Holley Double pumper 750 cfm
Mechanical secondaries
396 stroker SBC
Performer air gap intake

I am finding when I rev to 3000rpm tuning in neutral that my air fuel ratio can fall as low as 11.0

My question and confusion is about the accelerator pump jet vs. primary jet. Which one if these do I change to lean out my AFR under acceleration? If the accelerator pump jet, what exactly does the primary jet do and when does it come in to play?? Or.. should I not remove fuel, but instead somehow add air?

Also, how does the accelerator pump cam come in to play?

Thanks!

 

I'm no expert on this but I fail to see how you can get accurate readings without a load on the engine.
You also haven't taken into account the role the power valves play which will pop open under acceleration as the engine vacuum drops.
So what jets are you using primary and secondary?
What power valves are you using?
What accelerator pump cams are you using and which hole 1 or 2?
Have you adjusted the existing accelerator pump cams so with the engine off and the throttle held wide open you can insert a .020 feeler gauge between the pump arm and the pump lever screw head?

 

Try your reading at cruise speed

how does it run in general? Dont get too hung up on perfect AFRs with a carb put it where the engines happiest. EFI slightly different

If you cam has a lot of overlap or watching ratios when circuits are transitioning that gauge can have you thinking things are off

Good way to see where youre at though.

 

At 3000 rpm in neutral the acc pump is doing nothing, so it's not a factor.
At 3000 rpm in neutral the vacuum is high, so the power valve should be closed and therefore not a factor.
At 3000 rpm in neutral, you are barely running off the main jets. Most likely still running partly off the transfer slots and the main jets.

As MelWff says, 3000 in neutral is pretty much a useless reading. You need a load on the engine.

What size primary jets are you using ?
What is the list number for your carb ? R4779 ??

 

To do this easily you need a data logger to graph your AF ratio.


But here is the method I used to tune my carb with AFR Tools.

I have 1 O2 sensor in each bank. (2-4-6-8) (1-3-5-7) Cylinders.
The following Test Runs were used.

1. 2nd gear starting @ 1200 RPM through to 5500 RPM (WOT)
2. 3rd gear starting @ 1800 RPM through to 5500 RPM (WOT)
3. 4th gear starting @ 2500 RPM through to 5000 RPM (WOT)

Steady state driving 3rd, 4th , 5th @ 2300 RPM

For steady state driving I adjusted the primary jets.
For WOT I adjusted the secondary jets.

For acceleration changes I adjusted the acceleration pump squirter's

For Power Value I adjusted based on vacuum readings @ 1000 RPM.

 

http://www.burtonmachine.com/tech

To get a nice smooth fuel injection quality fuel curve, you’re likely going to have to modify some passages in the carb. Make a chart of idle, 1000rpm, 1200, 1500, 1800, 2000, 2200, 2500. With the engine warm, run the car at those speeds in neutral, just sitting in the driveway. At each of those speeds, carefully hold the throttle steady, and watch the wideband. Throttle movement will cause the readings to jump, so take your time, and let it find a steady number. Don’t just jot down the first steady number you see. Wall wetting from accelerator pump, leanout from slowly throttling down, etc can take a good 20 seconds or more to “clear out”.

BTW 11.0 is rich.

I used the above to dial on a nice 13.8 though transition and a cruse AFR of 14.5

 

Thanks everyone. I'm still trying hard to learn the operation and tuning of carbs. They didn't exist on cars when I started driving, so it's all new to me. There's definitely a LOT going on in there!

This is my carb:
https://www.holley.com/products/fuel...arts/0-76750BK

Everything is stock. I'm not sure what size jets it came with yet, but I haven't changed anything.

I've been reading all I can and watching videos (I actually read the link above just by coincidence before) and even so, did not know that at 3000rpm you are not operating off of the primary jets. I also did not know that operating with load vs neutral would make any difference. I still don't entirely understand why that is so, but it is great information to have.

All I can say for sure is that the car runs MUCH better when there is more air coming in. If I create a small vacuum leak with a pin-hole in a vacuum port cover, for example, the engine just runs perfect.

I wish there was a good speed shop here that I could go in to and get help and training, but all there is are dyno's for imports. There are no carb tuning shops here unless you take the carb off and send it out to them.

I've got to fix an issue with my distributor timing being out of phase today, then I'm back to the carb. I'm hoping correcting the phasing issue will resolve some of my issues.

My cam is not too tall, by the way. It's fairly standard, and I get up top 15" of vacuum on the gauge. I just can't get it to idle well without creating a small vacuum leak, no matter what I do to the idle screws and timing.

 

Carb tuning is a lost art. Those that know exactly how everything works are far and few between, with each year we lose a few. I know most of it but not all.

Do you have a dragstrip near you ? If so you might want to take it there and ask someone for help. Tell them you will pay them for there time but most guys won't take a cent and be glad to help you.

Hope you get it worked out

 

if you click on the link you provided and select numerical listing you will find your carburetor specifications
http://documents.holley.com/techlibr...al_listing.pdf

You have 72 primary and 80 secondary jets
you have 6.5 power valves.

Those 80 secondaries maybe on the rich side and you may find out that 78 or 76 work better.

 

Do the rear blades have a screw to adjust how closed they are?

That is how I set my idle.