400 ci G block with C heads, iron manifold and a Quadrajet and Pertronix Ignitor. Plug gap .030, initial timing 2.5 degrees BTDC in accordance with my CSM.

My car starts fine and runs well. Idle speed at 850, timing is at 2.5 and all is well, until it warms up.

Once the car is warm and it starts pulling vacuum from the Thermostatic Vacuum Switch (by now 18 mm Hg are available), the idle climbs to 1200 indicated on same tachometer. I also cannot adjust the fuel mixture screws to any effect nor can I reduce the idle any further using the low idle screw.

Is my vacuum advance working too well, broken, or just needs adjustment (if possible)? How do I test this?

Scott

 

Why is the TVS allowing full vacuum? It's not supposed to allow manifold vacuum until the engine is overheating.

You could just disconnect the vacuum advance from the TVS and connect it to the ported vacuum source on the carburetor, then you would not have vacuum advance at idle speeds.

 

X2. Bypass the TVS and run vac advance on ported vacuum. If idle behaves normally you've found the culprit.

TVS should only switch to manifold vacuum if coolant temp gets in 220 degree range.

You can try taking the thing out and cleaning grunge off it, but my guess is it's gotten old and weak.

 

^^^THIS. The TVS is supposed to provide ported vacuum to the distributor until engine temp reaches 250-258 deg F, when it switches to full manifold vac to raise idle speed and cool the engine. If it's opening at lower temps, the TVS is bad.

 

It has been in the 90’s nearly every day, my car runs around 200-210 degrees. But the fast idle starts climbing as the thermostat opens up and eventually gets to 1200.

So my car is not running too hot, and the TVS is not doing its job, and I can bypass to test. After I switch to ported vaccum and I disconnect from the manifold source, I’m assuming plugging it at the manifold is required.

Thanks gentlemen.

 

Manifold source would need to be capped, but if you want to keep the thing in place for looks, use an old emissions cheat trick. Leave the manifold vacuum line on the TVS, but put a BB in the vacuum hose to plug it.

I'm concerned you're running 210 coolant temps. 210 is close enough to TVS switch point that the thing may be working correctly, just a little early. Any idea what thermostat is in there? Switching to a 180 will help. If it has a clutch fan is the clutch working? Radiator in good order?

 

Have not pulled the thermostat yet. Clutch fan seems to be working, when cold it only spins < 1/2 turn. The shroud has a good chunk taken out of the bottom, about the size of a small loaf of bread. The rad was recored in the past, according to receipts.

Keep those tips coming!

Scott

 

If you are running ported vacuum, do not bypass the TVS permanently. The whole point of the TVS is to prevent your motor from overheating. Bypassing it temporarily for troubleshooting purposes is fine, but driving the car that way long term is asking for trouble. Note that the TVS system is discussed in the ENGINE COOLING section of the Chassis Service Manual. There's a reason for that.

I should also note that Glenn is correct, the 1968 TVS is supposed to switch at 220 F, not 250 as I suggested above. I was thinking of the switches on later cars. If your car is running at 220, that's why it's switching to full manifold vacuum. The system is functioning as it was designed. Is your cooling system operating correctly? Do you have a clutch fan?

 

I haven’t seen 220 yet and the advance starts coming on once warmed up, well below max temp. Idle RPM rises steadily once it warms until it reaches 1200 at max temp.

Clutch fan operates correctly, fan shroud is missing a piece out of the bottom and I haven’t pulled the thermostat yet.

To test the TVS can I put a vacuum gauge on the top port and map vacuum increase to temp increase. Assuming the switch and the vacuum is either on or off, and if I see a slow rise in vacuum as the temp comes up the TVS is bad?