Secondary Stumble

yellow rose said
It may be a power valve issue, and it can be explained that both carbs are lean at that spot of the fuel curve.

FWIW, if you set the power valve opening by idle vacuum it will be wrong.

You need to set your power valve opening by cruise vacuum. For example if you cruise at 16 inches of vacuum, you’ll use the 10.5 power valve, because that is all Holley now offers. Really, at 16 inches it should be 12.5-13.5 but you can get them.

The “Holley” method of using idle vacuum was wrong from the beginning, and it’s still wrong.

12many has an excellently thread where he documents tuning his power valve as outlined above. It is worth looking for that thread and reading it.

I agree!!! I actually hooked up a vacuum gauge to the engine so I could see the vacuum drop while I was driving around. I made numerous runs at different speeds while cruising, then mash on the gas. When I finally got the right power valve the engine responded so nice. I had been fighting that stumble for almost a year!!!

PS - the accelerator pump also gives the engine a shot of gas at lower cruise speeds. I messed with pump cams quite a bit trying to get rid of the stumble. The accelerator pump will help some with the stumble but the power valve really gives the engine an immediate boost when you need it.