Can't lower idle
Thread Starter
Intermediate
Joined: Jul 2002
Posts: 36
Likes: 0
Can't lower idle
I have been trying to figure this out on my own (reading past posts and Lars tech papers) and I think I am close but could use suggestions. I am working on a 73 with a 350 that was recently rebuilt with a mild comp cam, roller rockers, headers, etc. I have the initial timing (without advance) to 14 degrees and an overall pretty close to 36 degrees.
Here's the question... When I reconnect the vacuum advance the idle comes up, as I expect. But now when I try to lower the idle the screw comes off the stop and I can't lower it beyond 1100rpm. I have tried unhooking the throttle cable and that isn't hanging it up. I have messed with the idle mixture screws and those are 2 turns out.
I am guessing this is a carb problem and the answer is to have someone rebuild it or swap it out but I thought I would post and see if anyone has a better answer
The carb numbers work out to a truck carb from 73 and the PO had made a mess of all things mechanical 
One more to add... I am running about 14i inches vacuum. Is that normal with a mild cam? Sorry, I don't know the exact spec on the cam, I'm trying to find out.
Thanks - Scott
Here's the question... When I reconnect the vacuum advance the idle comes up, as I expect. But now when I try to lower the idle the screw comes off the stop and I can't lower it beyond 1100rpm. I have tried unhooking the throttle cable and that isn't hanging it up. I have messed with the idle mixture screws and those are 2 turns out.
I am guessing this is a carb problem and the answer is to have someone rebuild it or swap it out but I thought I would post and see if anyone has a better answer
The carb numbers work out to a truck carb from 73 and the PO had made a mess of all things mechanical One more to add... I am running about 14i inches vacuum. Is that normal with a mild cam? Sorry, I don't know the exact spec on the cam, I'm trying to find out.
Thanks - Scott
Last edited by ZIMMC5; May 25, 2014 at 09:36 PM.
Thread Starter
Intermediate
Joined: Jul 2002
Posts: 36
Likes: 0
It has what I think is called a divorced choke? The setup on the intake that as it gets hot pushes a rod that opens the choke. Seems to work? When I cold start it snaps closed and when warm I think is wide open. I haven't specifically checked but I believe the butterflies are opening correctly? I ran out of gas tonight while tuning
I have been doing some more reading and the idle mixture screws aren't really doing anything except if I close them both the engine dies. Other than that they can be 1 turn out or 8 and it doesn't really effect the engine? Sounds like that may be a problem that the jets are too small? I didn't want to mess with that until fixing the idle. Although maybe they are related???
Thanks again!!!
Racer
Joined: Aug 2006
Posts: 330
Likes: 3
From: Wellington, New Zealand
If you can screw in and out from 2 to 8 turns on the mixture screws and it has no effect then you have fuel entering the system from somewhere other than the idle circuit.
It could be float level or possibly stuck power needles. Can you see any weeping from the boosters etc?
It could be float level or possibly stuck power needles. Can you see any weeping from the boosters etc?
Drifting
Joined: Mar 2011
Posts: 1,886
Likes: 9
What year is your car?
Are you running a Quadrajet?
Does it have a solonoid control for the idle speed?
Then it also has a slow idle adjustment screw on the forward side of the float body on the driver's side. This was used to minimize the engine runon that was caused by the retarded (no vacuum) advance setting and large idle throttle opening.
Are you running a Quadrajet?
Does it have a solonoid control for the idle speed?
Then it also has a slow idle adjustment screw on the forward side of the float body on the driver's side. This was used to minimize the engine runon that was caused by the retarded (no vacuum) advance setting and large idle throttle opening.
Thread Starter
Intermediate
Joined: Jul 2002
Posts: 36
Likes: 0
If you can screw in and out from 2 to 8 turns on the mixture screws and it has no effect then you have fuel entering the system from somewhere other than the idle circuit.
It could be float level or possibly stuck power needles. Can you see any weeping from the boosters etc?
It could be float level or possibly stuck power needles. Can you see any weeping from the boosters etc?
Thanks for the ideas!
Thread Starter
Intermediate
Joined: Jul 2002
Posts: 36
Likes: 0
What year is your car?
Are you running a Quadrajet?
Does it have a solonoid control for the idle speed?
Then it also has a slow idle adjustment screw on the forward side of the float body on the driver's side. This was used to minimize the engine runon that was caused by the retarded (no vacuum) advance setting and large idle throttle opening.
Are you running a Quadrajet?
Does it have a solonoid control for the idle speed?
Then it also has a slow idle adjustment screw on the forward side of the float body on the driver's side. This was used to minimize the engine runon that was caused by the retarded (no vacuum) advance setting and large idle throttle opening.
Thanks for the ideas!
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Thread Starter
Intermediate
Joined: Jul 2002
Posts: 36
Likes: 0
Thanks for the help guys! I ended up buying a Holley 4165 to replace the quadrajet. I figured I would go to an easier to maintain carburetor. The qjet is a good carb, and Lars was willng to help with a rebuild but they seem to require some specialized expertise to maintain. By comparison the holley seems pretty easy to work with. Just my 2 cents, I know there are many opinions about this (I have read most of them here on the forum ).
I'll post and update early next week when I get the holley installed.
Thanks!!!
).I'll post and update early next week when I get the holley installed.
Thanks!!!
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Thanks for the help guys! I ended up buying a Holley 4165 to replace the quadrajet. I figured I would go to an easier to maintain carburetor. The qjet is a good carb, and Lars was willng to help with a rebuild but they seem to require some specialized expertise to maintain. By comparison the holley seems pretty easy to work with. Just my 2 cents, I know there are many opinions about this (I have read most of them here on the forum ).
I'll post and update early next week when I get the holley installed.
Thanks!!!
).I'll post and update early next week when I get the holley installed.
Thanks!!!
Scott
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Something is preventing the throttle/throttle plates from returning to fully closed (or idle) setting. That could be due to:
1) the choke mechanism or choke spring is keeping the choke plate closed (even when the engine is warm);
2) the throttle plates (butterfly valves in the base plate) are binding on the gasket and/or intake openings;
3) the throttle cable is misadjusted so that it cannot completely close;
4) something else I can't think of....
1) the choke mechanism or choke spring is keeping the choke plate closed (even when the engine is warm);
2) the throttle plates (butterfly valves in the base plate) are binding on the gasket and/or intake openings;
3) the throttle cable is misadjusted so that it cannot completely close;
4) something else I can't think of....
Drifting
Joined: Feb 2005
Posts: 1,879
Likes: 38
From: Los Gatos CA
The point others are trying to make is air must be getting in somewhere.
With the idle screw backed off she should idle down and die.
Either the throttle blades are not closing or you have an intake vacuum leak.
My guess is the fast idle adjustment because you said the idle speed screw backs off but the throttle does not close.
Good luck...
One more thing: When the engine is warm, is the choke plate vertical? Some times people will turn up the fast idle to compensate for mis-adjusted choke operation.
With the idle screw backed off she should idle down and die.
Either the throttle blades are not closing or you have an intake vacuum leak.
My guess is the fast idle adjustment because you said the idle speed screw backs off but the throttle does not close.
Good luck...
One more thing: When the engine is warm, is the choke plate vertical? Some times people will turn up the fast idle to compensate for mis-adjusted choke operation.
Last edited by Jeff_Keryk; May 31, 2014 at 09:38 PM.