RPM drop at idle

[smilys]

I recently purchased a 1973 340 Challenger, I have no history of the engine build other than it was done by Zig Zeigler it has a mean lope and rough idle. Any way trying to tune her up I noticed when the break pedal is applied while in park the Idle RPM drops? Car has a factory intake and a new eldelbrock 1406 carb. The factory Charcoal canister is installed but some of the vacuum lines not hooked up. Is this drop in RPM's a vacuum leak ?


thanks in advance

[734406pk]

Possibly a vacuum leak. Disconnect and plug the power brake vacuum line a plug it off completely. Start the engine and apply the brakes and see if it makes a difference. Take note... the brake lights come on causing an amp draw. The alternator will load the engine slightly to compensate for the drop in voltage.

[cudabob496]

wonder if power brake vacuum line is hooked up to right port?

[smilys]

Possibly a vacuum leak. Disconnect and plug the power brake vacuum line a plug it off completely. Start the engine and apply the brakes and see if it makes a difference. Take note... the brake lights come on causing an amp draw. The alternator will load the engine slightly to compensate for the drop in voltage.

I tried that, and no RPM drop. Of course the breaks didn't work either

[Chryco Psycho]

PCV should be connected to the large port on the carb base so the fumes are evenly distributed to all cylinders
 the brakes should be connected to the runner going to 7 or 8 cylinder .
 the carb is one of the worst to use with a lopey cam you probably are making very little vacuum & hitting the brakes can affect the carb having the metering rods rise from low vacuum which will affect idle . The 1406 has very poor adjustability for engine with low vacuum

[smilys]

PCV should be connected to the large port on the carb base so the fumes are evenly distributed to all cylinders
 the brakes should be connected to the runner going to 7 or 8 cylinder .
 the carb is one of the worst to use with a lopey cam you probably are making very little vacuum & hitting the brakes can affect the carb having the metering rods rise from low vacuum which will affect idle . The 1406 has very poor adjustability for engine with low vacuum

That's how it's hooked up, I do have the original Thermo-Quad. Previous owner just swaped it out prior to me buying the car, said he couldn't get a good idle out of old carb. Maybe I'll have it rebuild but never really had much success with the Thermo-Quad either.

[734406pk]

I tried that, and no RPM drop. Of course the breaks didn't work either

So no rpm drop with the vac booster disconnected....Check the vac booster for leaks with a vacuum tester. The diaphragm may have a leak. Consider a Holley style carb for this engine. Stock type AFB's and TQ's are not as adjustable to meet the demands of an aggressive camshaft grind. These OEM style carbs are for stock to mildly modified engines.
 

[moper]

The booster has an internal leak. The leak will have more of an effect on something with a lumpier camshaft.

[734406pk]

The booster has an internal leak. The leak will have more of an effect on something with a lumpier camshaft.

Small blocks can be less tolerant/more sensitive of vacuum leaks with an aggressive camshaft.   

[smilys]

The booster has an internal leak. The leak will have more of an effect on something with a lumpier camshaft.
[/quote

Tested the booster, yup its shot time for a new one. Thanks for the help gents.]

[smilys]

One more question, my catalog has two different brake boosters, one is black other is anodized gold in color. Which one would be the correct original replacement?

[cudabob496]

I'd consider losing the power brakes.

[734406pk]

One more question, my catalog has two different brake boosters, one is black other is anodized gold in color. Which one would be the correct original replacement?

The factory unit was usually painted black as far as I remember. The gold anodize might be for some engine compartment dress-up bling. Your choice, they should both operate the same mechanically (and not leak).

[Chryco Psycho]

i thought black was 70  maybe 71 & gold after that ?

[734406pk]

i thought black was 70  maybe 71 & gold after that ?

Could be, but too long ago for me to remember. I recall black paint or rusty, That was it.