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Old 10-12-2012, 11:34 PM
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Vacuum Question

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.

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Old 10-13-2012, 09:01 AM
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air leak,

Is the Modulator good, sounds like the diaphram is leaking and too much air getting into the intake. A hand pump vacuum tester is handy to have , I have seen guys suck on the tube to see if it holds vacuum, not aa me, if the modulator hose is long enough, run the engine, pinch the hose so it won't leak, pull of the hose put your finger on the end . unkink the hose and you should feel some vacuum in the hose. ,a lot of times a leaking modulator will allow oil to get into the hose..
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Old 10-13-2012, 11:49 AM
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Quote:
Originally Posted by pukas84 View Post
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?
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Old 10-13-2012, 08:28 PM
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Since the engine runs better with it getting more air then you need to check your idle mixture. Sounds like it's rich. What's your timing at?
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