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Old 01-27-2009, 04:36 PM   #1
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need help adjusting dwell on m/c solenoid

84 z28 305H carb#17084201 Rochester quadrajet. Just got done rebuilding my first quadrajet put on car and fired right up! I set tps to .5v at curb idle was .6v and goes up to 4.2 wot. Tested m/c solenoid across terminals and had 20 ohms and is clicking when key is turned on. I set the lean mixture solenoid screw using a 1.304 tube spacer that I made, lean screw is out 3-5/8 turns. I adjusted rich solenoid screw to get 1/8" travel. I then used a gauge that I made from 5/32 rod and bent into candy cane shape gauge that measures 1.756" from tip to tip to set iab valve which was 4-1/2 turns out from shut. Also set float level to 11/32". Soaked in carb dip prior to rebuild. Didn't bother to remove factory plugs from idle mixture screws. I thought the factory setting was good enough and that I would just adjust the iab to get proper dwell. Ran car at high idle for awhile to get up to normal temp. Hooked up dwell meter to diagnostic wire. At low idle, dwell reads 54 on 6cyl scale. Tried opening up iab to seven turns and needle did not move at all. At 2000-25000 rpms, it reads between 30-35 at seven turns out, up in the 40's when iab is set back to initial 4-1/2 turns. Do I need to remove carb and drill out idle mixture plugs and screw in further? Car does seem to load up at idle after a while. I replaced o2 sensor two years ago. Could it be acting up? Also need to add that the charcoal canister was unplugged from carb and carb inlet tube was capped.

Last edited by dirtpoor1; 01-27-2009 at 05:18 PM.
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Old 01-27-2009, 07:29 PM   #2
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Re: need help adjusting dwell on m/c solenoid

Where are you attaching the leads to measure dwell? We've had "issues" with how people have measured dwell on this board before. Stuff that's had a few of us beating our heads against a wall before we figured out the problem wasn't the carb, it was how the dwell meter was hooked up.

Always drive the car for a while before you adjust the mixture screws or IAB. You need to be CERTAIN you are in closed loop and I'm not absolutely sure you are guaranteed to get in closed loop just idling in the driveway for a while.

Your initial bench adjustments on everything are definitely within the realm of sanity. Don't see anything that says "whoah, he's way off the edge of the map on that one!"
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Old 01-27-2009, 07:33 PM   #3
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Re: need help adjusting dwell on m/c solenoid

had green lead of dwell meter hooked to the diagnostic wire back by the blower motor, had black lead of dwell meter hooked to neg terminal of battery. What exactly is open and closed loop? thank you.
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Old 01-28-2009, 07:19 AM   #4
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Re: need help adjusting dwell on m/c solenoid

You're hooking up the dwell meter properly, so that's not it.

Closed loop is when the ECM is using the O2 sensor to provide feedback on how to trim the mixture by adjusting the duty cycle of the mixture control solenoid. Engine temp is only one of the factors that the ECM uses to determine if it's time to go into closed loop.

If it's not looking at the O2 yet (i.e. it's still in open loop) it's running off a pre-programmed duty cycle table programmed into the ECM. Your adjustments at the carb won't make any difference in the duty cycle you are reading. That's why it's important to be certain you're driven around enough that you're in closed loop before making any carb adjustments based on duty cycle.
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Old 01-28-2009, 07:50 AM   #5
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Re: need help adjusting dwell on m/c solenoid

You'll know its in closed loop when choking the airhorn with a rag causes a change in dwell reading. Dwell should increase as it attempts to lean the mixture. Sounds like your motor's not quite fully warmed up.
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Old 01-28-2009, 08:48 AM   #6
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Re: need help adjusting dwell on m/c solenoid

OK, I'll try driving it around for a while and see what happens then. It seemed a little odd that it would go to 54 on the 6cyl dwell scale and just sit there like a rock. Is 7 turns out on the iab too much to be in the accepted adjusting range?
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Old 01-28-2009, 09:27 AM   #7
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Re: need help adjusting dwell on m/c solenoid

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Originally Posted by dirtpoor1 View Post
OK, I'll try driving it around for a while and see what happens then. It seemed a little odd that it would go to 54 on the 6cyl dwell scale and just sit there like a rock. Is 7 turns out on the iab too much to be in the accepted adjusting range?
For it to go into closed loop the CTS has to report that it's warm enough AND the O2 sensor has to start sending readings to the ECM that it can 'understand'. This means the O2 has to be hot enough and the readings have to be within its narrow band that the ECM can adjust from. Until everything is properly warmed up and the mixture is 'close' it won't go into closed loop. With the CTS warm enough but no feedback from the O2 it will still adjust dwell based on its 'last known good' data or the default settings of the ecm (if its lost power).

If its stuck on 54 either the O2 isn't sending good data yet or the ECM is trying to lean out a rich mixture. If opening a vacuum source lowers your dwell reading you''ll know you're in feedback mode and if you can bring it near 50% by adjusting IAB, you're there. I wouldn't go much more than 7 turns out though.

If you're not in closed loop try setting the IAB at 2 turns out and give it about a minute for the dwell to respond. If no response add half a turn out. Keep going til you get to 7. You'll know its good when the dwell meter begins wavering back and forth +/- 10 degrees or so.
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Old 01-28-2009, 09:44 AM   #8
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Re: need help adjusting dwell on m/c solenoid

question: does car need to be in drive at idle to check dwell? thanks.
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Old 01-28-2009, 10:07 AM   #9
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Re: need help adjusting dwell on m/c solenoid

I usually set mine in park. I've set them in drive before and didn't have an appreciable difference between the dwell in park or drive.
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Old 01-28-2009, 02:01 PM   #10
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The factory instructions (sticky above) says to have it in Drive, but I don't recall which way I set mine, or checking it in Drive vs. Park.
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Old 01-28-2009, 04:37 PM   #11
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Re: need help adjusting dwell on m/c solenoid

Had the same exact issues.

I was never able to get the darn thing to work properly until all the vacuum leaks were found and plugged.
Then it happened again. No closed-loop, or so I thought.
This time there was a major vacuum leak in this 4-nippled thing near the alternator top-bracket.

What I am trying to say is that when you think that the car is not in closed-loop, it may simply already have such a bad air leak, that when you purposely try to make that dwell meter move, it doesn't.

Seth
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Old 01-28-2009, 08:09 PM   #12
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Re: need help adjusting dwell on m/c solenoid

Quote:
Originally Posted by NoTransistors View Post
Had the same exact issues.

I was never able to get the darn thing to work properly until all the vacuum leaks were found and plugged.
Then it happened again. No closed-loop, or so I thought.
This time there was a major vacuum leak in this 4-nippled thing near the alternator top-bracket.

What I am trying to say is that when you think that the car is not in closed-loop, it may simply already have such a bad air leak, that when you purposely try to make that dwell meter move, it doesn't.

Seth
Exactly. Even if the dwell is high it could only mean that the O2 parameters are so far out of whack that the ECM doesn't know which way to adjust-it's not yet getting a good signal from the O2.
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Old 01-28-2009, 08:09 PM
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