Edelbrock 600cfm carb idle will not stay steady

[HeavyHauler]

I have a edelbrock 600cfm carb on a stock 350 chevy except for the usual edelbrock intake manifold. The idle goes up and down and will not stay steady. Also if I shut the engine off and try to restart it then it is very difficult to get started again...I try pumping the gas peddle once or twice but it will not start .....but if I prime the carb by putting gas into the carb from a container it will fire right up but when I get it started the idle will not stay steady. I wanted to be able to drive the truck around while I rebuild the original big block 396 for the truck.

This is a old carb that was on the truck when I purchased it from the last owner that I am just trying to troubleshoot what is wrong with it. Could it possibly be the timing chain and not the carburetor at all?

[aerotest]

Spray some carb cleaner down the bowl vent while it’s running. Sound like your idle circuits are gummed up.

['68OrangeSunshine]

Check for manifold leaks.
Do you have power brakes?

[HeavyHauler]

Originally Posted by aerotest

Spray some carb cleaner down the bowl vent while it’s running. Sound like your idle circuits are gummed up.

I will give that a try tomorrow.

[HeavyHauler]

Originally Posted by '68OrangeSunshine

Check for manifold leaks.
Do you have power brakes?

No power brakes but I will find out how to check for manifold leaks.

['68OrangeSunshine]

Originally Posted by HeavyHauler

No power brakes but I will find out how to check for manifold leaks.

The carb cleaner spray trick works. When the RPMs bump up a little, after getting hit with the volatile vapors, you've found your leak. Also check all the tubing...vacuum canister, PCV etc. An air leak in a power brake reservoir can cause that, but you've ruled that out.
The Edelbrock 1405, 1406 are pretty simple carbs. Easy to fix with the available EDL-1477 kit.

[Steeveedee]

If the carb has any kind of age I'd check the primary throttle shaft for a leak. It's quite common,and will give the idle characteristics you are seeing. Other things may be wrong with it, too. You can have it or get it rebuilt, and put in new bushings. It's cheaper to have them put in since the tools cost money and you may only do it once. I usually rebuild my own and have a carb shop do the bushings. Last time the guy squawked about not having the whole job. YMMV

[Caddylackn]

Check your centrifugal advance weight bushings and weak springs in distributor. Replace if worn, make sure they are greased. You may be getting unwanted distributor advance creeping in at idle. You can check this with a timing gun. Make sure your centrifugal advance is starting above 1,000 rpms.

[HeavyHauler]

I took the carb off the engine and I am going to take it apart and fully clean it. I have found that it is a 1405 600cfm carb.

[HeavyHauler]

Originally Posted by Steeveedee

If the carb has any kind of age I'd check the primary throttle shaft for a leak. It's quite common,and will give the idle characteristics you are seeing. Other things may be wrong with it, too. You can have it or get it rebuilt, and put in new bushings. It's cheaper to have them put in since the tools cost money and you may only do it once. I usually rebuild my own and have a carb shop do the bushings. Last time the guy squawked about not having the whole job. YMMV

How do I check to see if the primary throttle shaft is leaking and needs new bushings?

[68 P.O.S.]

Sounds like you have a couple potential issues. First things first, timing. If your timing isn't correct, everything else will be off as well. GM engineers found Chevy engines run best at 36 degrees total timing, 38 at higher elevations. Set your total timing accordingly and then move on to your carb. If you plan on sticking with that carb, your best bet is to clean it out real well and throw a rebuild kit at it. Then set the float levels to specs and regulate the fuel pressure to 5.5 psi to prevent flooding. Next, put a heat insulating gasket under it, to prevent fuel percolation, which causes the hard starting after warm up you're reporting. You also have to set the transition slots in the throttle bores to be a square to ensure it's only idling on the idle circuit. What is your idle rpm set at, or the range its bouncing around at? I'm wondering if it's set too high and allowing the mechanical advance to dip in n out? Idle should be 750 rpm for auto and 600 (I believe) for manual. Which distributor are you running, original points or HEI? I agree with Caddylackn, make sure the distributor advance weights are well lubed. After messing with the carb, hook the vac advance up to manifold vacuum. Then after you get it running well, use a vacuum gauge connected to manifold vac to set the idle mixture screws for highest vacuum. Hopefully this fixes your issue and the throttle shafts are fine. If you don't already have it, download a copy of the Edelbrock instructions. All you need to know is in there as well.

[Steeveedee]

Originally Posted by HeavyHauler

How do I check to see if the primary throttle shaft is leaking and needs new bushings?

It would have been easier if it was still installed and you could spray carb cleaner or something onto the shaft. But since it is off, open and close the primary butterflies while pulling the throttle linkage forward and backward at the linkage. It it moves from front to back while you rotate the butterflies you will see that they don't close all the way. Maybe even try just back and forth at the shaft without opening or closing the butterflies. It probably has detectable movement if the body of the casting is worn.

[HO455]

Throttle shaft bushings wear in a front to back direction on the drivers side of the carburetor. This wear is accelerated by having improperly installed springs or too heavy of return spring. (Most aftermarket springs fall into this category).
You can feel the shaft wear if you try to move the throttle shaft forward and aft. Anything more than 1/16" of play could be problematic.

[HeavyHauler]

Originally Posted by 68 P.O.S.

Sounds like you have a couple potential issues. First things first, timing. If your timing isn't correct, everything else will be off as well. GM engineers found Chevy engines run best at 36 degrees total timing, 38 at higher elevations. Set your total timing accordingly and then move on to your carb. If you plan on sticking with that carb, your best bet is to clean it out real well and throw a rebuild kit at it. Then set the float levels to specs and regulate the fuel pressure to 5.5 psi to prevent flooding. Next, put a heat insulating gasket under it, to prevent fuel percolation, which causes the hard starting after warm up you're reporting. You also have to set the transition slots in the throttle bores to be a square to ensure it's only idling on the idle circuit. What is your idle rpm set at, or the range its bouncing around at? I'm wondering if it's set too high and allowing the mechanical advance to dip in n out? Idle should be 750 rpm for auto and 600 (I believe) for manual. Which distributor are you running, original points or HEI? I agree with Caddylackn, make sure the distributor advance weights are well lubed. After messing with the carb, hook the vac advance up to manifold vacuum. Then after you get it running well, use a vacuum gauge connected to manifold vac to set the idle mixture screws for highest vacuum. Hopefully this fixes your issue and the throttle shafts are fine. If you don't already have it, download a copy of the Edelbrock instructions. All you need to know is in there as well.

I checked timing and initial is at 12 BTDC and 36 total.

[HeavyHauler]

I totally disassembled the carb,cleaned cleaned everything,set the floats because they were waaaaay off,put it back together and set the idle mixture screws and idle. It runs really well now. Took it for a drive and it ran the best it ever has since I purchased it couple months ago. Now I just need to fix the brakes and make them stop better.

[HeavyHauler]

Originally Posted by Steeveedee

It would have been easier if it was still installed and you could spray carb cleaner or something onto the shaft. But since it is off, open and close the primary butterflies while pulling the throttle linkage forward and backward at the linkage. It it moves from front to back while you rotate the butterflies you will see that they don't close all the way. Maybe even try just back and forth at the shaft without opening or closing the butterflies. It probably has detectable movement if the body of the casting is worn.

The truck is now running like a champ but I will check this anyways to see what kind of movement it does have currently.

[HeavyHauler]

Originally Posted by HO455

Throttle shaft bushings wear in a front to back direction on the drivers side of the carburetor. This wear is accelerated by having improperly installed springs or too heavy of return spring. (Most aftermarket springs fall into this category).
You can feel the shaft wear if you try to move the throttle shaft forward and aft. Anything more than 1/16" of play could be problematic.

Awesome and thanks for the tip. I will check the play anyways just to see how much it has. My truck is running really well now.

[HeavyHauler]

Thanks to everyone who took the time to add their knowledge. My truck is now running really well. This is a great forum!!!

[68 P.O.S.]

Glad you got it running well and glad we could help. Congrats! So did the tuning make the fluctuating idle disappear?