I've noticed on my stock L79 that I feel a bit of a late kick of the vacuum secondaries and I think I've figured out why but I need help correcting it.

The L79 runs a 4150/4160 Holley hyrid (it's actually a 4160 with a modified 4150 rear metering block with jets instead of a metering plate). If I recall correctly it has the middle of the road plain spring in the vacuum secondary can. The engine is completely bone stock and in the past the secondaries opened seamlessly and the car simply accelerated hard from a slow roll to and past redline.

More recently, however, the car will accelerate relatively well, then really provide a kick in the pants about about 4500 or so. I can actually feel the secondaries open (or at least I assume this is what I'm feeling). It doesn't really bog, it just really provides a stronger surge at the top end.

It occurred to me the other night that the vacuum secondary action is controlled by vacuum at the top of the car and that the vacuum measured there is to some extent affected by the air cleaner. A while back I installed an K&N filter so I'm probably getting less of a vacuum signal at the top of the carb which would be causing the secondaries to open slightly later than they used to with a paper filter.

Does this make some sense? If this is the case, I would want to move to a slightly SOFTER spring wouldn't I so that I'd get the secondaries open with less of a vacuum signal to get it back to roughly the same opening point as I would have had earlier with a paper filter, right?

Of course a real easy test of this theory would be to reinstall a paper filter and see how it accelerates and I do plan to try that. I'm just trying to determine if my line of thought is correct.

About the same time I recurved the distributor to have 34-36 degrees in all by 3000 so that may be a contributing factor but I don't think that would have much affect on my 4500 and up surge. Prior to the recurve, the acceleration curve was more linear but not nearly as quick.

Rich,I would try either of the yellow springs.

I've always heard that if you can feel the secondaries "kick in" that the spring is too soft and the secondaries are opening too early. If adding a lighter spring does not improve things, you might try a stiffer one to make sure you are not feeling a dead spot

I have heard that sometimes you need to fatten up your jetting when going to a K&N do to the better flow.

something to think about anyway.

Rich

makes sense to me, the passages for the vacuum secondaries are actually in the venturies themselves. so any change in velocity or airflow through the venturies would effect the secondary opening point.

what size is that carb?

im thinking its 600cfm or less, and if so that 327 should be able to use every bit of cfm it has. it sounds like a purple or tall yellow spring would be a better match for your combo.

Holley says that if you can feel the secondaries open, the spring it too light. It should be a smooth transition from primaries to WOT.

I would think that a K&N filter would give you a stronger wacuum signal if anything. The vacuum is generated by air moving through the carb. An example would be like blowing over the end of a straw and watching the fluid get pulled up into it. More air, would provide more vacuum. If this is the case, you should try a stiffer spring to counter act the increased vacuum signal.

Okay, I'm glad I asked because it sounds like my though process was going the wrong way perhaps.

The carb is an oddball at 585cfm. It never seems to run out of breath even up to 7000rpms.

I found an interesting quote from the instruction sheet that came with my spring kit:

"An installation with an open element air cleaner will require a weaker spring than one with a restrictive snorkle type air cleaner."

This makes sense since the vacuum signal used by the secondary system is BETWEEN the air filter element and the top of the venturi (not at the base of the venturi where true engine vacuum is present). If the filter has little to no restriction, the vacuum signal at the top of the carb is weaker than when the filter has a greater amount of restriction.

So I'm back to my original belief that a slightly softer spring will get my vacuum secondary timing back to where it should be while using a less restrictive K&N filter.

We'll deal with jetting changes another day....