Slow idle return...
Thread Starter
Race Director
Joined: Apr 2003
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From: Tampa, Florida
Slow idle return...
If I set my idle then rev it up a little its slow to return to baseline.
Example....Idling at 850 rpm then rev'ed to 2000rpm. It hovers around 1500rpm for awhile then sloooowly comes back down.
I've checked at the linkage and the stop is against the screw...
Example....Idling at 850 rpm then rev'ed to 2000rpm. It hovers around 1500rpm for awhile then sloooowly comes back down.
I've checked at the linkage and the stop is against the screw...
Team Owner
Joined: Aug 2006
Posts: 24,125
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From: Columbia Missouri
Do you have a throttle return spring in place? Is it a single spring or a double set? Is it getting old and easy to move with your fingers?
I would suggest a trip tot eh parts store...buy a new double spring set of throttle return springs.
I would suggest a trip tot eh parts store...buy a new double spring set of throttle return springs.
Tech Contributor
Joined: Aug 1999
Posts: 14,373
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From: At my Bar drinking and wrenching in Lafayette Colorado
Slow return to idle is usually a timing issue. There are 2 things that can cause it:
1. Vacuum advance control unit not properly matched to engine idle vacuum level. If the unit is too "stiff" it will produce full advance when you rev it a little, and then it will gradually retard the timing (and lower the idle speed) as manifold vacuum degrades. To test for this, unplug the vacuum hose and see if the idle becomes steady. If it does, you need a better matched vacuum unit. You can contact me for a part number.
2. If the idle still acts the same with the vacuum disconnected, chances are good that the centrifugal advance is a little sticky. Put a timing light on it and see what the timing does as the idle is slowly coming down - see if the timing is slowly retarding as well. If it is, you need to clean up and fix your centrifugal advance.
1. Vacuum advance control unit not properly matched to engine idle vacuum level. If the unit is too "stiff" it will produce full advance when you rev it a little, and then it will gradually retard the timing (and lower the idle speed) as manifold vacuum degrades. To test for this, unplug the vacuum hose and see if the idle becomes steady. If it does, you need a better matched vacuum unit. You can contact me for a part number.
2. If the idle still acts the same with the vacuum disconnected, chances are good that the centrifugal advance is a little sticky. Put a timing light on it and see what the timing does as the idle is slowly coming down - see if the timing is slowly retarding as well. If it is, you need to clean up and fix your centrifugal advance.
Thread Starter
Race Director
Joined: Apr 2003
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From: Tampa, Florida
Thanks for the help. I do have a double return spring and it does return fully.
Lars, can you recommend a good timing light. I saw an Actron model with the rpm readout and a dial. Is that acceptable?
Speaking of vacuum advance....Where should I attach the hose? I know to attach it to manifold vacuum but at the moment it runs to the front of the carb up high. The hose is over a foot long. I'd imagine as short as possible is best?
thanks,
Jim
Lars, can you recommend a good timing light. I saw an Actron model with the rpm readout and a dial. Is that acceptable?
Speaking of vacuum advance....Where should I attach the hose? I know to attach it to manifold vacuum but at the moment it runs to the front of the carb up high. The hose is over a foot long. I'd imagine as short as possible is best?
thanks,
Jim
Last edited by jim2527; Mar 19, 2007 at 11:01 AM.
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
The Actron light is good - I use one on my Tuning for Beer Tours, and I've had good luck with it.
Attach the vacuum advance hose to any manifold vacuum source - it doesn't matter where. All the ports on the carb are manifold vacuuum except the one ported vacuum nipple (location depends on year of carb). If you have a 67-74 Q-Jet, the ported vacuum nipple is the one on the forward driver's side of the carb - about in the middle of the float bowl. If you have a 75-80 Q-Jet, the ported vacuum nipple is on the forward passenger side corner of the baseplate at a 45-degree angle. All other ports are manifold vacuum. I've never seen hose length to have any effect on vacuum advance performance/function.
Attach the vacuum advance hose to any manifold vacuum source - it doesn't matter where. All the ports on the carb are manifold vacuuum except the one ported vacuum nipple (location depends on year of carb). If you have a 67-74 Q-Jet, the ported vacuum nipple is the one on the forward driver's side of the carb - about in the middle of the float bowl. If you have a 75-80 Q-Jet, the ported vacuum nipple is on the forward passenger side corner of the baseplate at a 45-degree angle. All other ports are manifold vacuum. I've never seen hose length to have any effect on vacuum advance performance/function.
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Todd - good point. If the car is a CA-emissions car and is equipped with the dashpot, the idle is designed to drop down slowly from an elevated idle rpm.
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Thread Starter
Race Director
Joined: Apr 2003
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From: Tampa, Florida
If I knew what it was I'd could tell you. I wasnt able to play around much today. The mechanical advance is moving very freely. How does the vacuum advance canister come out? It seems the screws are blocked?
Also, it appears the points are too tight. Thats based on 'eyeing it' up. I'll pick-up a dwel meter tomorrow and post results of my garage tune.
Also, it appears the points are too tight. Thats based on 'eyeing it' up. I'll pick-up a dwel meter tomorrow and post results of my garage tune.
Tech Contributor
Joined: Aug 1999
Posts: 14,373
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From: At my Bar drinking and wrenching in Lafayette Colorado
The dashpot is a little canister that has a rod up against the throttle lever. It is not the idle solenoid... It pops out when the throttle is opened. When the throttle rests against it upon return to idle, the dashpot gradually bleeds down and collapses. It has no wires to it.
Have you identified it as being bad? Or are you just changing it out? Make sure you use a unit that is compatible with your idle vacuum level. To change it, put a long hose on it and suck on it to pull the point breaker plate. This will rotate the plate so you can access the screws. Once the plate rotates, block the hose with your tongue and pull the screws out. Wash and rinse your mouth with beer and swallow.
Dwell won't cause the problem you describe. You also can't eyeball it. You can set the gap to .016" or use a dwell meter.
Dwell won't cause the problem you describe. You also can't eyeball it. You can set the gap to .016" or use a dwell meter.