Strange vaccuum ??? - Team Camaro Tech
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post #1 of 6 (permalink) Old Jan 25th, 03, 01:49 PM Thread Starter
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Ok, here is a wierd one. I have been tinkering with my old buggy for a while now. Lot of different things doen mostly considered a major tune up. When I went to check timing a couple of onths after we set initial timing, this is what happened. When I removed the vaccuum hose to the distributor the engine accelerated instead of decreasing in rpm. I dont have a dwell meter but am going on what I hear. This just doesnt make sense to me. Anyone have any ideas why this would happen? I tried to connect the vaccum hoses to differetn ports and each time when all vaccuum hoses were connected the car would drop rpm's pull a vaccuum hose and the rpm's increase. Here are the specs. 73 350 bored .30 over. rebuilt Roch Q-Jet. major tune up all around.

73/74 mix. (Its a mutt) 73 350 bored 30 over, Th350 tranny. With the Lord willing, may we never have to drive a MiniVan!

The ole Mutt
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post #2 of 6 (permalink) Old Jan 25th, 03, 02:03 PM
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Scott - what you have happening is normal.
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post #3 of 6 (permalink) Old Jan 25th, 03, 02:26 PM
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To kind of explain what DJD said what you experiencing is a mild surge in terms of RPM from leaning out a bit. Nothing to worry about at all. Essestially, you are giving the motor just a little bit more oxygen but the amount of fuel ingested is the same. What happens is the extra oxygen increases the speed of travel of the flame front, thereby increasing power stroke speed at idle only. The end result is your motor speeding up a little bit. Hope this helps to explain what you are seeing a little better. Scott
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post #4 of 6 (permalink) Old Jan 26th, 03, 04:29 AM
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To confirm what DjD and psalm69 are saying...did you experience the RPM surge with the vacuum port open or plugged. If open, then they are right. You just leaned the idle circuit out a bit causing the RPMs to increase. This is probably normal, but might indicate a rich idle. The important thing is that without vacuum advance, the timing is somewhere between 8 and 12 BTDC and with vacuum it's around 15-20 BTDC.


[This message has been edited by dnult (edited 01-26-2003).]
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post #5 of 6 (permalink) Old Jan 27th, 03, 11:26 AM Thread Starter
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Guess I was confused. I was lead to believe that when you take the vaccuum hose off it would actually bog down a bit. This was supposed to be the case because the vaccuum advance would not have a vaccuum on it at the distributor. Makes sense what you guys are saying anyway. I wont get concerned with it.

73/74 mix. (Its a mutt) 73 350 bored 30 over, Th350 tranny. With the Lord willing, may we never have to drive a MiniVan!

The ole Mutt
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post #6 of 6 (permalink) Old Jan 28th, 03, 07:30 AM
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If you have a stock advance system then there is no vacuum to your distributor vacuum advance at idle. The vacuum advance is ported vacuum, vacuum obtained above the throttle plates, not manifold vacuum obtained from below the throttles. At idle the throttles are closed and there is no ported vacuum. Leaving that hose uncapped at idle should have no effect on your idle mixture. Now if you have it connected to manifold vacuum, then you will affect your mixture as described above.

Mark Canning
1969 Indy Pace Car
350/300HP RPO Z11
My 69 L48 - 350/300HP Engine
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