Optimizing a Holley 4175

[TylerW]

Hey guys:

A few years back I snagged a nice Holley 4175 out of a junkyard van and it's been floating around looking for a home since. Well now it will go on a mild 383 in a heavy vehicle.

For those who are not familiar with them, this is a 650 cfm spreadbore Holley intended as a Quadrajet/Thermoquad replacement. In it's baseline form it's probably calibrated lean since it was sold for use on emissions-equipped vehicles. 562 jets and a dual-stage power valve.

My eyes start crossing when you get into 6.5, 8.5, 100.5 etc etc power valves, idle feed restrictions and all that because I'm simply not used to tuning Holley's. I'd like to know if there's a proven jetting/PV combo that would be a better fit on that MILD 383.

This is not a carb I want to be taking apart all the time trying different combo's. A couple times maybe, but that's it. I know some of you will say there are better Holley's to use, but the engine already has a 400 spreadbore intake on it and I'm not changing it.

Thanks!

 

[hotstuff1]

Whats the list # on that carb , so we can look up the spec.s as a starting point

 

[TylerW]

7855...thank you.

 

[hotstuff1]

Holley 7855 is a 1975-76 440 eng replacement carb pri jets 62 , sec, jet plate 45 , pri p/v 8.5 , pri nozzle .028 ,sec spring plain . I would change the p/v to a 6.5 and try it . Hope this helps , let us know how you turn out .

 

[redfastback]

Put a vacuum gauge on it. What does it read in park and in drive?

 

[66durgederp]

I run the same carb on my 318, and ran it on my 360 in my old D100. all I can say is get the nylon bowl screw washers, and blue reusable gaskets. you will be taking this apart more than you want to. those 2 things will allow you to and be hassle free. I run a 6.5 single stage power valve in the front 7.5 in the rear , 62 primaries, and 76 secondary jets. .031 acc pump discharge nozzles on both sides, blue acc pump cam primary, dark brown in the rear. if this thing will be driven in ANY temps below 40 degrees DO NOT run the green 30cc acc pump diaphram. it gets stiff and will not work. All I can say is start with factory jetting as mentioned above, give it a good run, and pull the plugs and inspect color EVERYTIME you make a change. porcelain should have a nice orange or light gray tinge to it (depending on what fuel you get, and stay consistent) once you get it dialed in its a kick *** carburetor. I love mine. i pulled 20 mpg in my 66 on a 600+ mile road trip a couple weeks ago. thats 3 mpg better than the thermoquad and the 4160 i ran before. good luck!