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Old 09-05-2012, 03:46 PM
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Edelbrock Idle screw wont go lower than 1200 rpm

Hey guys Im having some trouble dialing in my new engine. Its a L31-R crate, Edelbrock 1406 electric choke 600 cfm carb, th375 tranny, Ive been planning on having a mechanic do it but the guy i want to tune it is on vacation till monday and i figured i could try to sort out the problem before i fork out the cash for him to do it. I cant get the idle to go below 1200 rpm in park and 900 rpm in drive. I have the Idle screw as far backed out as it can go before the linkage stops making contact with it. The engine was fully warmed and not sitting on the fast idle screw. I checked for vacuum leaks with some carb cleaner but couldnt find any change in idle when sprayed on hoses, ports, brake booster, vacuum modulator on tranny. Ive removed the throttle cable to see if that was holding it up but it wasnt the problem. any suggestions what to check now? I read to check the choke maybe ill go check that while i wait for some suggestions.

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Old 09-05-2012, 09:35 PM
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The Secondary throttle blade may be cracked open allowing air in.
Last Q-Jet I installed had that problem.
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Old 09-05-2012, 09:36 PM
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If it's idling that high and the idle screw is no longer touching then either the timing is way over advanced or it's got a major vacuum leak inside the power brake booster. Check the timing and see where it's at. Keep in mind the mechanical advance could be stuck in full advanced position.
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Old 09-05-2012, 10:43 PM
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Ive been trying to watch the engine as close as possible while I mess with it. Its at 10 degrees initial but with that high idle Im thinking some mechanical is coming in as part of that 10 degrees initial. I think you guys are on the right track for sure. A mechanic told me to pinch the booster vac hose to the maifold I did it and it didnt make any difference? would it have changed idle speed or died if that was the vac leak source? also I did have the line off the booster when i swapped out the engine could I have messed something up in there?

As for the blades sticking open should I be able to spot it or is the gap so small I might not see it with my blurry eyes I read to blow smoke into the engine and it would come out where the leak is....I blew smoke into the carb manifold vac port and it came outta the carb but i was pretty much blowing into it so i figured it would come out there no matter what..with the car off should smoke not be able to pass up through the blades?
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Old 09-05-2012, 11:31 PM
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Heres some pictures just to give you an idea of my set up...Some stuff is kinda rigged atm but heck it all works good except this dang vac leak or whatever the problem turns out to be. Let me know if you se anything that looks out of order or could be causing the problem. Ive allready had some major headaches with this swap that were just really hard to track down simple problems...had a valve cover plugged instead of vented and it made a scrEeEeEech from hell.

Ill try to get some good pics of the carb tomarrow maybe down inside it so you can see if you spot any problems...I think the plate over the back butterflies is allways open if thats the secondary throttle blade youre talking about. I dont know the anatomy of my carb verry well yet but hopefully after this ill know it like the back of my hand



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Old 09-06-2012, 05:55 AM
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Out of curiousity why is the vac advance hose not connected?
Or is it disconnected while you try to find a vac leak?
You'll want to pop the distributor cap and grab the rotor and check the mechanical advance, see if it sticks or if it has any places that are sticky
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Old 09-06-2012, 06:45 AM
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Quote:
Originally Posted by GUITAR1989 View Post
Ive been trying to watch the engine as close as possible while I mess with it. Its at 10 degrees initial but with that high idle Im thinking some mechanical is coming in as part of that 10 degrees initial. I think you guys are on the right track for sure. A mechanic told me to pinch the booster vac hose to the maifold I did it and it didnt make any difference? would it have changed idle speed or died if that was the vac leak source? also I did have the line off the booster when i swapped out the engine could I have messed something up in there?

As for the blades sticking open should I be able to spot it or is the gap so small I might not see it with my blurry eyes I read to blow smoke into the engine and it would come out where the leak is....I blew smoke into the carb manifold vac port and it came outta the carb but i was pretty much blowing into it so i figured it would come out there no matter what..with the car off should smoke not be able to pass up through the blades?
------------------------------------------------------------------------
If there was a leak in the Booster, pinching the hose would've lowered the Idle speed.

I'm talking about the rear Secondary butterflies on the bottom of the carb may be open allowing air to flow thru there. This would make the Idle adjustment on the front Primary butterflies ineffective at slowing the Idle speed.
Edelbrock 1406 - Edelbrock Performer Carburetors - Overview - SummitRacing.com
The adjustment screw is usually on the bottom side of the linkage & requires removing the carb on some types or using a 90* degree angle screwdriver.
http://s.shld.net/is/image/Sears/spin_prod_520831301
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Old 09-06-2012, 11:25 AM
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I had a q-jet fast idle problem as you describe. It turned out to be a vacuum leak at the power brake booster\hose and a leak at the rear of the carb where a vacuum tube screwed in the carb. My vacuum leak was so bad the primary towers were dumping fuel (drips) with the blades fully closed.

good luck, bt
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Old 09-06-2012, 01:04 PM
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The vac advance is just unplugged cause it gives way to much advance at the moment. I need to block out some of the travel on the vac canisters rod.
thats why its just unplugged for now. Im going to go out and check the brake booster again and adjust my choke then ill check the secondaries. I couldnt really see them open at all but ill double check it thanks for the help guys ill let you know what happens.
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Old 09-06-2012, 02:08 PM
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Hey guys just checked the choke and its operating fine. I also looked into the carb to see if it was just dripping fuel and i couldnt see any excessive fuel coming out. I pinched the brake booster hose and it didnt do anything. I took it off and plugged it to make sure there was no air flow and nothing. One odd thing I notice is when i use the brakes it has a direct connection to the idle. When I pump them it seems to surge the idle a bit.

Now im going to look into checking and adjusting the secondaries.
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Old 09-06-2012, 04:23 PM
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Well I popped off the carb and the secondaries look like theyre closed. maybe a hair open but its so close I cant even tell really. I see the linkage and spring that connect to the secondaries but still need to find out more about adjusting it. wich im not really finding anything on the subject
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Old 09-06-2012, 07:33 PM
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Here are few links I found useful when I was trouble shooting my q-jet.

Carburetor Tuning Basics - Classic Trucks Magazine

Q-Jet idles too fast - Corvette Forum (power piston may be sticking & it's vacuum operated)

High Speed Idle - How To Fix Idle Problems - High Performance Pontiac Magazine

Here is a q-jet reference book.

Amazon.com: How to Rebuild and Modify Rochester Quadrajet Carburetors (S-a Design) (9781932494181): Cliff Ruggles: Books Amazon.com: How to Rebuild and Modify Rochester Quadrajet Carburetors (S-a Design) (9781932494181): Cliff Ruggles: Books

Hope this helps and good luck. bt
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Old 09-06-2012, 10:26 PM
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If the engine rev changes when you operate the brake pedal it's likely due to not having a check valve in the line.
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Old 09-06-2012, 11:24 PM
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I think it does have a check valve right where the hose goes into the booster. I pulled the carb and cleaned its linkages and just messed with them to get a better idea of how they work together. I hooked up my vac gauge after putting the carb back on and its sitting steady at 17 inches at idle. should the needle fluctuate with a vac leak? it still wont idle down but i was able to get the idle to drop by turning on the passenger side secondaries linkage. when i turned it the other way the engine started running like crap....im gonna mess with that linkage more and see if i can figure it out. It was just to late to really mess with it tonight.
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Old 09-07-2012, 07:05 PM
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Edelbrock Idle screw wont go lower than 1200 rpm

Try pushing the brake pedal with the vacuum gauge hooked up. Is there any change in vacuum readings when pushing the brake pedal? I would replace the check valve Dorman makes a replacement. Advance Auto or Auto Zone should carry it for about $4.00.
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