Vacuum pulse near air intake troubleshooting

[Fairtax4me]

Originally Posted by Ryan G

anything on inside of air filter should be pulling vacuum. Atmosphere is on outside of filter.

The air filter should never make that big of a difference. If there is an excessive pressure drop across the filter it’s either too small or it’s clogged and needs to be replaced. A very small pressure drop across the filter is normal when the engine is at WOT at full operating Speed (Approx 3,500rpm in this case), but it’s should be hardly enough to make a difference. If you were to compare the difference inside to out, it should ideally be less than 0.5” mercury.

Kinda splitting hairs on that small of a change to call that “vacuum”.
In actuality we’re not dealing with vacuum at all. Our perception of vacuum only comes from the change in air velocity that creates a pressure difference. That difference is greatest behind the throttle plate when it is closed.
When the plate opens, that difference gradually decreases as the plate opens farther.
Once the plate is open all the way there is little to no pressure difference across the plate itself, but there is a ridge in the carburetor throat that creates a pressure drop right before the main jet, which is what creates the lower pressure that pulls fuel through the jet.