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| Off idle vacuum advance https://slantsix.org/forum/viewtopic.php?t=17105 |
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| Author: | REDNECKMOBILE [ Mon Apr 17, 2006 8:18 am ] |
| Post subject: | Off idle vacuum advance |
I'm confused as to what to do about the way my Slant is running. The motor surges up and down when going along with the throttle just a little off idle. I think I have found the problem but not the cure. The ported vacuum goes to full advance just as the throttle is cracked and I have screwed the vacuum screw to the limits with no change in the action. The motor is in a 1985 D-100 that I converted to a mechanical/vacuum advance distributor. I replaced the carb with a 1971 Carter BSS and the distributor came from a 1976 Dart that has the springs changed to give 28 degrees full with a 8 degree intial. My question is do all Carter BSS ports behave this way by giving full vacuum as soon as the throttle is opened. I can't find my vacuum gauge to get a reading and if need I will purchase one to get a reading. I don't see how getting full advance of idle is proper, most motors I have messed with got their vacuum advance in the mid-range, not off idle. |
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| Author: | Reed [ Mon Apr 17, 2006 8:28 am ] |
| Post subject: | |
That sounds right. Every carburetor I have worked has the same behavior on the ported vacuum source- no vacuum at idle but full vacuum immediately upon opening the throttle. Sounds like you need to fine tune the vacuum advance pod on the distributor. |
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| Author: | DusterIdiot [ Mon Apr 17, 2006 8:56 am ] |
| Post subject: | Correct... |
The port in the carb is above the throttle plates, so when they are shut (or close to it) at idle you get 'no vaccuum' at the port. Once you crack the plates open (off idle acceleration...) they will get whatever vacc. is available in the intake plenum... in the same vein you don't need vacc. adv. on full throttle so the vacc. reading in the intake will be close to "0" and thus no vacc. adv. Are you having ping problems down low? You'll have to mount up a vacc. gauge and see where your truck's 'sweet spot' is for "idle", "off idle", city cruise, highway cruise, "passing acceleration" then tune the vacc. can to be: all out a little after passing acc. , and be all in at "highway cruise"...you may have to switch cans, or tailor the springs so you don't have too much adv. during off-idle to city cruise... -D.Idiot p.s. do you have a part number for your distributor (if it's a Feather Duster/Dart lite unit...it won't be kind to your truck...) |
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| Author: | REDNECKMOBILE [ Mon Apr 17, 2006 10:38 am ] |
| Post subject: | Re: Correct... |
Quote: The port in the carb is above the throttle plates, so when they are shut (or close to it) at idle you get 'no vaccuum' at the port. Once you crack the plates open (off idle acceleration...) they will get whatever vacc. is available in the intake plenum... in the same vein you don't need vacc. adv. on full throttle so the vacc. reading in the intake will be close to "0" and thus no vacc. adv.
Thanks for writing, so far I have no pinging just the timing surge. The distributor came off a 1976 automatic Dart and the canister can be adjusted all the way in and out with no change as to when the canister pulls open, which is as stated before, just as soon as the throttle cracks I get full advance. I'm hoping someone will have a list of canister numbers and their related vacuum numbers so that when I do find my gauge I can get one that works OK.
Are you having ping problems down low? You'll have to mount up a vacc. gauge and see where your truck's 'sweet spot' is for "idle", "off idle", city cruise, highway cruise, "passing acceleration" then tune the vacc. can to be: all out a little after passing acc. , and be all in at "highway cruise"...you may have to switch cans, or tailor the springs so you don't have too much adv. during off-idle to city cruise... -D.Idiot p.s. do you have a part number for your distributor (if it's a Feather Duster/Dart lite unit...it won't be kind to your truck...) |
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| Author: | DusterIdiot [ Mon Apr 17, 2006 10:54 am ] |
| Post subject: | Some here...but... |
http://www.slantsix.org/forum/viewtopic.php?p=73272 Be aware that if you get one that's listed for a pre-73 model it will change the throw in a bad kind of way for the newer EI distributor. My last 'experiment' was fine with a new style adv. arm, the old style one allowed the static timing to go from 12BTDC to 2 ATDC at startup...then once 'it came on' it only gave me 12 deg. of vacc. advance at the high range (boy that didn't help the mileage...). Good luck, -D.Idiot [/quote] |
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| Author: | REDNECKMOBILE [ Mon Apr 17, 2006 11:15 am ] |
| Post subject: | Re: Some here...but... |
Quote: http://www.slantsix.org/forum/viewtopic.php?p=73272 [/quote]Be aware that if you get one that's listed for a pre-73 model it will change the throw in a bad kind of way for the newer EI distributor. My last 'experiment' was fine with a new style adv. arm, the old style one allowed the static timing to go from 12BTDC to 2 ATDC at startup...then once 'it came on' it only gave me 12 deg. of vacc. advance at the high range (boy that didn't help the mileage...). Good luck, -D.Idiot I'm thinking that possibly the spring in the canister has broken and allowing the rapid advance but I think I still need to know my vacuum numbers before going after another one. |
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