Rich idle, would this help?

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PA Dodger

PA Dodger

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I'll start this thread by saying-Yes I did search, and research this issue.
My 340 has a problem with the idle being overly rich at idle. The carb is a 650 cfm mech secondary Holley. The car runs really well but has always been rich at idle. I rebuilt the carb and still have the same problem.
The stats: timing is 20* at idle/car idles at 900 when warm/vacuum is 10ish at idle/PV is 6.5/idle mixture screws are 1 1/2 turns/primaries are set "square" on the transition slot/secondaries are set per the Holley rebuild specs/cam is .485" lift-220* [email protected] -266*duration-110-104 LSA.
I read where opening the secondary butterflies will help this condition. When I opened the secondaries a half turn I had to back the primaries down to get the idle back to normal. My primaries were already OK to the transition slot I should have been OK running off the idle circuit. It seemed to help the rich idle but I don't understand how. What I added to the secondaries I took away from the primaries. Is this the same thing as drilling the primary blades?
Any other suggestions? Vaccuum secondary carb? smaller carb?
 
Same thing, just lets more air that does not have fuel mixed with it yet.
Your procedure had the exact expected results from what you described doing.
 
Rich as in eye burning? What type of fuel pump? Floats set correctly? Do you have a fuel pressure gauge? If so , what pressure is it at? Smell your oil, might be dumpin fuel into the block and blowin it out the back.
 
Probably need to jet down in the front, or open up the primary air bleeds a little.
 
Seems like a high level of ported vacuum for idle ....where it ported vacuum now???? I suspect that at least the progressives are in play with that amount of airflow at idle. (Before you adjusted the primaries, did you look down the primary throats when idling and see if any fuel was dripping/spitting out of the auxiliary venturi's? If so, it was pulling some fuel from the mains too.)
 
Trailbeast-so the benefit is it can be tweeked instead of guessing what size hole to drill. Sounds like a win to me.
Pntastar69-Pretty close to eye burning. Mechanical pump. 8-9 pounds of pressure. Floats set correctly. Before the carb rebuild I noticed the secondary squirters would drip a little for about a minute after shutting the car down. Don't notice it now.
Rusty-I didn't think the jets had much affect on the idle circuit. Do I need to drill the air bleeds out. How do I tell how much?
Snake- My mistake. The vacuum reading was in park. I'll have to check it in gear.
BBM-It's not my engine build. I don't know what the previous owner did with it. It runs great except at idle.
nm9s-I probably misrepresented the vacuum at idle since I didn't check it in gear. I never noticed any dripping while it was running.

I know the car was raced before so maybe I need to check the main jets and lower them a number or two. Opening the secondaries a bit helped the problem but I don't want to do this if it screws something else up. I'm just tired of smelling like a tailpipe when I take the car out.
 
Don't drill anything would be my suggestion. Should not be required on that engine.

If the idle mixture screws kill the engine when all the way in, you are on the idle circuit.

Float level, no dripping boosters, make sure that is correct or not happening.
 
I had to open my secondaries a bit to get rid of my rich idle problem. This was on a new out of the box Holly 750 double pumper. Brian at Indio Motor Machines who had my cam ground told me to do so. Works great. Just open it up enough to lean it out not too much.
 
crackedback- The idle circuit is working fine with the idle mixture screws. Floats are fine, no booster leaks.
340 fastback-I'm happy to hear this worked for you and was suggested by your engine guy. Now I know its got some validity.

I agree with crackedback that this isn't some radical engine. This seems like it should be an easy fix. I was thinking I have too much fuel pressure or maybe rejetting the primaries if this would help like RRR suggested.
 
I have always understood that fuel pressure should be at 7 psi maximum.
 
Well Lemmie ask you this......what's your basis of comparison? It's runnin rich compared to WHAT? A 2013 fuel injected car?
 
RustyRatRod said:
Well Lemmie ask you this......what's your basis of comparison? It's runnin rich compared to WHAT? A 2013 fuel injected car?
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Been playing with cars for years. The only other car I had with this problem was a '70 Challenger with a built 383. I had to drill the butterflies on the Eddy carb. My '74 Cuda with a mild 360 and dual quad intake and 625 Eddys runs and idles perfectly. While I'm not an expert at carbs I'm not a novice either. Although I have tossed around the idea of fuel injection on this car.
 
Jetting has nothing to do with the idle circuit.

I would verify fuel pressure.

How's the mag pick up clearance if it's a reluctor wheel style.

I would isolate the intake tract, take everything that draws vacuum off and plug it. See if that changes anything. Add back each item one at a time to see if it causes a change.

Power brake boosters are sometimes an issue.
 
Unless the cam is very radical, the low vacuum could point to a vacuumleak somewhere, which could be masked by a richer idle-setting in the carb.
 
You need a regulator to drop the pressure down to 6 - 6.5 for a holley. You're blowin fuel past the needle seats. How do I know?
 
Have you check vacume now in gear,you said in P you had 10 i bet when you drop it in gear it will drop down to 6-7 if so your power valve needs to be lower.
 
PA Dodger said:
Trailbeast[/U I know the car was raced before so maybe I need to check the main jets and lower them a number or two. Opening the secondaries a bit helped the problem but I don't want to do this if it screws something else up. I'm just tired of smelling like a tailpipe when I take the car out.
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Regarding the latter, at 9 psi, the fuel is blowin by the needle seats and guess where it's going, right into the oil and out the back. Did ya smell your dipstick for fuel smell? If so, once you correct the problem, you need to change the oil and filter a few times to remove the gas or you'll keep washin down the cylinders.

I went thru 2 Carter mechanical pumps due to the high pressure until I installed a regulator which stepped it down to 6psi. No mo eye burner.
 
Mark, I picked up a regulator and oil a few days ago. That's one of this week's projects. That's where I'm starting the troubleshooting. From there I'll check for vacuum leaks and recheck the vacuum at idle in gear. We'll see how it goes from there.
 
No one mentioned the mixture screws. 1 1/2 turns out is a starting point. Then adjust them to highest vacuum or rpm. Do this after setting timing. Set pressure to 6.5.
 
Updates-installed the fuel pressure regulator set to 6 psi. Changed the oil mainly because it was time for it but also to address any fuel washing issues. The idle mixture screws were set to 1 1/2 turns then tweaked for highest rpm. After all the tweaking I ended up almost at 1 1/2 turns anyway. Nothing helped yet. The car is off to the alignment shop so I'll try more tuning/troubleshooting when it gets back. First thing will be another vacuum check.
Just an observation-when I rebuilt the carb I pulled a 3.5 power valve out and put a 6.5 in. I thought the 3.5 was out of line, but maybe not? The car runs strong. There was a video of it running a 13.0 at a NY dragstrip.
 
PA Dodger said:
Updates-installed the fuel pressure regulator set to 6 psi. Changed the oil mainly because it was time for it but also to address any fuel washing issues. The idle mixture screws were set to 1 1/2 turns then tweaked for highest rpm. After all the tweaking I ended up almost at 1 1/2 turns anyway. Nothing helped yet. The car is off to the alignment shop so I'll try more tuning/troubleshooting when it gets back. First thing will be another vacuum check.
Just an observation-when I rebuilt the carb I pulled a 3.5 power valve out and put a 6.5 in. I thought the 3.5 was out of line, but maybe not? The car runs strong. There was a video of it running a 13.0 at a NY dragstrip.
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Hum u pulled the power valve you know if you drop it in gear and vacume falls to below 6.5 its going to idel rich.
 
Snake said:
Hum u pulled the power valve you know if you drop it in gear and vacume falls to below 6.5 its going to idel rich.
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Snake, I have read many differing opinions on this subject. I have checked many different sources and have come up with - the power valve has no affect on the idle circuit. I'm all ears if you have an opinion you want to share. Once I get the vacuum checked correctly I will choose the power valve for off idle performance.

From a different thread.
doktor_x said:
The power valve shouldn't be doing anything at idle. This is the reason for turning the idle screws all the way in to check if it's blown. If it's intact, closing the fuel path across the idle screws cuts off the fuel source and if it keeps running, the fuel has to be coming from somewhere else. Its purpose is to add fuel when the engine is under a load (low vacuum).
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I'm kinda wondering now whether it's a rich carb or weak ignition. Timing is right at 20* but the spark isn't awe inspiring.
TrailBeast- I read someone post on one of your threads that your ignition setup cleaned up their engine. Have you seen this to be the case?
 
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