Originally Posted by pukas84
I have a small block Chevy with a demon carb. I Have the vacuum tank hose, trans modulator hose and brake booster hose going to a connector screwed into the back of the manifold. Idle is a little rough. I was messing with my car yesterday and forgot to plug the modulator hose in and the engine was running smooth. When I plugged the modulator back the idle was rough again. Any suggestions on why this happens? Mahalos.
What is the initial timing set at? What's the cam specs?
Might be pointing to the idle circuit being too rich. The added air from the open modulator fitting leaned it back out and the idle smoothed out. Have you adjusted the idle mixture screws to the highest vacuum/idle speed? Is the curb idle screw adjusted so far in that the idle mixture screws don't do anything (or very little) when the screws are turned in or out?