Finn5033

is a holley 750cfm carb too big for a mildly built 70 olds 350? I am not looking for a discussion on who makes the best carb just what size to use. Thanks for any tips. I have a nice holley on my engine but it needs to be tuned. I'd rather spend the time tuning a correctly sized carb if indeed the 750 is too big.

455man

I think that depends on the brand. The quadrajets were 750 cfm but only used as much as the motor could take. I think a holley would open up all 750 cfm and would be too much. 600 cfm would be closer to what you need.

s i 442

I put 750 Hollies on everything!
Make it work because it can be done!

jag1886

If it's a DoublePump it's to big if it's a vacuum it will work but a 600 would work better for a mild engine.

oldcutlass

Which 750 is it as I somewhat agree with Jag.

Finn5033

It is a 80508s-7. 750 cfm vacuum secondary

oldcutlass

My next question is what intake are you running? That and the 3310 are basically the same. I would consider the Quick Change Vacuum secondary kit.

Finn5033

Edelbrock performer rpm intake and 1" spacer

oldcutlass

Why the spacer?

Finn5033

the spacer was another thing the guys put on during the dyno. They said the engine seemed to like it. My builder was a very nice guy but I am really starting to question a lot of the things he did and told me. I had a 600cfm edelbrock and he convinced me to get the Holley. Since then everyone else I talk to says the carb is too big. That is why I am asking questions on here trying to figure things out. The other thing I question is why a factory rated 310hp engine would only gain 10hp from a hotter cam, big block valves, rpm intake, headers, and high comp. pistons? Before the build was done my builder said the engine would make around 375hp with these upgrades, ended up being around 320hp

oldcutlass

With your engine the 750 should work well.. Maximize your timing and watch your vacuum advance at cruise. I don't know what to say about your HP as I don't know how it's set up.

Grace Stinespring-Welch

the spacer may be there for throttle linkage clearance? i.d.k. i built a buick a few years ago it ran great around town,i took out and got on it to see what it would really do, and the throttle linkage jammed against the egr valve and it stuck wide open. that was a scary ride.

Finn5033

Ok, if you think the carb size is ok then I will do what is needed to get it dialed in correct. I just wanted to be sure before I started putting money into it.

Finn5033

I don't believe that was the issue. The first couple of runs on the dyno were without it. It just seemed the engine ran better when they tried the spacer. Man that would be scary though

oldcutlass

Oh and when you change to the Holley, have someone sit in the car and press the gas pedal to the floor while you look at the carb to make sure your getting WOT.

Finn5033

Will do, thanks

Finn5033

Ok, engine is running pretty good. Adjusted the floats and idle but it still seems rich. I was told my carb has either #72 or #74 jets in front and the rear are 82 or 84. Could anyone tell me from experience what jets I should try for my engine? I know it may take some trial and error but I can't even find a starting point. I have tried calling holley tech line for the last 3 days and can't get a hold of anyone. Thanks for any help

Specs are 70 350 with stock high comp. pistons for around 9.8:1 comp. Cam is Elgin 214/224 .050 dur. Lift is .472/496. big block valves, performer rpm intake, holley 750cfm vac. Secondary carb.

oldcutlass

When you adjusted it did you use a vacuum gauge or tach? What's your idle speed (try to keep it below 800) and timing set to? Are you running vacuum advance and is it connected to ported or manifold vac? I know with the a/f mixture screws a little is a lot.

s i 442

You need to find a hot rodder to adjust your engine, I would be pretty POd if I was promised 375hp then only saw a 10hp gain from stock!
A 3310 carb has #72 jets in the front you could drop to a 70 jet and while its apart put in a 4.5 power valve.
I think your timing set up needs a real good adjusting and that will clear up alot of problems present and future.
I just went from an Edelbrock 600 to a Quick fuels technology 750 and it was amazing the "seat of the pants" difference that I felt!

MDchanic

Well, for one thing, the nominal compression ratio on the 310 HP motor was 10.25:1, so your new build has reduced compression compared to the original.

For another thing, the 310 HP number is SAE Gross Horsepower, while your dyno was probably measuring SAE Net Horsepower.
Read this article.

- Eric

Finn5033

That is a good point Eric, I am not sure if it was stating net or gross for the dyno. I should check into that. The other thing is the Holley they threw on for the dyno was probably set up for optimum max conditions on a dyno.

TripDeuces

If I'm not mistaken vac. secondaries come with a jet plate on the secondary side, not actual jets. Can you physically see a metering block on the secondary side?
No metering block = no jets.
You also mentioned it 'seems rich'. Was there ever a backfire and now the power valve is blown out? That would richen it considerably.

Finn5033

Yes, used a vacuum guage optimum vacuum after playing with it was 16 vac. Vacuum advance is connected to the carb so must be ported. And idle is around 700. Timing is at 36 total which was what it liked best on the dyno. Thats not to say that doesn't need to be changed now that its in the car. I was with a pretty knowledgeable friend and we played with the idle screws for a while. He is a former drag racer and he thinks it needs to be jetted down. He said he can't remember the specifics of the jet sizes though.

Finn5033

There was never a backfire. I am not knowledgable about the carbs, here is a link to the carb I have. Hopefully this answers some questions. http://www.holley.com/0-80508S.asp

TripDeuces

From that picture is easy to see that there is no metering block in the rear so you have a metering plate there, hence no jets in the rear. Yes I said hence, lol.
An easy way to get the proper power valve size is to hook a vacuum gauge up and run it into the passenger compartment and observe it while you drive. Drive around as you normally would without getting on it. Half throttle at most along with everyday stop and go stuff. Observe the lowest reading. Put a power valve in it that is 2 inches less than your lowest reading. Example: If you never see less than 8 inches around town then a 6.5 or 5.5 power valve would be best.

oldcutlass

Before you change jets, are both your a/f mixture screws turned the same amount out from the seated position? How many turns out are they? You have great vacuum, timing is good. When you connect your vacuum advance does the idle speed rise?

This how your carb is set currently:
CARBURETOR SPECIFICATIONS:
CARBURETOR #
PRIMARY METERING JET
SECONDARY METERING JETS OR PLATE
PUMP DISCHARGE NOZZLE
PRIMARY POWER VALVE
SECONDARY DIAPHRAGM SPRING

0-80508S - Your carb
70
21 PLATE*
0.031
65
Black

0-80783C - This is a 650 CFM
67
73 JETS
0.028
65
Black


*21 PLATE is equivalent to 75 JET

TripDeuces

If you read the last page of the tech. info on your link you'll see you have 70 jets in the front and a 21 metering plate in the rear which is equivalent to 75 jets. That's more than enough for your combo.
I'm just not a fan of 750 Holleys on 350 CI engines. Especially unmodified engines. My first choice would have been a 650 for a street ride. You asked for opinions so there's mine.

Finn5033

I apreciate the info and your opinion, it is the same opinion I get from a lot of other people. Question on the jets though, I was told by the guy that I bought it from that the primary jets were either 72 or 74. Are those bigger or smaller then?

TripDeuces

The 72 or 74 would be bigger than the originals which were 70's. That could be your rich condition right there. Just for comparison I ran 72's with a Holley 780 on a 455 with a Torker, ported heads and a mild cam.

oldcutlass

Those are bigger, I was assuming you had an unmodified carb. IMHO the 750 in it's stock form should have worked fine on your engine. Your power valve is ok based on your vacuum. The only difference as per my post above between the stock version of your carb and a 650 is jet and discharge nozzle size.

Finn5033

Thank you for this info, this is exactly what I was trying to find. The carb has definately been modified then. I think I will get it back to stock form and go from there. Thank you all for the great info and help

Finn5033

That is good to know. Now I will need to decide if I should set it up like the 750 or the 650. You guys really helped me out, thanks again

droldsmorland

Read your plugs & your pipes, before and after all this tweaking. Its a simple way to see if you need more or less jet.

ramrod31

I had a 570 on my 455 with a 432 gear and a turbo 400 it was great
I have a vac sec 750 on mu built 350 its fine all depends on the combo and what you are doing with the car a qjet is best in my mind for street
mpg /performance