Running lean on light throttle

[Steven Munoz]

So I’ve done a bunch of research I have a 350 and a holly quick fuel 600 vacuum secondaries carburetor right off of idle my AFR gauge Will go 15-16-17 to 1 with only light throttle if I jam on it it leans out for a split second which makes me think it is the Excelerator pump squirter but I’m like throttle it doesn’t make too much sense I have messed around with the idle feed restrictors they are currently at 43 size and if I’m already going and hit the throttle it does not lean out. So I thought it was the Jets part most the time it’s running at 12 1/2 13 to 1 when I’m on the highway or driving around town it’s just when I move the pedal under light acceleration it leans out! Any ideas? Please anything helps!

 

[75gmck25]

Since these are vacuum secondaries, I’ll go out on a limb and suggest that your secondaries are coming in too quickly, and I know that is adjustable on your carburetor.

Bruce

 

[Backfoot100]

Us old farts called it an off idle stumble. You young guys and your fancy gauges don't know what that means....LOL.
You're on the right track with the Accelerator pump. It isn't the nozzles though. Its the adjustment of the pump arm to the pump diaphragm.
By the book, the adjustment says that there should be .015" gap between the pump arm and the diaphragm at full throttle or something like that. I found out a long time ago that's just plain horse ****. Adjust that pump arm so there is no slack in that pump linkage sitting at rest. Just take any slop out of it but no more than that. Even the slightest throttle movement should engage the diaphragm and give a fuel shot to the nozzles.

It also helps to adjust the idle mixture as follows....
Most people adjust the idle mixture screws in neutral to the highest vacuum (best way) or the highest RPM. That can work but the best way is make sure its warm first off. Then adjust to the highest vacuum (best way) or highest RPM while in gear (loaded).
Setting the idle mixture in neutral doesn't take into account the drop in RPM being loaded in gear so you set the idle mixture under the same conditions. Just make sure you have a friend that can keep the brakes on or you better have the wheels well chocked with the emergency brake on. And whatever you do, don't rev it up or you could go for ride. Just setting the idle mixture and nothing else.

I'm about 99% sure that'll fix your problem.

 

[Steven Munoz]

Yes I’m aware of all of that and it is done correctly. I appreciate the help with that. I’m fairly sure it’s not the Excelerator pump system because if I give it a pretty heavy pedal it does not have any hesitations or leanness Because it around 13 five but just trying to leave a stoplight in traffic it goes real lean I believe it’s part of the transition circuit

Us old farts called it an off idle stumble. You young guys and your fancy gauges don't know what that means....LOL.
You're on the right track with the Accelerator pump. It isn't the nozzles though. Its the adjustment of the pump arm to the pump diaphragm.
By the book, the adjustment says that there should be .015" gap between the pump arm and the diaphragm at full throttle or something like that. I found out a long time ago that's just plain horse ****. Adjust that pump arm so there is no slack in that pump linkage sitting at rest. Just take any slop out of it but no more than that. Even the slightest throttle movement should engage the diaphragm and give a fuel shot to the nozzles.

It also helps to adjust the idle mixture as follows....
Most people adjust the idle mixture screws in neutral to the highest vacuum (best way) or the highest RPM. That can work but the best way is make sure its warm first off. Then adjust to the highest vacuum (best way) or highest RPM while in gear (loaded).
Setting the idle mixture in neutral doesn't take into account the drop in RPM being loaded in gear so you set the idle mixture under the same conditions. Just make sure you have a friend that can keep the brakes on or you better have the wheels well chocked with the emergency brake on. And whatever you do, don't rev it up or you could go for ride. Just setting the idle mixture and nothing else.

I'm about 99% sure that'll fix your problem.

 

[Steven Munoz]

Since these are vacuum secondaries, I’ll go out on a limb and suggest that your secondaries are coming in too quickly, and I know that is adjustable on your carburetor.

Bruce

Vacuum secondaries opening when I’m at like 10% throttle you think?

 

[Steven Munoz]

Us old farts called it an off idle stumble. You young guys and your fancy gauges don't know what that means....LOL.
You're on the right track with the Accelerator pump. It isn't the nozzles though. Its the adjustment of the pump arm to the pump diaphragm.
By the book, the adjustment says that there should be .015" gap between the pump arm and the diaphragm at full throttle or something like that. I found out a long time ago that's just plain horse ****. Adjust that pump arm so there is no slack in that pump linkage sitting at rest. Just take any slop out of it but no more than that. Even the slightest throttle movement should engage the diaphragm and give a fuel shot to the nozzles.

It also helps to adjust the idle mixture as follows....
Most people adjust the idle mixture screws in neutral to the highest vacuum (best way) or the highest RPM. That can work but the best way is make sure its warm first off. Then adjust to the highest vacuum (best way) or highest RPM while in gear (loaded).
Setting the idle mixture in neutral doesn't take into account the drop in RPM being loaded in gear so you set the idle mixture under the same conditions. Just make sure you have a friend that can keep the brakes on or you better have the wheels well chocked with the emergency brake on. And whatever you do, don't rev it up or you could go for ride. Just setting the idle mixture and nothing else.

I'm about 99% sure that'll fix your problem.

It also doesn’t help that I have a 273 rear axle ratio in combination with the weight of the truck and what not makes it more complicated than if it was a super torquey gear

 

[75gmck25]

I do not have experience with tuning a Holley like yours, so I may be totally wrong.

Transition from idle to primary - When the primary throttle plates are full closed, the transfer slot in the bore should look like a square (equal on all sides). However, if you start out by adjusting idle mixture and get it way off, and then try to fix the idle with the idle speed screws, the plates may be too far closed or open. If the plates are off at idle, it will screw up the transition from idle to primary jets and you get a part throttle bog.

Transition from primary to secondary - On a Quadrajet the secondary throttle plates move mechanically because they are connected to the linkage, but the secondary air doors on the top are pulled open by vacuum, and those flaps determine the secondary side air flow. If the air door spring is set too "loose" it will allow air to start flowing too quickly and it will lean out the carburetor as secondary comes in. I think this secondary transition point is easy to adjust on your carburetor.


Bruce

 

[Steven Munoz]

I had completely forgotten about the transition stop being a square I’ll have to look at that I know I did in the past but I cannot remember the results! I’ll have to pull off the carburetor on my next day off and take a peek

I do not have experience with tuning a Holley like yours, so I may be totally wrong.

Transition from idle to primary - When the primary throttle plates are full closed, the transfer slot in the bore should look like a square (equal on all sides). However, if you start out by adjusting idle mixture and get it way off, and then try to fix the idle with the idle speed screws, the plates may be too far closed or open. If the plates are off at idle, it will screw up the transition from idle to primary jets and you get a part throttle bog.

Transition from primary to secondary - On a Quadrajet the secondary throttle plates move mechanically because they are connected to the linkage, but the secondary air doors on the top are pulled open by vacuum, and those flaps determine the secondary side air flow. If the air door spring is set too "loose" it will allow air to start flowing too quickly and it will lean out the carburetor as secondary comes in. I think this secondary transition point is easy to adjust on your carburetor.


Bruce

 

[Ricko1966]

I like the transitions absolutely as shut as I can get them, if you are pulling fuel out of the transition circuit at idle you don't have the fuel you need when you open the transition ports. Is your distributor running on manifold vacuum, not ported vacuum.If not try running manifold vacuum, tgen reset your idle. A whole lot of carburetor problems are actually ignition problems.

 

[Steven Munoz]

I like the transitions absolutely as shut as I can get them, if you are pulling fuel out of the transition circuit at idle you don't have the fuel you need when you open the transition ports. Is your distributor running on manifold vacuum, not ported vacuum.If not try running manifold vacuum, tgen reset your idle. A whole lot of carburetor problems are actually ignition problems.

So my engine wants 22° of timing at idle it won’t really run too well anything underneath that and with mechanical advance it’s already at 36° so I am not running a vacuum advance

 

[Ricko1966]

There's your problem.Set the distributor up right first then work on the carb.How much centrifugal is built into your distributor?

 

[Steven Munoz]

There's your problem.Set the distributor up right first then work on the carb.How much centrifugal is built into your distributor?

It’s not unheard of to run at 22° distributor is set up correctly I’ve checked and double checked that more times than I’d like to admit

 

[Ricko1966]

So how much centrifugal is in that distributor? Only 14 degrees centrifugal?. I disagree very strongly that your distributor is set up correctly, but it's your truck.

 

[Steven Munoz]

So how much centrifugal is in that distributor? Only 14 degrees centrifugal?. I disagree very strongly that your distributor is set up correctly, but it's your truck.

I appreciate you’re help but I’ve triple checked everything and just had the dizzy out 20 min ago to double check and it just doesn’t like 14 ish degrees it is currently sitting at 20-21at idle. And yeah it sits about 34-36 total

 

[Ricko1966]

The 14 degrees I mentioned is centriqual not initial. If you are running 34 total and 21 initial now things just got even worse because it's only got 13 in the dizzy.