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Discussion Starter #1
Hello everyone. I've ran into something unsual today with my 1967 coupe with inline 6, C-4 auto trans and autolite 1100 carb.
When I fired it up this morning, the car barely managed to keep idling at around 500rpm's. as it started to warm up, the rpm's went up all the way to 900. to recheck, I started it up again after 5 hours and same thing. When I press the brake paddle and release it, the rpm's go up to 1100 and back down to 900. The idle screw is set to the lowest idle. Im not sure where to look. Anyone ever ran into this before?
 

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Do you have power brakes?

The fact that the idle changes when you push the brake pedal makes it sound like you've got power brakes and something in there is leaking vacuum.

The power brake booster runs on vacuum power, I think from the port on the passenger side of the intake just under the carb. The hose then goes over to the booster. Make sure it's tight on both ends and not leaking. Also make sure that the fitting where the hose goes into the booster isn't broken or loose.

If it's not the fittings or the hose I think that the problem is probably going to be internal to the brake booster and you're likely going to have to get that rebuilt or buy a new one.

That's the only thing I can think of that would change the engine RPMs just by pressing the brake pedal. You could try unplugging that hose and plugging it and see if that fixes your RPM problems. It's not going to stop very well, but it might help you make sure the engine's running right while it's idling in your driveway and not going anywhere.
 

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Discussion Starter #3
Its drum brakes all around but there is a vacuum hose running from booster into the intake manifold. Both hose and connections are tight with no leaks. That leaves me with the option of getting new brake booster. By any chance does transmission has anything to do with high rpms at idle? it slipped couple days ago.
 

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With the engine running take a pair of pliers and squeeze the vacuum line closed that goes to the power booster and see if that drops the idle. If it does you need a new booster. Do the same with the small hose that goes to the vacuum modulator on the transmission as it could have a hole in the hose or could be a bad modulator.
 
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