I have a 427 BBC with a 750 Holley / vac secondaries. I have been dropping the main jet sizes and recording it with a wide band O2 sensor. I noticed when dropping the last 2 jet sizes, the engine has developed a hesitation under hard acceleration. I can hear the engine rev up momentarily when the accelerator pumps squirt, then it hesitates, then it seems to catch. I think I need to increase the secondary plate sizes... Does that sound right or am I out in left field?

Thanks for your input in advance.

 

If it just started happening when you made your latest jet change it makes sense that the problem is connected to that. What's your wideband sensor telling you?

 

You leaned it out too much and that is the result. Record the run with the LM-1. A/F vs RPM, it should show it momentarily going lean. Why are you trying to jet down, better fuel mileage?

 

With the LM-1, I noticed that I had the best performance when AFR was in the 12.5-12.8 range at WOT. Some engines make more power with a little higher AFR ( 13.0-13.5 ).
Generally, the higher the compression ratio, the lower the AFR at WOT. With a turbocharger, supercharger or nitrous injection, 11.5 to 12.0 is acceptable.

You've got a wideband O2 sensor but you didn't tell us what it recorded...

 

Sorry for the delay, I had some things to take care of...

Here is the RPM / AFR of a typical acceleration with the current jetting:



It is a lean stumble, however I'm trying to determine what circuit needs attention. Thanks for your comments.

 

bump... anyone? Bueller?

 

calling all Holley experts...

 

What's your vacuum at idle and which power valve are you running?

 

vacuum at idle is ~14 and power valve is a 7.5 i think

 

What jets are you down to? Is the rear plate still stock?

You can extend the accelerator pump shot by varying the pump cam or its position. Try rotating the cam to the #2 hole and readjusting accelerator pump.

Might need to open the pump squirter a few .000's. Do you have any others to test with?

I'd work on the primary side only at this point unless you've been playing with the vacuum can too.

The PV is best tuned looking for some mild hills and see if it transitions well as vacuum drops without feeding it more throttle. It ought to seemlessly add fuel.

From what you're describing, you may be too lean on primaries some...and/or need more pump shot. You can tune light throttle cruise with the idle feed restrictions and you can tune WOT with the power valve channel restrictions. PVCR's. You can open them up to add more fuel...but 99% of the time this isn't necessary.

Tell us where you started and where you're at right now.

Auto or stick car?

What gears?

Cam?

 

with JIM

The accelerator pump can only supply fuel to the engine for a few seconds and your record shows a much longer lean peak, so I would bet on main jets
( primary, maybe secondary too, I'm not sure that the secondary opens at such a low RPM )...
But an important information doesn't appear in the diagram : Throttle Position Sensor. The optimum AFR depends on it, and of course the corresponding carb circuits too.
First make sure your accelerator pump works like it's supposed to. See Lars papers or Holley papers for adjustments.
Then, if you don't have a TPS, richen your primary main jets by 2 sizes and make a new record, at WOT.
From 1500 to 3500 RPM should be enough ( 2nd or third gear, engine at normal operating temperature ).
It takes a few seconds before the main jets / PV come in and all the extra fuel supplied by the accelerator pump is gone. Then and only then, the diagram shows the AFR of the main jets circuit.
If it helps but it's still lean, try to increase your jets by 2 sizes again. Don't try to compensate inadequate main jets with bigger squirters.
Try to keep the same difference of number of sizes between primary and secondary jets. Holley web site has all the information about the stock jets for every carb they sell.
If your main jets are more than 4-6 sizes bigger ( or smaller ) than the stock Holley jets, something else is likely wrong ( power valve not working or inadequate, modified carb plate ? )
So please, if you can, first post what primary and secondary jet sizes you have in and what sizes the stock jets were. Holley carb list is a good basis to start from.


If you still have a short initial lean peak after that, I mean only during the first second after you floor the gas pedal, a green or orange cam and/or a bigger squirter could help.