Its a Holley Street HP Series 750. The primary jets are 69's and the secondaries are 74's. The motor is a 350 bored .60 over with an Isky 270 Cam that has lift of .465", advertised duration is .270 degrees, .50" duration is 221 degrees and lobe center is 108 degrees. The heads are a crappy late 70's casting. It has Hooker LT's with true dual exhaust and Moroso Spiral Flow mufflers. My car tends to bog down a little and I'm assuming I need something more along the lines of a 650 cfm w/ mechanical secondaries. Any help would be appreciated. Thanks

 

750 sounds like a good choice for that engine. I used a Holley 3310 (750 VS... similar to your 750 SA) on my 350 when it was alot tamer than yours and it was excellent. Worked just as well once I put better heads and cam on it later too. 750cfm is an excellent carb for an engine like yours.

 

The carbs size in of itself is not the problem.

A few things to consider:

carb jetting I suggest 70-72 primary 80-83 sec.

accelerator pump shooters i suggest .031 to .035"
accel pump cams: pink? requires trial and error tuning.
accel linkage adjustment: Fuel must start coming out the nozzles as soon as you move the throttle. Adjust the linkage as necessary.

Ignition timing at idle and advance curve: This is uber, uber critical.
Generally the stock/default curve in a stock/new distributor is not right at all for a performance motor with a high perf cam in it. You need to dial it in.
I suggest more initial/base timing at idle up to 24deg static base timing at idle
yet the same 34-36deg at max advance. This requires custom recalibration of the distributor curve usually shortening the mechanical advance curve travel limit. This makes a huge difference in throttle response.
(more initial @ idle, less advance travel, 34-36 max mech advance.)
Then 10 to 12deg of vacuum advance for efficient cruising.
The combined distributor advance curve is very critical.


Auto trans:
Torque converter stall speed: I suggest a 3000 stall converter for your car.
A stock converter is too low a stall.

manual trans:
What minimum rpm are you trying to get good throttle response from? Cammed up motors need rpm.
Ignition timing curve and carb accelerator pump fine tuning is uber critical.

What is the cylinder head casting number (between the valve springs) ?? With this I can recommend a spark plug (type and heat range) for you.

Again the carb size is right but will need dialing in.

Investment in a Air fuel ratio gauge to help fine tune your carb is invaluable.

http://www.innovatemotorsports.com

 

Thank you for all the information. It still smells somewhat rich when driving even with the jets I have. I don't know if I want to go any smaller. I even have power valve blockoff's on both ends.

 

PV plugs (especially primary PV plug) would make it run rich under cruise when jetted properly for power.

 

So I should put the power valves back in? It just seemed they were really loose. I think I would need to get some power valves with stiffer springs on them.

 

6.5 primary PV is typically a good starting point for a mild build like yours. What was in there?

I'm not too keen on secondary PV's.

 

Too be honest I am not sure. I do have one laying in the garage that is 6.5 but I don't know if it was off this one. So do you think My car might run a little smoother with the primary back in and NOT a secondary? It does take about a minute of me giving it gas for it to finally idle on its own. Could this help with that problem?

 

how many rpms u gonna be cranking w/ this motor

 

Well it pulls pretty good up to 6k but it stumbles down low.

 

a 350 cubic inch motor cranking 6k needs 608 cfm and 7k needs 708 cfm. so i personally would say u have a bitt too much carb for ur app. u can reasearch some cfm calculators and see for ur self

 

It's actually a 361 if it makes a difference. Can you post what the formula is for calculating cfm please? I'm also still running open longtubes right now. Still haven't gotten the exhaust done yet.

 

check this out.
http://www.4secondsflat.com/Carb_CFM_Calculator.html

 

Yeah so like I originally thought I may just go buy a 650 dp. Then when I finally get better heads and a bigger cam than I can throw the 750 back on.

 

thats what i would do.

 

That formula assumes a single plain intake where all cylinders share a common plenum and draw from all 4 venturi equally. With a dual plane intake, you need a little more flow capacity.

A 750 isn't going to hurt you.

 

id say the 650 would be perfect.

 

750 would be a better choice, going off past experience.

 

I have experience with both. Both will work when properly tuned. Both will work poorly when not properly tuned.
If you real concern is making your carb work for you consider sending it out to a carb builder with all of your specs instead of buying yourself another problem to fix.