Vacuum secondaries spring

[FredG]

You learn something new with these cars everyday. A little while back I put a new carb on. I replaced the 870 with a new 870 Holley. I kind of suspected the previous cab had been monkeyed with a lot and when I opened it up my suspicions were correct. The new carburetor ran great except for one difference. The secondaries on the new one opened gradually where on the old one they opened much quicker and boy could you feel the difference. It pinned you to the seat. Turns out that someone had replaced the spring in the vacuum secondaries with a lighter one. A little research showed that lighter cars should have lighter springs. What a difference in the acceleration. Here is a picture of the springs. If you have the original vacuum advance spring in your car you might want to try a lighter one and boy will you feel a difference when you accelerate. The lighter spring is on the right and it's smaller.

Fred

[patrickt]

Or, you install an adjustable vacuum secondary housing, then you can get it just perfect with the turn of a screw.

[FredG]

The adjustable was going to be my next option after I see how much of a change the spring made. What did you use as the determining point for the opening. Best info I could find was right before the engine bogged when you punch it.

Fred

[Grubby]

On a car as light as a Cobra, I suspect the secondaries weren't opening all the way. They may not have been opening at all with the as built spring in place.

John

[patrickt]

Originally Posted by FredG

The adjustable was going to be my next option after I see how much of a change the spring made. What did you use as the determining point for the opening. Best info I could find was right before the engine bogged when you punch it.

Fred

Yep, you open it up so the secondaries "flop" open, and cause a bog, and then you close off the screw a bit, test it out, close it off some more, test it out, close it off some more, test it out, the bog is now gone, so, open it up, test it out, either leave it where it is or ease it back a hair and you're done. Now, if you change your air cleaner to a low restriction type, you must change your setting or you will lose power noticeably.

[Gaz64]

Lighter cars "can benefit" from a lighter spring. There is no rule that says "should have" a lighter spring.

There are many variables involved.

Vehicle weight, gearing, engine capacity, carb size.

An 870 on a 482 needs to spin 7200, before it flows 870 at 1.5 inches, assuming 85% VE.

Secondary spring free lengths are not all of the answer when comparing springs. Coil count, wire diameter, compressed length when fitted, spring rate etc.

Go lighter on the spring until full throttle acceleration is felt as a bog, then come back one. Or go to the lightest spring, (white) then come up until the bog disappears.
The QFT adjustable secondary is an easier option.



Gary

[FredG]

Based upon all I have experienced with the different spring rates on the vacuum secondaries, I am guessing there are a lot of owners out there with carbureted cars and vacuum secondaries that have yet to experience the true violent acceleration these cars can produce. The new carb with the heavier spring brought a tameness to the car. The older one with the lighter spring brought the acceleration to a get your attention and you better be careful level.

Fred

Fred

[Gaz64]

That could all be correct, depending on the colours of the two springs.

If one is black, and the other plain(silver), one carb is still factory stock, one is jetted on a dyno, A/F logged through out the session etc.

A friend of mine, and my car were on the same dyno, same day. Operator did not believe they could be so different. I said his IS stock, mine looks stock.

I run a 850 Annular DP (custom carb) on my baby engine. No Vacuum secondaries for me.

[FredG]

I have a brand new polished and blue 870 with vacuum secondaries so I will have to live with it. I did go with the QFT adjustable vacuum secondary. Put it on yesterday and it's a definite improvement right out of the box at the default setting over the Holley springs. Very smooth secondary fuel delivery. I will adjust it some for the performance I like.



Fred