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06-18-2011, 04:33 PM
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CC Member
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Join Date: Nov 2007
Location: Clayton,
IN
Cobra Make, Engine: ERA 838
Posts: 1,122
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Not Ranked
750 vac secondaries partially opening
I was doing some fine tuning on my Cobra and suspected the secondaries on my Holley 750 vac secondary were not opening. I put a zip tie on the vac pod rod and observed that the secondaries are only opening about 25%.
I ordered a set of springs last night. Not sure which spring is in the carb now. The engine was dyno tuned by the builder.
Would you go 2 spring sizes softer or just 1? I have a turkey pan and it is a pain to change springs and would like to get it on the first guess.
John
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06-18-2011, 04:42 PM
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CC Member
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Join Date: Jan 2008
Location: Williamsport,
PA
Cobra Make, Engine: Kellison Stallion 468 FE
Posts: 2,703
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Not Ranked
you observed this how?
you will not see secondaries opening with the car in neutral and whacking the throttle.
full travel will only be happening with the load on the engine. strap your best friend to the intake and go for a spin....
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Fred B
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06-18-2011, 04:53 PM
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Half-Ass Member
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Join Date: Jun 2005
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 22,004
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Not Ranked
Quick Change Kit
Yep, you have to kind of scare yourself with the secondary opening check.  Did you know that Holley has a Quick Change Secondary Kit to make it easier on you? Holley Performance Products Quickchange Vacuum Secondary Housing Cover*20-59
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06-18-2011, 05:24 PM
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CC Member
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Join Date: Nov 2007
Location: Clayton,
IN
Cobra Make, Engine: ERA 838
Posts: 1,122
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I put a zip tie on the vac secondary shaft that pulls them open. It was loose and slipped down the shaft as they opened. I took the car on the road and did a few full throttle passes.
Performed the test twice. The vac pot only pulls in about 25%
Vac secondaries are not supposed to open when free reving a engine.
John
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06-18-2011, 09:07 PM
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CC Member
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Join Date: May 2006
Location: Portland,
OR
Cobra Make, Engine: ERA FIA, 1964 289->Webers
Posts: 3,689
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If the dyno tuner knows what he's doing it should all be in good shape. I'd go a step at a time with the spring if that's the route you wish to go. Is the spring stock size for the carb? If it is, it may be telling you the carb is a bit big.
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ERA FIA 2088
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06-18-2011, 09:07 PM
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CC Member
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Join Date: May 2008
Location: Brisbane,
QLD
Cobra Make, Engine:
Posts: 2,797
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Quote:
Originally Posted by Grubby
I was doing some fine tuning on my Cobra and suspected the secondaries on my Holley 750 vac secondary were not opening. I put a zip tie on the vac pod rod and observed that the secondaries are only opening about 25%.
I ordered a set of springs last night. Not sure which spring is in the carb now. The engine was dyno tuned by the builder.
Would you go 2 spring sizes softer or just 1? I have a turkey pan and it is a pain to change springs and would like to get it on the first guess.
John
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A bit like a jetting change, go two steps first, could be correct first go.
Then if necessary, come back one.
You could even find another two steps lighter is necessary, especially if you have a brown or black spring in there now.
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Gary
Gold Certified Holden Technician
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06-19-2011, 08:12 AM
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CC Member
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Join Date: Jun 2008
Location: Chester Springs,
PA
Cobra Make, Engine: Kirkham 289 FIA #690, FRPP 427 Boss engine
Posts: 764
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Not Ranked
I agree on the two steps, but be prepared as you may need to back off to one. Just the fun of carb tuning. Good luck.
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RCR GT40 SOLD to Fast 5
Kirkham #690 289 FIA
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06-21-2011, 05:11 PM
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CC Member
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Join Date: Nov 2007
Location: Clayton,
IN
Cobra Make, Engine: ERA 838
Posts: 1,122
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My carb had a black spring (the stiffest). No wonder the secondaries were not opening. I just installed a purple spring and will test drive when the rain stops. I opted to go three sizes.
Next report to follow in a few days.
John
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06-21-2011, 05:14 PM
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CC Member
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Join Date: May 2006
Location: Portland,
OR
Cobra Make, Engine: ERA FIA, 1964 289->Webers
Posts: 3,689
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That would explain it. Good luck!
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ERA FIA 2088
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06-21-2011, 07:15 PM
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CC Member
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Join Date: Apr 2007
Cobra Make, Engine: # 757 ERA 427 SC , 482 Al. big block
Posts: 896
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Grubby ... FWIW , a general rule is : lite car , lite spring ... heavy car , heavier spring . I have the kit on my 427 Galaxie that Patrick talked about and it makes dialing in the secondary opening a snap ... and I have dual quads on the Galaxie . Another rule of thumb ... if it feels like someone threw a switch when the secondaries open , then the spring is too lite . When they open , it should be a smooth transition .
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06-25-2011, 10:16 AM
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CC Member
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Join Date: Nov 2007
Location: Clayton,
IN
Cobra Make, Engine: ERA 838
Posts: 1,122
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The purple spring is right. The secondaries fully open with no hesitation. I can really feel them come in between 3500 and 4000rpm.
I have attached pictures to show how I could confirm the secondaries fully open. There is a black zip tie around the secondary butter fly shaft. The first two pictures show it before it is driven and the last shows the zip tie slipped on the shaft to show they fully opened.
With the black spring the shaft turned about 5 degrees.
John    |
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