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Username Post: 383 very rich at idle        (Topic#351882)
LSS1333 
Poster
Posts: 29

Reg: 05-13-15
07-05-18 09:47 AM - Post#2739050    

Have a Quickfuel 670cfm carb on my 383. It's extremely rich at idle. Also, has a hesitation when aggressively giving it gas in neutral. Car is not drive able yet. Initial timing is at 10 degrees initial. Vacuum advance (full manifold) is adding about another 12 for a total idle advance of 22. Using a stop to limit advance, but more can be added, if needed. Total timing is around 32 degrees.

Engine starts fine, but runs rough. Idle mixture screws 1 1/4 turns from fully seated. Float level is half-way up sight glass.

Vacuum gauge is 99% steady and registering 13"Hg, but this is kinda low for cam (211/219). Should be in the 16-17"Hg range.

There is no spitting/backfiring, but I did see a fine "mist" of fuel come out of carb last time I turned the engine off. Also, engine almost acts like it wants to diesel for a spit second upon shut down

What should I do?

I'm thinking of turning idle mixture screws in to only 1/2 turn from seated and lowering float level, but everywhere I read says mixture screws should be 1 to 1 1/2 turn off seat.





Edited by LSS1333 on 07-05-18 09:53 AM. Reason for edit: No reason given.

 




grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17178
grumpyvette
Age: 70
Loc: FLORIDA USA
Reg: 03-16-01
07-05-18 10:11 AM - Post#2739055    
    In response to LSS1333

start with the basics, verify your float settings are correct and you have no more than about 5 psi at the carb inlet port,
fuel level should just barely show on the lower edge of the sight plug window, yeah, you can ignore the basics but the result is always problems


http://garage.grumpysperformance.com/index.php?thr...

http://garage.grumpysperformance.com/index.php?thr...

http://garage.grumpysperformance.com/index.php?thr...

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


Edited by grumpyvette on 07-05-18 10:18 AM. Reason for edit: No reason given.

 
Rick_L 
Honored Member
Posts: 27389
Rick_L
Loc: Katy, Tx, USA
Reg: 07-06-00
07-05-18 11:07 AM - Post#2739059    
    In response to LSS1333

Verify fuel pressure and float level as suggested.

With the vacuum gauge connected, turn the idle screws inward until the engine stumbles or vacuum decrease. Then start turning the idle screws outward until the highest vacuum is reached.

Some Holley carbs, especially more recent ones in my experience, can run with the screws all the way in and they are still rich.

There are a couple of other things you can do but start there and report back.



 
IgnitionMan 
Valued Contributor
Posts: 3369

Reg: 04-15-05
07-05-18 02:42 PM - Post#2739086    
    In response to LSS1333

How far are the primary idle transfer slots exposed at curb idle?

One of the reasons engines run rich at idle is, that if they want more air, which forces the throttle plates to be held farther open than they should be, the idle transfer slots are too far exposed, not the normal .020 inch, and the engine idles off the transfer slot fuel, fully rich.

A number of things can force the throttle plates to be too far open at idle.



 
Shepherd 
Frequent Contributor
Posts: 1342

Loc: Lake George, NY
Reg: 11-11-15
07-05-18 03:13 PM - Post#2739090    
    In response to IgnitionMan

Good info,common issue with Holley style carbs, especially with big cams. As recommended, I drill a .080 hole in the primary throttle plates to get the idle up some without uncovering the the transfer slots, sometimes even up to .125, some carbs you can play with cracking the secondaries slightly.



Edited by Shepherd on 07-05-18 03:17 PM. Reason for edit: No reason given.

 
IgnitionMan 
Valued Contributor
Posts: 3369

Reg: 04-15-05
07-05-18 04:52 PM - Post#2739098    
    In response to Shepherd

I really don't advise opening the secs to try to get more idle air. They need to literally only be cracked one eighth of a turn from actuatelly bound up closed. Just enough to stop them from binding.

I am a former Holley factory race carb tech.



 
Shepherd 
Frequent Contributor
Posts: 1342

Loc: Lake George, NY
Reg: 11-11-15
07-05-18 05:35 PM - Post#2739107    
    In response to IgnitionMan

Agreed, that is what I did on a highly modifified small block, big cam, big compression, just cracked the secondaries, couldn't get a clean idle otherwise, really wasn't a street car but the owner insisted driving it around town. Lol



 
Junkyardwarrior 
Contributor
Posts: 331

Loc: Yuma AZ
Reg: 02-25-15
07-06-18 08:26 AM - Post#2739177    
    In response to Shepherd

It's all true. Standard procedure is bunk. I have a Proform 750 CFM carb on my Nova with a 350 and it is only out maybe 1/8th of a turn on the idle screws. Any more and the thing is so rich you can't breathe in the car.

"8 cylinders of why the f&@% not" - me


 
IgnitionMan 
Valued Contributor
Posts: 3369

Reg: 04-15-05
07-07-18 10:51 AM - Post#2739277    
    In response to Shepherd

"just crack open the secs" ISn't what I was saying.

On a Holley, or any other carb, if there is no adjuster for the secs plates to hold them the slightest open from binding, they will do just that, BIND IN THEIR BORES.

On a Holley, I take the stop screw down to full slack on the stop, then, loosen the plate screws, align and actually LIGHTLY bind the plates in the bores, re-tighten the plate screws to the shaft, and then, bring the stop screw up to about 1/8th of a turn past the bind point, so the plates just do not bind in their throttle bores.

That is my ONLY reason to do that to the secs on a non-4 corner idle carb, no matter the make and type.

I do the same with every carb. I NEVER use the secs as an idle supplement, unless the carb is specifically built as a "4 corner idle" type.

It has to be very severe to need more air for any engine, but it does happen with very large cams. For them, and NO 4 corner idle in place, I do then drill the bypass holes in the primary plates, and set the secs up as I just described above, paying particular attention to the primary idle transfer slot exposures.



 
Shepherd 
Frequent Contributor
Posts: 1342

Loc: Lake George, NY
Reg: 11-11-15
07-07-18 04:58 PM - Post#2739308    
    In response to IgnitionMan

I was able to adjust the secondaries on a quick fuel carb,as I said just open a crack. This was a while back.so the details are foggy now. I do recall it was a four corner set up. But I see what you are saying.




Edited by Shepherd on 07-07-18 05:00 PM. Reason for edit: No reason given.

 
IgnitionMan 
Valued Contributor
Posts: 3369

Reg: 04-15-05
07-07-18 06:34 PM - Post#2739316    
    In response to Shepherd

Big difference between the 4 corner idle and all other carbs. It can get very confusing, and make for more trouble than a person can think of.



 




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