Component selection.

[slowcamaro]

Hows does one decide on a starting point on jets? Is it a cubic each/rpm deal like cfm rating?

How about accelerator pump shots sizes?

Needle/seats? How do I identify these?

As stated some of the parts i've come across the info such as cfm, and power valve starting points.

I guess I would like some recommendations on where to start tuning this carburater, but would also know the tought process behind each selection.

My engine is a 406 sbc.
10.5:1 compression
221cc intake runner 2.08 valves
244/244 hyd roller cam .597 lift
victor jr intake

As you can tell im still learning about the carb. Here is what I know.

850 DP - 4781

Primary side
76 jet
6.5 power valve
30cc pump shot (i think)

Secondary side
82 jet
8.5 power valve
50c pump shot.

Now in my notes from when I purchased the carb it has "31" for both primary and secondary sides written down, but has no details on what part they are referencing.

I was having issues with fuel getting past the needle and seat on the primary side and swapped the needle and seat from my 650 into the 850 and that cured the problem. I think one was marked "101" and one was marked "H". Without double checking im unsure which is which.

Once again I am fairly ignorant on the subject and would like to learn, so be gentle and try not to cost me too much.

[Shaker455]

I build, modify, & tune custom Holleys so I can get you started.

What do you have for a dist and were's the timing set and how's it curved?

Your 4781 is a bit big for your combo and it usally takes a bit of mods to help them fit better.

I would have suggested something a bit smaller like a modified 950

I would suggest 78/84 with a 4.5 PV not knowing how much vacuum your engine produces?

Jeff

[slowcamaro]

I have a HEI setup. 36* in by 3300 rpm.

I believe vacuum at "idle" was 10-12" mind you idle wasn't very consistant as I have yet to really get the carb very close at all.


So what i've gathered so far is that im likley not too far off on my jet sizes, but the power valves are off If my vacuum readings hold true once I have the idle closer to where it needs to be. I noticed you said 4.5 PV singular do you not run a power valve in the secondaries? How is this exactly blocked off and what benefits does one gain? Does secondary jet size need increased when this is accomplished?

I appreciate your respond, would you mind helping me find answers to my other questions, as you seem quite knowledgable on the subject. I'll try to make them more direct this time.

Needle and seats - The number or letter stamped on the flats corresponds to the size how? How does the numeric size of the needle/seat effect performance and which circuit does it affect. How does a "101" compare to a "H"?

Accelerator pump shots - I know how to identify these, but how do I select the correct size for the application? Should I be running 30cc primary and secondary or is the 50cc in the rear ok?

Any clue as to what the "31" could be in reference to? Would it be nozzle size? Im not findin much that includes the numbers "31" besides ".031 accelerator pump discharge nozzles"

Yes the 850 wouldn't have been the carb I selected if I was purchasing a carb new either, but I couldn't beat the price (free). And at the time I was a broke college student who spent every penny building the engine to pass the class. I'm assuming you meant a modified 750, which is likely where I would have began if I hadn't stumbled across this deal.

[slowcamaro]

Any new input?