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Discussion Starter #1
Yesterday on my way to work my 66 suddenly started running rough. Rough idle, throttle lag, rough running at easy cruise, although when I give it the gass all smooths out and it runs fine. I've checked out all the ignition components from rotor to plugs and all looks good. Is this behaviour indicative of a manifold or carb vacuum leak? Here's the strange thing: I can unplug any vacuum line and it doesn't change a thing. Seems to me that if all is running right, I'd get rough running when I unplug a vacuum line.

Does this sound like a vacuum problem to you? All my lines and connections look perfect. Could it be a leak in the manifold seal? Any ideas on how to check this out? Any other ideas on what's happening?

Details: 289 with 302 heads, C4, Holly 4160 (vacuum secodaries, elect choke), stock 4V manifold, stock disti, manual brakes, pwr steering, no A/C

Any advice would be appreciated, thanks.
 

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It sounds to me like you're on the right track.

I would expect it to run differently with the vacuum hoses off too. Take some spray carb cleaner and spray all around the base of the carb, around the intake manifold to head surfaces and see if it changes things. If spraying that around changes the idle then you've got a vacuum leak in that area.

Just for grins did you check to make sure your carb bolts are tight?

Another possibilty is that there's some dirt or something in your carb, but the vacuum seems the most likely culprit.
 

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Check your coil wire and connections. I had some corrrosion on a coil connection and mine acted just like yours.
 

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Discussion Starter #4
Carb cleaner? Hadn't heard that one, although it sounds like an interesting troubleshooting trick. Is the idea that the carb cleaner plugs the leaks and it runs better, or that it gets injested through the leak and runs worse? Also, will carb cleaner strip the paint off my freshly detailed engine?
 

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Discussion Starter #5
Hey, you may be on to something there! I pressure washed the engine bay a few weeks ago, maybe a connection or two got wet and now corroded. I'll check that out tomorrow. Thanks
 

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It gets ingested and changes the idle much like it does when you spray it down the carb. I don't know whether it will eat your paint or not. Best try in an inconspicuous place before going ape $*** with it.

Others have mentioned stuff besides carb cleaner, but I can't remember what was recommended. I'll see if I can find that.
 

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As stated before, carb cleaner to find the leak.

Another possibility. It happened to me once. Check your idle mix screw. I once had my rattle close and started making the car run rough. Don't ask me how it happened...it just happened.

Slade
 

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Did the c4 still shift normally?
Maybe the hose at the vacuum modulator fell off?
 
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