dizzy switch
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
The dist. already has a mechanical advance in it, along with the vacuum advance. Unless this is an race vehicle that never runs on the highway or you don't care about gas mileage, why would you not use the vacuum advance? It only works at little loads and little throttle settings anyway and helps improve gas mileage..
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Freshman User
Joined: Jan 2005
Posts: 25
Likes: 0
From: so cal
This truck is a weekender, so gas mileage isn't a great concern. I've been having trouble at idle, the port on the side of the holley carb is pulling vac and advancing the timing to max.
I've opened up the rear throttle plates to lower the mains, that helped. I also adjusted the vac adv screw on the dist, that helped. With that done, it doesn't run that great at idle. I have used different intial timing from 10-15 btdc but that has had little effect.
Since the dist has both vac and mech adv, can I just block the vac adv port?
I've opened up the rear throttle plates to lower the mains, that helped. I also adjusted the vac adv screw on the dist, that helped. With that done, it doesn't run that great at idle. I have used different intial timing from 10-15 btdc but that has had little effect.
Since the dist has both vac and mech adv, can I just block the vac adv port?
Posting Guru
Joined: Oct 2005
Posts: 1,235
Likes: 2
have you adjusted the vacuum advance can? there's a allen head screw that's accessed through the vacuum port on the advance cannister where the hose goes on it. i think it's a 3/16th allen wrench but i'm not sure. turn the screw clockwise until it stops and that is zero advance. every complete turn counterclockwise is 7 degrees advance i think. the mallory website had all this info when i was having some trouble with mine- it turned out the screw was stripped and wasn't limiting the advance...
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
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From: Long Island USA
Hash, what cam did you use?
When you get into the bigger cames (like the Crane 941) you have to do certain things to get the vacuum port to not pull vacuum at idle, which you seem to already know if you're playing with the secondary plates. HOWEVER - that is a band-aid and will still ruin your idle because the secondaries don't have an idle circuit and to make up for it, you need to richen the front idle circuit way up, which doesn't help the mix AT ALL...
Bear, if he has a big cam, think it's time to drill the primary throttle plates so they can be shut to where the vacuum advance port doesn't pull vacuum anymore?
When you get into the bigger cames (like the Crane 941) you have to do certain things to get the vacuum port to not pull vacuum at idle, which you seem to already know if you're playing with the secondary plates. HOWEVER - that is a band-aid and will still ruin your idle because the secondaries don't have an idle circuit and to make up for it, you need to richen the front idle circuit way up, which doesn't help the mix AT ALL...
Bear, if he has a big cam, think it's time to drill the primary throttle plates so they can be shut to where the vacuum advance port doesn't pull vacuum anymore?
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Originally Posted by hash
This truck is a weekender, so gas mileage isn't a great concern. I've been having trouble at idle, the port on the side of the holley carb is pulling vac and advancing the timing to max.
I've opened up the rear throttle plates to lower the mains, that helped. I also adjusted the vac adv screw on the dist, that helped. With that done, it doesn't run that great at idle. I have used different intial timing from 10-15 btdc but that has had little effect.
Since the dist has both vac and mech adv, can I just block the vac adv port?
I've opened up the rear throttle plates to lower the mains, that helped. I also adjusted the vac adv screw on the dist, that helped. With that done, it doesn't run that great at idle. I have used different intial timing from 10-15 btdc but that has had little effect.
Since the dist has both vac and mech adv, can I just block the vac adv port?
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Freshman User
Joined: Jan 2005
Posts: 25
Likes: 0
From: so cal
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Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Originally Posted by krewat
Bear, if he has a big cam, think it's time to drill the primary throttle plates so they can be shut to where the vacuum advance port doesn't pull vacuum anymore?
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