We are talking about a 66 coup with a rebuilt 302 (500miles) that has 351w heads, moderate cam, Edelbrock performer intake, Edelbrock 600cfm carb (mechanical secondaries), Mallory unilite dizy, and duel exhaust with long tube headers.

The problem: the engine runs a little rough. It almost seems as though it is a sporadic miss in a cylinder however I have discovered that it can be smoothen out or made worse depending on how I adjust the idle mixture screws. No matter how much I adjust the IMS the rough running will not go away, its still there during cruse and full throttle however not nearly as noticeable. One last bit of info is that when I hit the throttle the carb stutters/coughs a little a spits a mist out of the carb (almost like what you would see with a back fire, but without the combustion and pop) before taking off and only happens when the throttle is pumped. I am at a loss to figuring this out, I was thinking it is the carb but have no idea what I need to do to fix it.

If I missed something or need to give more detail, let me know.

P.S. I am well aware of the Holly/Edelbrock rivalry that goes around and I do not care either way, plus I am not about to go buy a holly carb to fix the problem. Lets keep the comments to the task at hand, fixing the problem.

Thanks, I appreciate the help.

 

Start with the basics:

Are the spark plug wires installed like the camshaft firing order?
Cap and rotor good?
Lead wires to coil tight?
Timing correct?
No vacuum leaks at the pcv hose or elsewhere?

Hook up a vacumm gauge and tachometer
start motor and set timing at about 12* at idle. Turn off motor.
Back off the throttle stop screw so that it does not open the butterflies at all.
Set the idle screws at 1.5 turns out.
start motor and adjust idle screws 1/4 turn at a time for best vacuum and highest idle.
The butterfly stop screw should not engage the butterflies until throttle is applied. If you can't achieve a decent idle you may need to go through the carb or ignition system.
Consult your cam manufacturer to make sure you don't have some crazy loping dragster cam.

I used to be decent at this kind of stuff. Now, I am a little rusty. Recently, I crossed the 5 and 6 spark plug wires on the HO firing order and didn't realize it. The car idled well and ran well until 4000rpms or so and it would do this missfire thing. I buddy of mine suggested I check it and I blew it off. It wasn't until further investigation that cylinders 5 and 6 weren't heating up like the others did I realize this was the case. So in short, start with the basics-we all make simple mistakes.

Holley has carburetor instructions on their website you can download. Print it out and take to the garage in case I missed something.

Keep us posted what you find. I like to learn from other peoples problems.

Dave

 

Hey, I just noticed you are in AZ too. Where at?

 

I live in Tucson, your self?

 

I was afraid you were going to mention a tac and vaccume gage... I guess that gives me an excuse to buy one eh?
Yeah, I figured I needed to look over the basics again but I didn't want to.

As far as the cam goes, its a Crane Cams Energizer Hydraulic with .469 lift on both intake and exhaust, and 266 duration on both. Future project is to get 1.7 rocker arms for the exhaust port but that won't solve the problem. However I am fairly certain that it holds a stock firing order. But I will play with the wires to see if its spark related at all.

I will post as soon as I play with the car some more and have some answers.

 

I live in Queen Creek. I travel to Tucson regularly for work.

That cam will not have any lope whatsoever. Don't forget the dwell meter. Many dwell meters have a switch to read rpm's.

 

the cam should be not much of a lope. you'll definetly have to get a vacuum gauge as that is the easiest to adjust carb (metering springs in your edelbrock) and the idle mixture as well as the secondary. some gauges do fuel pressure at the same time.
on top of what Ocho said, check fuel pressure and floats.
most fuel pressure pumps are set for 7psi which the holley like, but more than 6 can cause an edelbrock to leak through the boosters. i think they like 4.5psi.

So first check ignition, then fuel pressure, then float level, then curb idle, then idle mixture screws

i don't know if you have a timing gun, if not that's a must have. since you have a mallory unilite you won't have points, that rules that out

 

Ok time for an update.

I as I had a verity of plug wires on the car I decided to get a new set, stuck with a standard set. Spark plugs were replaced when I put the engine in the car, about 500 miles ago, and they looked black. Hooked up a tac to the engine and it was idling at 1000rpm with a sporadic drop in rpms by about 50-100 (this was notices as a slight jostle in the engine when watching it). Checked and reset the timing to about 10 BTDC. Hooked the vacuum gauge up to the manifold vacuum port to discover that the needle was sporadic at idle bouncing rapidly between 12-15psi but would smoothen out and almost go away once the rpms reached 2000. From there I tried to set the IMS on the carb as best I could, however in the process i discovered that depending on how I mixed the left bank would change how much the vacuum gauge would jump back and forth.

My opinion is as fallows. The vacuum leak is on the left side of my carb causing a fluctuation in the vacuum, causing the vacuum advance on the dizy to fluctuate the ignition advance, causing the drop in rpms. The fact that the carb is not adjusted properly has little effect on this problem and the vacuum problem needs to be addressed first before attempting to adjust the carb.

What are your guy's thoughts? Am I on the right track or is there something I overlooked and another issue that needs to be addressed?

P.S. The engine was rebuilt 500 miles ago and the E7 heads I put on it were sent to the machine shop to be machined and rebuilt prior to being put on the engine, so by all means the internal components "should" be fine.

 

Your idle is high. Have you backed off the butterfly stop screw? When the throttle is partially opened it starts to take the idle mixture screws out of the picture. One good way to check for vacuum leaks is to use a spray can of carb cleaner and spray it at the area you think is leaking. Be carefull you don't start a fire. If the idle smooths out temporarily you have found your leak.

 

if i read that correctly, one of the mixture screws has affect on idle and vacuum, whereas the other one doesn't change anything?
if that's the case you could say it might be a vacuum leak, but i'd be more pressed to say that the side which has no affect has a blockage (dirt etc).

but yes. try the carb cleaner first like Ocho says. I usually use WD40 ... whatever i have lying around that doesn't leave marks. spray at carburetor base, intake manifold gaskets. if you have any vacuum hoses hoooked up to the manifold check them as well, especially if they're sitting on the same runner that is feed by that carb side