Spreadbore carb idles too fast
Thread Starter
Pro
Joined: Jun 2009
Posts: 613
Likes: 3
From: Fleetwood PA
Spreadbore carb idles too fast
I just put a Holley spread bore(6853 DP) on my 68 327....I know it should have a Q-Jet, but the Q-Jet that was on it was not the right year anyway, plus it was in really bad shape. I had this spread bore sitting around, so I figured I'd give it a rebuild and throw it on.
Go it to start, but the idle is way fast. I have the mixture screws about 3/4 turn out,and the idle screw is all the way out, and it's still at 2000 rpm. The choke and fast idle cam are off the carb so it can't be that. Checked the butterflies and they aren't sticking. Can't find any Vac leaks. I even pulled the distributor and completely disassembled it and cleaned it.
I'm usually pretty good with this stuff, but this one has me stumped.
Any ideas would be greatly appreciated.
Go it to start, but the idle is way fast. I have the mixture screws about 3/4 turn out,and the idle screw is all the way out, and it's still at 2000 rpm. The choke and fast idle cam are off the carb so it can't be that. Checked the butterflies and they aren't sticking. Can't find any Vac leaks. I even pulled the distributor and completely disassembled it and cleaned it.
I'm usually pretty good with this stuff, but this one has me stumped.
Any ideas would be greatly appreciated.
Instructor
Joined: Mar 2002
Posts: 140
Likes: 1
From: east hampton ct
Assuming you don't have a big vacum leak check the secondary throttle blade adjustment. If you turn the carb over there is a small screwthat sets the location of the secondary throttle blades. A ssmall adjustment can make a big difference. Your mixture screws should also be out about 1 1/2 turns to start. When you have the carb off set the primary transfer slot opening at .020 to .030 exposed. Thats the opening you will want to install the carb with. Right now with the idle turned up you are runnung off the idle transfer circut instead of the idle circuit. After setting the transfer opening see how many turns it takes to bring the opening to about .040. That will give you the range available to set your idle speed. Anything much more than a .040 opening will start the transfer circuit. Any adjustments more than that need to be set useing the secondary blade adjustment
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
Also check the clearance between the throttle shafts and their bores in the throttlebody. Too much clearance there is the same as a big vacuum leak. This could be why the carb was "sitting in the shelf" in the first place.
CF Community Team
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Thread Starter
Pro
Joined: Jun 2009
Posts: 613
Likes: 3
From: Fleetwood PA
Linkage is good also. When I rebuilt it I bead blasted everything and washed and blew it dry. All linkage was lubed and checked for movement. If I turn the idle screws all the way in on just one side I can kill the engine so they are good. I removed all the vac accessories and plugged them....same problem. My Vacuum gage is bouncing between 8 and 12 HG which is way low, and It shouldn't be bouncing that much, I'm just boggled. When I find it I know it's going to be something stupid, but I'm at a loss.
Heel & Toe
Joined: Jul 2010
Posts: 21
Likes: 0
From: Bohemia NY
Had a similar problem with a 4175 Holley spread bore. 1st problem was the bolt blocking off hot air choke was holding secondarys open slightly, Fast ideal can be turned down, mine was just under choke assembly 1/4 open end did the trick carb runs like a clock. Q-jet gave me lots of problems it had to go, dispite all the negative feed back about the Holley spread bores i think it runs good on my Stock L82.
