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Old 12-12-2002, 11:41 AM
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Post vacuum advance problem

i have been struggling with the ignition on this motor since i have had it. i have replaced the springs, weights, and vacuum advance. then i started playing with the timing. motor still had a miss at various rpms and throttle positions. at wot it would hesitate but not miss, but it started to miss after 5gs. finally i got to guessing and disconnected the vacuum advance. everything works fine, no miss and she will pull until she blows. when disconnecting the vacuum advance the motor idles about 150 higher. my question is why is my vacuum ADVANCE, retarding my timing? could the weights be on backwards, but i would think the motor would run horrible now. for now i just drive it without the vacuum advance. any ideas or suggestions?

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Old 12-12-2002, 03:08 PM
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what is the motor chev,ford,doge, foriegn.? is it single points.elec.coil pick up ? and what is the timing set at with the vac unpluged and the vacuum port pluged up.?
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Old 12-12-2002, 03:23 PM
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it is a chevy motor with the hei ignition. i do not have a timing light. timing is set right before any drag occurs whiles starting (8-10). i am using the stongest springs in the pack so i know it is not over advancing. when i drove it today it ran stonger than it ever has(without vacuum advance). i can only imagine how it could run with it working properly. there is no detonation or valves pinging under a load either.

[ December 12, 2002: Message edited by: tonylsg ]</p>
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Old 12-12-2002, 03:34 PM
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before this problem started have you ever pulled the dist out of the motor. because it sounds like you are advance one tooth on the cam forwards. try this pull the cap off and move the motor around to tdc line the mark on the balancer up with 0 on the timing tab. the rotor on the dist should be lined up with the drivers side back carb bolt. also make sure that the number one piston is all the way up. the car will run but will have a funny piss to it and lack real power.
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Old 12-12-2002, 03:46 PM
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the motor did this when i first got it. we played with the timing and set it to 8 degrees at idle. well about two months later i rebuilt the top end(cam, lifters, heads). the motor definantly runs stronger and went from 4gs max to 5gs. since i took the vacuum advance it runs like a dream. i had it up to 5700 and she was still pullin. scared the daylights out of the girlfriend, and she could stand my high 13's supercoupe. so there is no lack of power now, and it is running really smooth also. would the weights being put on backwards cause this? i can just tell that the vacuum advance is causing my ignition to retard at the rate it should be advancing.
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Old 12-12-2002, 04:18 PM
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Do you have the vacuum advance connected to ported or manifold vacuum?

You really might want to consider having the distributor set up on a distributor machine to get the advance curve and amount of advance correct.
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Old 12-12-2002, 06:21 PM
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what do you have this motor in ?

you really need to by a timing light with a dail back nob.
moving the weights from one side to the other side would not cause the timming to retard. and by having the heavy springs on the advance is only keeping the weights from moving. what carb are you using and have you checked to make sure you are hooking it up to the right vacuum port. it really sounds like, when you have the vacuum advance hooked up it advancing it way to far.
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Old 12-12-2002, 06:39 PM
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It is very possible that when you are disconnecting the vacuum advance the motor is idling up because you are opening the manifold port on the carb. This is like opening the throttle a tad bit. Have you tryed plugging the port to see if it idles down.

You need to get a timing light to verify what the timing is doing.

Also, that miss above 5000 rpm could be valve float. It will sound like the motor is missing. I really dont think that the timing retarding(if it really is) would cause this.

Chris
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Old 12-13-2002, 04:33 AM
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I`ve seen this before, what`s going on is your connected to a direct vacuum source at idle and it`s pulling way more advance than you need at idle, over advancing the timing will cause the engine to idle down due to the mixture being combusted way before the piston gets there. you should have the vacuum advance connected to a port that only pulls vacuum when you accelerate.
Set your base timing at 12 degree`s BTDC, with the advance disconnected, make sure you use the lighter springs to get full advance of 36 to 38 degree`s all in 3000 RPM, plug the vacuum advance into a port that pulls vacuum only on acceleration and the thing will run like satans after you. I used plug directly into a full vacuum source at idle when I was young and still learning the ropes, then one day it dawned on me,, why do you need vacuum advance at idle? what purpose does it serve? Vacuum advance is for part throttle crusing where the engine can use more advance to give it better emissions and MPG.

[ December 13, 2002: Message edited by: DoubleVision ]</p>
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Old 12-13-2002, 06:40 AM
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i guess i need to clarify that the vacuum is set to a timed port(only works when accelerating), this is where it has always been. there is a full vacuum port but i do not use, like you guys said who needs full vacuum advance at idle. it doesn't make any difference which port it is on. same problem. i am running an edelbrock 600 carb.
i thought for the longest time that i was experiencing valve float, but now that the vacuum advance is disconnected there is really no problem with that or anything else. i know it is weird and probably something minor. when i think about it the motor quits missing under full throttle(no vacuum) but still runs out at about 5000, no vacuum advance 6000. i am confused please help.

i also wanted to add that according to the weights and spring kit there are two different types of distributors(counterclockwise and clockwise). i believe mine to be clockwise but i am not sure how the weights are set up. i just duplicated what i took out, and remember the problem existed then to. i will check that today and let you guys know.

[ December 13, 2002: Message edited by: tonylsg ]</p>
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Old 12-13-2002, 08:32 AM
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ok you guys were on the right track. it turns out that setting the timing by advancing it until your motor drags when starting is an absolutly horrible idea. atleast it is when your motor does not drag until somewhere around 20 degrees. so what was happening is the motor was over advancing. i still do not have the timing light but it will turn 6000 with out any miss. first time that has happened since i have owned it which is about 20000 miles. just wanted to say i really appreciate. thanks.
p.s. i am on way to get the timing light now. stupid not to have one to begin with.
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