Rough Idle at Stop light/Sign from brakes?

[jgf100]

'70 302, 650cfm Edelbrock, rpm performer intake, recently rebuilt heads. All 4 drum brakes
I have an interesting problem. When stopped at light/sign, engine seems to stall as if starving for fuel. I have to give it some fuel to idle better. I noticed that If I press down on brake harder than normal, it idles better and I dont have to give it fuel. I dont see how it can be related, or can it?

Thanks

[jzjames]

Automatic transmission? When you throw it in neutral (at the stop sign) is it idling OK?

[cep62]

If you're running rich the brake booster acts like a vacuum leak leaning the mixture . Maybe.

Try adjusting the carb with a vacuum gauge to get the highest reading .
You'll have to go from side to side a few times.

[jgf100]

Automatic transmission? When you throw it in neutral (at the stop sign) is it idling OK?

I have the auto c4. It does idle better when in neutral.

[jgf100]

If you're running rich the brake booster acts like a vacuum leak leaning the mixture . Maybe.

Try adjusting the carb with a vacuum gauge to get the highest reading .
You'll have to go from side to side a few times.

I don't have a brake booster. The only lines I have hooked up are the transmission to intake, carb to pcv on valve cover, and the port on the front of carb to distributor mechanical advance.

[jzjames]

I have the auto c4. It does idle better when in neutral.

Verify the vacuum line to the modulator valve on the transmission. Or could be the modulator valve itself?

I admit I dont know much about transmissions, I have a c4 in my truck.
- the “stall speed”? -
Does the truck tend to creep forward alot at idle rpms?

[jgf100]

I have the auto c4. It does idle better when in neutral.

Verify the vacuum line to the modulator valve on the transmission. Or could be the modulator valve itself?

I admit I dont know much about transmissions, I have a c4 in my truck.
- the “stall speed”? -
Does the truck tend to creep forward alot at idle rpms?

It doesnt creep much at all at idle. How do I diagnose faulty modulator valve?

[basketcase0302]

="jgf100"

I have the auto c4. It does idle better when in neutral.

Verify the vacuum line to the modulator valve on the transmission. Or could be the modulator valve itself?

I admit I dont know much about transmissions, I have a c4 in my truck.
- the “stall speed”? -
Does the truck tend to creep forward alot at idle rpms?

It doesnt creep much at all at idle. How do I diagnose faulty modulator valve?

Some really good troubleshooting advice above. I 2nd the vacuum issue and it can be ruled out by running vacuum gauge in the cab while driving, (lot of racing applications it's a normal gauge to have). Run a piece of 1/8" vacuum line into the cab, (you can get a cheap vacuum gauge for around $15 from any "big box" auto store maybe even wallyworld). Watch the gauge when the C-4 shifts.
And there are two places to pay particular attention to in regards to the vacuum line to the modulator valve. The two small pieces of rubber tubing that connect the steel line at the intake manifold and at the modulator valve itself.

Has there been any recent transmission work done?

Also like above don't rule out a vacuum leak elsewhere, (the 302 is bad about leaks at the end rails where the rubber/cork gasket seals the intake to the block). If...you find the rubber or cork there eliminate that material with a fat bead of RTV silicone to seal those two area's for good.