Pontiac Enthusiast magazine (Vol. 1 No. 2) and was written by Peter Serio:

{...} In the previous issue of Pontiac Enthusiast, we looked at the 1968-69 manual transmission vacuum advance valve. In the next issue, I will cover the transmission-controlled spark systems used from 1970-72. For now, it's time to turn to the TVS, or thermostatic vacuum switch.
{...}
In all applications, the TVS serves as a safety device to help prevent overheating. The switch is located at the front of the intake manifold, threaded into the coolant passage. There positions inside the TVS related to coolant temperature. Standard vacuum flow is ported vacuum from the carburetor to the distributor vacuum advance. Whenever the engine coolant temperature rises above 230 degrees F, the TVS switches the distributor advance from ported to full manifold vacuum. This advances the timing about 20 degrees at idle, allowing the engine to cool down to normal operating temperature. After the engine cools, the system returns to ported vacuum.

1. "front of the intake manifold" describes the location of the thing I photographed and quoted above. I don't know if that is a coolant passage. It would make sense that the thing in my photo is a TVS, based on its shape, layout, and location...but its behavior sure seems like a check valve (despite not looking like any of the other vacuum check valves). Its hookup (with the other lines going to the charcoal canister) doesn't support the TVS idea either.

2. How does advancing the timing help the engine cool?

3. I wonder where I can find the issues of Pontiac Enthusiast magazine mentioned above, containing the other articles (especially the one about the manual transmission advance)...

[theholycow]

Good news, everyone!

I moved the distributor advance to the nipple on the carburetor, as well as the vacuum gauge. I can confirm that that one is ported vacuum. The vacuum gauge showed exactly what it should for ported: 0 vacuum with throttle closed and similar to manifold with any throttle opening.

When I pulled the vacuum line off the distributor my excess idle speed went partially down. Ported vacuum also gives me that result (obviously). So now with ported vacuum I get a reduced (but still not correct) idle speed. It was ~1500, now it's ~1000. Properly it should be 550-650.

Anyway, after a few miles I was satisfied that it was indeed ported vacuum. I moved the vacuum gauge to manifold vacuum (the one that previously went to the automatic transmission modulator) and left the distributor on ported.

Now:
- the vacuum gauge works right
- idle is partially better
- no spark knock!

[GasSavers_Erik]

Is your idle speed problem that you cannot locate the adjustment to reduce it?

Or that it will not run smoothly/stay running if you adjust it down below 1000 rpms?

[theholycow]

I cannot locate the cause of the excess idle speed. It may be possible to reduce it by turning the idle screw but that should not be necessary; something is causing the higher idle and I'd prefer to identify the real cause rather than burying it.

So, bad news: The skipping/bucking reared its ugly head, but not in the same context as before I started messing with ignition advance. Before, it would happen with heavy throttle at any RPM but especially low RPM (1000-1500). Today it happened at high RPM (2000-2300) with heavy throttle while getting on the highway in 4th. It didn't do that yesterday when I got on the highway and I don't know what the difference is...ambient temperature?

[theholycow]

Originally Posted by theholycow

I was thinking about my spark knock/vacuum advance situation, how I can't make it knock when it's cold but can when it's hot and how my fuel economy has improved this way. I had an idea to use a thermal vacuum switch to change the vacuum source so I can have the current configuration when cold but another configuration when hot. I googled to see if anyone has done it and I found this:

http://firstgenfirebird.org/FAQ/engine/carburetor.html


1. "front of the intake manifold" describes the location of the thing I photographed and quoted above. I don't know if that is a coolant passage. It would make sense that the thing in my photo is a TVS, based on its shape, layout, and location...but its behavior sure seems like a check valve (despite not looking like any of the other vacuum check valves). Its hookup (with the other lines going to the charcoal canister) doesn't support the TVS idea either.

2. How does advancing the timing help the engine cool?

3. I wonder where I can find the issues of Pontiac Enthusiast magazine mentioned above, containing the other articles (especially the one about the manual transmission advance)...

Hey wait a minute...HEY....what if it IS a thermal vacuum switch, and its line to the charcoal canister is the source of vacuum rather than a consumer of vacuum? Or what if someone hacked it by teeing the two lines together that currently go to the canister and one should actually go to manifold vacuum...

Edit: Negatory on that last thought, the lines do belong teed together.

[GasSavers_Erik]

Hmm- I wonder if your fast idle is caused by the fast idle cam being engaged and is preventing the throttle butterfly from closing all the way.

The other cause could be a throttle cable that is adjusted too tight.

In light of this info (idle rpm decreased when you switched from manifold to ported vacuum at the dist advance), it would seem that running ported vac to the dist actually decreased efficiency at idle- and that is why the idle rpm decreased (less rpm = less work being done by the fuel at idle).

As far as the skipping and bucking, I vote you temporarily disconnect the vacuum advance and take it for a test drive.

[theholycow]

I think my fast idle cam is not to blame. My cold idle does seem to go down in steps properly, and I spent a whole lot of time trying to fix the fast idle cam last fall. Also, my ported vacuum reads 0 with the throttle closed; that should mean that the throttle really is closed, right?

Idle RPM decreased not only when I switched to ported vacuum, but also when I removed vacuum from the distributor entirely.

Good idea, disconnecting vacuum advance entirely for a test drive. I may try that. I've decided for now that I'll keep driving the way it is and see if the symptom recurs, and if so, if there is a pattern to when it happens.

[GasSavers_Erik]

Originally Posted by theholycow

Also, my ported vacuum reads 0 with the throttle closed; that should mean that the throttle really is closed, right?

I agree with your reasoning, but what about when the idle is extra high when it is cold (fast idle cam is engaged)- is there ported vacuum then? If so, that would support the idea that the butterfly is completely closed now. Maybe you could also investigate the secondary butterfly linkage- to be sure the secondaries are closing nice and tight.

Originally Posted by theholycow

Idle RPM decreased not only when I switched to ported vacuum, but also when I removed vacuum from the distributor entirely.

That supports the idea that the timing needs to be more advanced at idle- which manifold vacuum would do for you.

[theholycow]

So, I didn't have to wait long. The bucking returned almost to the way it used to be, on the way home today. I guess I won't be sticking with straight ported vacuum.

Originally Posted by Erik

I agree with your reasoning, but what about when the idle is extra high when it is cold (fast idle cam is engaged)- is there ported vacuum then?

I didn't try that. I'll hook manifold vacuum to the distributor and the vacuum gauge to ported, and see how it looks at 1500 idle.

Maybe you could also investigate the secondary butterfly linkage- to be sure the secondaries are closing nice and tight.

The secondaries have an air valve, basically exactly like a choke, that is completely closed almost all the time. I've never been able to load it enough while stationary to observe them opening (if they do at all). I don't know if the secondary butterfly would matter at that point. I'll see if I can check it anyway.

That supports the idea that the timing needs to be more advanced at idle- which manifold vacuum would do for you.

But then my idle would be 1500 again.

[theholycow]

Every time I drive it, its behavior is different. I drove it around town today. It idled anywhere from 700 to 400 RPM. The vacuum gauge read 14-15 at these nice low idle speeds. According to the gauge, that means late timing, which I think matches everything we've discussed and the current configuration.

Also, it burbled almost like a hot-cammed performance V8.