We used to call that an “Italian tuneup”. That’s a problem that disappeared with electronic fuel injection, especially when it evolved into electronic port injection. Near-perfect metering makes one realize how marginally (and richly) carburetors metered fuel.
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I have owned several vintage C4s. I had to replace a couple of vacuum modulators. An additional symptom was slight loss of ATF and exhaust smoke. Replacing the VM, vacuum line, and topping up the Type-F made everything well.
yep. i over rev’d my 66 mustang w/289/c4. ruptured the modulator. smoked like a banshee. pulled heads and replaced rings. whew, talk about a carbon ridge in cyl top. was easy job for high school kid. than friend came over and told me about the VM.
I have seen Chevrolet pickups that smoked but the oil level was rising and the cause was a leaking modulator valve. The modulator was tapped into manifold vacuum near the PCV valve where the hose sloped downward to the valve. When the engine was turned off the transmission fluid would drain down into the engine.
But regarding delayed shifting when cold, line pressure will increase when ATF is cold and the increased pressure will extend shift points on old non electronic automatics.
Yup!
Just one more bit of evidence that “the good old days” weren’t necessarily… good.