Carb Experts: What spread is needed on jets?

[67GTCOUPE]

Just wondering what spread is required or suggested on a 750 holley vac secondary?

It came with 6 sizes apart ie? 72/78.

Can I run 70/74 or should I use 70/76???????

Thanks

 

[Mach1steve]

I used similar

On a built 351C I used front 70's front & back 80's on a 650 Double Pumper Holley.
You have to play with it.
The fronts are easy, keep droppin em until it runs like **** when you accelerate then go up 2 sizes.
The backs, you just keep goin until it bogs when they open, or black **** comes out under WOT, then back off 2 sizes.
thats my rule of thumb anyway.
I';m sure there's more scientific methods, but it's worked super for me for 25 years.
Mach1steve

 

[dennis112]

I recommend only playing with one set at a time, starting with the front.

Similar to Steve, I have a 750dp and my jet spread is 72/82.

Not sure what you need in the rear for a vac secondary, but it sounds like the 70 should get you close in the front.

 

[67GTCOUPE]

Thanks for the info.

I am going to try 70/76 combo and see how that does.

 

[10secgoal]

Always keep the same it came with until you start drilling holes in the metering block. Until then, you don't want to change it.

 

[Mach1steve]

ooops ... I didn't notice

I didn't notice that you said a 750 VAC SEC....
thats different.
The secondaries use a metering block, and you have to buy the complete metering block I think to change the secondary jetting.
In would try going to a lighter spring in the secondary actuator, this will allow the secondaries to open a little sooner, which may work great for you.
Try a "Yellow" spring (they are color coded).

Mach1steve

 

[D.Hearne]

You can convert a metering plate secondary to metering block, but you need the block, gaskets, new longer fuel line to make the swap.