Engine shudder

[Barry Wolk]

I had my Treadle-vac rebuilt years ago but the problem persisted. It operates the brakes just fine, but the car shudders at a light when the brakes are applied, and smooths right out when you let your foot off the pedal. Clearly the booster is causing a massive vacuum leak when activated.

Mine is an early car with the sleeve valve booster. They later went to a poppet-valve booster, which is more grabby, but doesn't lose vacuum.

Does anyone else's car do this?

[Shelly Harris]

I'm getting a shudder while standing at the light in D, but its not affected by the brake pedal. The idle in N is fine. I think it's all in the carb settings. when I start to accellerate it starts to shudder a little bit more but then it quickly goes away with accelleration and the ride from then on is ok, except for the trans slipping a bit. I'm thinking the problem must be some poor funtioning accelleration jet whose job is to give a little squirt when getting off from a stop. Car idles fine in Neutral but in D is the problem.

[Barry Wolk]

You just don't notice it in neutral because it's not under load. What rpm does your car idle at? The correct speed is 425 rpm. Higher idle will mask the problem.

It sounds like a vacuum problem at idle and a problem with the piston within the carb on accelleration. Unrelated problems that will give you similar stumbling. Do you have a vacuum gauge?

Set the idle low and start the car. Spray carb cleaner at the base o the car, along the intake manifold gaskets and at your vacuum connections. A vacuum leak at any of these junctions will cause the rpms to change when you spray on the cleaner.

[Nick DeSpirito]

Barry,

When my car is running and it is in park and I step on the brake, the engine makes (I don't know how to describe this) a giggling sound. Like blub-blub-blub. When I take my foot off the brake it goes away. Is that what you're talking about?

Also, (not related to the above) in my opionion, the tranny takes too long to shift from second to third. I find myself easing off on the gas to get it to shift. Being used to newer cars, I don't know if this is the norm for cars of this era. What do you think?

[Barry Wolk]

That's the brake booster causing a vacuum leak. The second is probably a sticky valve body port. A thorough cleaning and reassembly should cure that.

[Nick DeSpirito]

So the blubbing when I step on the brake in park is normal?

[Barry Wolk]

It's normal for an early car with the sleeve-type Treadle-vac, but not the later type poppet-valve unit.

[Shelly Harris]

Later report.... I just drove 45 miles and now snacking at McDonalds and fumbling with the iPhone.... all shudder symptoms previously described have disapeared !!! Now idling under load and nice with no hesitation and laboring on accelleration. Amazing...eiother the carb cleaned itself up or I have an intermittent vacuum problem. Mean while I have a smile on my face and a great running car... According to the mechanical tack on the dash I idle somewhere between 500 - 600.

[Nick DeSpirito]

I think the problem with most of us is that we don't get our cars out to "flex their muscles" enough. I've found that the more I drive my car, the better it runs.

[Barry Wolk]

I think it'll come back next time you use it. Vacuum leaks at intake manifold, carb and other locations can seal themselves as the metal parts of the engine expand. Let us know if symptoms return.