Vacuum secondary spring question

[Texasdoc]

I'm tuning the secondaries on my Holley 670. Right now I can feel when the secondaries open. It doesn't bog or hesitate, it just pulls harder. The Holley instructions state to use a lighter spring until you feel it open, then go back one. Since these cars are so light, and since I don't have a bog, do I need to go to a stiffer spring? Or just leave it in place? Some day I can try to record 15-60mph times and compare, but just wondering.

[jhirasak]

Texasdoc,

For Holly carbs with vacuum secondaries, you want the lightest possible spring that does not cause a bog. That is why they recommend that you keep replacing the spring with progressively lighter ones until you actually can feel it slightly bog as the secondaries open. The slight bog indicates that the secondaries opened slightly too early for optimum performance. By replacing that spring with the previously stronger spring, you maximize the performance of the carburetor over its full operating range. Holly spring kits are readily available and this is an easy task to accomplish.

[RET_COP]

Good post jhirasak, light cars like ours are very forgiving and will tolerate quick opening sec. as well as faster timing curves. Tune both together and let'er rip

[Texasdoc]

Right, I have the spring kit and have been doing that. The instructions say that you shouldn't feel the opening of the secondaries. If you feel it, you will get a bog and should back off to a stiffer spring.

I don't get a bog, but I also feel it when they open. Which is better - feel the opening without the bog, or don't feel them open.

I realize it should be measured with a stop watch to see which one is actually fastest - I just don't have the time (or police free straightaway) to compare now.

I'm going to the Friday night drags at Texas Motor Speedway tomorrow night - if it doesn't rain. I want to get it going as fast as possible. I just want to beat CoralCobra who is going too . I won't have time to compare different springs, so I have to go by concepts. Holley says that the car is faster if you can't feel them open.

[patrickt]

Originally Posted by Texasdoc

Holley says that the car is faster if you can't feel them open.

I believe this to be true. I have an adjustable vacuum secondary on my Holley (see this thread: Theoretical Vacuum Secondary Question ) and my "informal street testing" would seem to back it up. You do have a zip tie on the shaft that shows you how far they're opening, right?

[jhirasak]

You should not feel a bog or a change in the rate of acceleration if the correct spring is used. If you feel a bog, the spring is too soft and allows the secondaries to open too early. If you feel a change in acceleration when the secondaries open, the spring is too stiff and the secondaries need to open sooner. If there is no spring available between these two conditions, I would elect to use the stronger spring and avoid a bog during acceleration.

[Texasdoc]

I don't have the paperclip. I may try that tomorrow to see how far they are opening. If they are not opening all the way, i'll use a lighter spring. I will try a heavier one too just to see.

[patrickt]

Originally Posted by Texasdoc

I don't have the paperclip. I may try that tomorrow to see how far they are opening. If they ate not opening all the way, i'll use a lighter spring. I will try a heavier one just to see.

A zip tie cinched down on the secondary shaft, on the opposite side of the vacuum secondary, that is snipped off leaving about an inch of a stem, is a wonderful indicator. The shaft rotates within the zip tie when the secondaries open, and then the zip tie stub "points" to how far the secondaries opened when the shaft rotates closed. Do a quick search on these forums for more info. I'm sure I've posted pics of mine many times before. I just keep the zip tie on there all the time and have it rotated out of the way when not testing.

[ERA Chas]

Originally Posted by patrickt

I just keep the zip tie on there all the time and have it rotated out of the way when not testing.

Testing for WHAT??
Beige paint, two roll bars covered with insulation on hundred year-old Yokomas---testing for low ET to the laundromat???

[patrickt]

Originally Posted by ERA Chas

Testing for WHAT??
Beige paint, two roll bars covered with insulation on hundred year-old Yokomas---testing for low ET to the laundromat???

Yes, I periodically push the zip tie over to measure the secondary opening, not on full WOT, but rather on partial acceleration. The secondaries should open partially even on a spirited trip to the oyster bar -- and I am sensitive to even the slightest change in the performance of engine. Yes, Chas. -- even the slightest.

[ERA Chas]

Originally Posted by patrickt

-- and I am sensitive to even the slightest change in the performance of engine. Yes, Chas. -- even the slightest.

And well you should be with only 200 asthmatic horses on hand...

[patrickt]

Originally Posted by ERA Chas

And well you should be with only 200 asthmatic horses on hand...

Ahhh, but I have two hands.

[Rick Parker]

Testing for WHAT??
Beige paint, two roll bars covered with insulation on hundred year-old Yokomas---testing for low ET to the laundromat???

That there is some funny s*it.
I use that stuff on my water pipes in the Winter

[DanEC]

I'm still waiting for a picture of the zip tie on the shaft. I know Patrick has one. I may try the zip tie trick myself.

[patrickt]

The first picture shows the zip tie in the normal setting when the vacuum secondaries are fully closed. The shaft will rotate within the zip tie itself as the secondaries open. Then, when the secondaries close back up, the zip tie will remain in the same position on the shaft that it held while in the fully open position. How far the secondaries are opening is then readily apparent. The second pic shows how the zip tie appears when you get back home after the secondaries have opened completely during your "test drive." Note: I took these photos with the zip tie tail "extra long" so it would show up better on the photos. You trim the end of the zip tie so it is as short as possible (like maybe an inch or so -- just enough to touch the idle screw).

[ERA Chas]

And this is what got you the pole at LeMans...?

[patrickt]

Originally Posted by ERA Chas

And this is what got you the pole at LeMans...?

Yep. And, I generally keep the zip tie completely rotated clockwise, so it's out of the way and doing nothing. But, from time to time, I'll just push it over against the idle screw, take the car for a normal drive, and then observe the position of the zip tie upon my return. Vac secondaries that are properly configured should open up somewhat even under "I'm just headed to the grocery store" driving conditions.

[ERA Chas]

Better you get on an empty stretch (or Englishtown) and hold redline in 3rd and 4th for 60 seconds (or until your Cole Haans are melting). Load is what tells you how much secondary you used.

[patrickt]

Originally Posted by ERA Chas

Better you get on an empty stretch (or Englishtown) and hold redline in 3rd and 4th for 60 seconds (or until your Cole Haans are melting). Load is what tells you how much secondary you used.

When I was checking the vac secondary operations after installing the adjustable housing outlined in this thread: Theoretical Vacuum Secondary Question I had to do something similar in order to get the adjustments perfect. That is not for the faint of heart, it is not particularly safe, and, frankly, I am not Stirling Moss when it comes to "at the limit" driving skills. One slip and my CC post count would have been carved in stone.

[ERA Chas]

Originally Posted by patrickt

One slip and my CC post count would have been carved in stone.

So what's the bad part?