Rough Idle Issue

[Bruzilla]

I installed a new carb return spring and cable kickdown setup on the 440 in my Roadrunner on Sunday. Took it out and drove it around, and aside from some issues with the kickdown coming in too soon, there were no issues. I ended the drive with an awesome burnout in front of my house that brought out a lot of my neighbors.

That night, we went to go out for dinner, and I noticed the idle was a little low. Since putting the Holley on a few weeks ago, the idle is at about 1150 when I start the engine, then when I hit the gas the first time and let off it drops to about 960 and stays there, and curb idle is in the mid 700s. When I started the car Sunday night, the initial RPM was in the low 900s and the curb idle was down in the 600s. When I started the car after dinner, the start idle was in the 700s, and the curb idle dropped to the low 500s and the car stalled. I got out and adjusted the idle to get it back to the mid 900s, and curb to the mid 700s, but within a few miles curb was dropping to the low-mid 600s.

I suspected a vacuum leak had cropped up, but I found nothing when I checked last night. Timing was also fine. I was thinking that maybe the two driver's side nuts on the carb base, that I removed to install the carb bracket and kickdown brackets, might not be tight enough and causing a leak, and I'll check that tonight. I'm also wondering if the new return springs aren't putting enough tension on the throttle arm.

Any other ideas?

 

[STREETREBEL]

I have had the same problem with my Eddy.
Finally took the top off and cleaned the dirt out of it.
Don't forget the little screens where the fuel enters.
If you have one.
May just be coincidence that it happen when put the new
spring on. I also adjusted my floats.

 

[Bruzilla]

I have had the same problem with my Eddy.
Finally took the top off and cleaned the dirt out of it.
Don't forget the little screens where the fuel enters.
If you have one.
May just be coincidence that it happen when put the new
spring on. I also adjusted my floats.

This is a Holley, and I have a new fuel tank, sender, and filter on the car, so I don't think fuel is the issue. This is something that happened between the time I shut it off Sunday afternoon and started it back up Sunday night. There were no issues that afternoon and the issues started immediately on start up that evening.

 

[PurpleBeeper]

Just an idea here..... IF you just put on a new return spring (probably stronger) then maybe it's pulling the carb more closed than it used to close. You may have also had a little "smutz" on the throttle blades that would hang them up slightly with the old/weaker spring, but now the new/stronger spring can pull the carb a little more closed. Maybe you could spray some carb cleaner on the ends of the throttle shafts & work the carb back and forth to remove any "smutz" & re-set the idle screw where it needs to be.

Otherwise, a new vacuum leak would explain that too.

 

[oldbee]

Also check if you have a little slop in k/d cable,maybe too tight?

 

[PurpleBeeper]

Also check if you have a little slop in k/d cable,maybe too tight?

Good point.

 

[beanhead]

I had the same issue recently, was fine and I romped on it from a stop pulling up the driveway..it suddenly didn't want to idle right, kept dropping like yours. Vacuum gauge read 3-5 inches lower than normal and slightly erratic, turned out to be a leak under the valley pan... couldn't find it through the standard checking methods and it drove me crazy for a while but when I pulled the pan off there was oil going right into the chambers...maybe check for oil on the plugs and hook up a vacuum gauge when it acts up

 

[Ski 61701]

Since you were putting brackets on the carb etc and snugging bolts down , I wonder if maybe you got a bad casting of a baseplate and you're developing a small hairline crack thats slowly growing?
Maybe take an unlit bottle of propane and gas the baseplate to rule out cracks?
With the quality of things nowadays, who knows??

 

[Bruzilla]

Since you were putting brackets on the carb etc and snugging bolts down , I wonder if maybe you got a bad casting of a baseplate and you're developing a small hairline crack thats slowly growing?
Maybe take an unlit bottle of propane and gas the baseplate to rule out cracks?
With the quality of things nowadays, who knows??

You're close! I think I found the problem last night.

I used to have an Edelbrock POS on there, and of course I had to use a 1/2" phenolic spacer to keep the fuel cool. I left it on there when I installed the Holley, and after reading the instructions from Holley (yes... I do read instructions regardless of what my wife claims) I bolted the carb on "hand tight".

When I installed the throttle return and kickdown brackets, I tightened the nuts on the driver's side of the carb like I would any retaining nut, but I didn't touch the passenger side ones. I found when I had to remove the Edelbrock, which was often, I would loosen up all four nuts and once I took the front two off the phenolic spacer would decompress a bit and the rear nuts would be tight again, so those spacers are bit spongy. I checked the driver's side nuts and they were tight, and then I checked the passenger side and they were nowhere near tight, so I'm thinking when I cinched down the driver's side, it compressed the driver's side of the spacer and lifted the passenger side of the carb just enough to create a small vacuum leak. Nothing major, just enough to penis up the idle.

I cinched down the passenger side and the idle seemed to steady out, but just then our afternoon deluge started and it didn't stop raining until after dark, so I hope to get a road test in today.

 

[Ski 61701]

Ive never used a phenolic spacer, do they have a tendency to "soften" with age? Hope your problem is solved and you'll have plenty of trouble free miles ahead of you

 

[Bruzilla]

Ive never used a phenolic spacer, do they have a tendency to "soften" with age? Hope your problem is solved and you'll have plenty of trouble free miles ahead of you

They're not exactly hard when new. They're basically layers of cardboard bonded together, so there is a bit of squish to them. Like I wrote earlier, you could loosen all four nuts to the point where you could take them off with you fingers, and by the time you get the front two off, the spacer would expand enough to make the two nuts on the back tight again.

 

[coloradodave]

Actually, "phenolic" refers to spacers that are made of plastic composite that is super hard. Like this: https://www.summitracing.com/parts/qft-300-4004qft/overview/
The wood ones are different and may have some compression ability, but I don't think that would be a good thing, considering how easy it is to warp or break an ear off a baseplate. Carburetors should always be tightened diagonally, a little at a time...

 

[Bruzilla]

The phenolic spacers on the market today are made of a host of different compounds. Holley's are plastic. Edelbrock's are a composite of nitrile rubber (the stuff they make surgical gloves from), cellulose fibers (wood), and plastic.

 

[Ski 61701]

Very interesting both Bruzilla and Coloradodave, thanks for sharing /explaining things a little more in depth

 

[coloradodave]

And, for you scientific type guys, here's the actual meaning of the word:
Definition of phenolic. 1 : a usually thermosetting resin or plastic made by condensation of a phenol with an aldehyde and used especially for molding and insulating and in coatings and adhesives —called alsophenolic resin. 2 : phenol 2.
Phenolic | Definition of Phenolic by Merriam-Webster
https://www.merriam-webster.com/dictionary/phenolic

I guess this puts this post to bed...lol

 

[Bruzilla]

I took the car out tonight... we actually got an evening without rain, and got on it pretty good. She's running just fine again, so I guess the problem was a vacuum leak from the carb base, which was fixed when I tightened the passenger side nuts.