Hey, I'm new here, but I've used this as my go to for information for a while now. I've got a question about carb selection. Curently my car has a holley 750 vac secondary carb on it. I converted it to a secondary metering block and installed a secondary power valve plug. It has 70 primary and 80 secondary jets with a 6.5 power valve up front. However, after looking around and having to replace fouled plugs once already I honestly think the carb is too big for the engine. Its a 70 l46 with an edelbrock scorpion intake. So I began looking around for a 650 carb and came across a holley aluminum ultra hp 650 carb. Its at a very good price but it does look a little racy for the street. Im not necessarily concerned with it not having a choke as the car is driven only in nice warm weather but i was curious if anyone knew the street manners of this carb? Sorry if thats a long and confusing read I'm new to this haha. Thanks.

 

What size engine are you running? I haven't heard anything bad about the 650 hp but you could simply running your float level too high on the 750 if your fouling out plugs.

 

Id think a stock 350 at the most would want a 68 main 80 way too big on secondary maybe a 74. think you are right though thats just too much for a stock 350. Can you tune it to run the car yes
650 probably a better choice and if it has the provision to change IAB jets and what not even better for fine tuning

Too bad I didnt see this earlier just gave away 3 600s and a 650

 

make sure the power valve is working .

 

Its a 350. I set the floats to just under the sight hole so I dont think thats the problem and I'll try rejetting it. Also the power valve currently is damaged as its backfired a few time which is also the reasoning i want to change to a new holley as the have the passage that protects power valves.

 

Just to save you some bucks....I've seen an improved power-valve kit for Holleys advertised in the last year.....and jet it down....but do one modification at a time or you'll be going in circles despising a good carb (been there...despised my Holley 1850......now I'm finally getting 22MPG on roadtrips in my '34/350cid/Holley 1850 4bbl/700R4/Pickup)

 

You have an old "Scorpion" intake ? It's rare. That's an old school 8,000+ rpm intake !!
You're probably getting a lot of reversion with that big tall single plane , and if the rest of the engine is stock I'm sure it would run better, and tune easier, with a good dualplane intake.
Your 750 carb would be fine also when setup correctly - I guarantee it.
If you're intent on keeping it, a small carb will crutch it alittle, but I hope you have a manual trans and 4.10 to 4.88 gears !!

 

I always thought it looked like a racy piece. Good to know! I would like to keep it though as it is a cool piece! Would a 650 really hinder it though especially if i dont wind it past 6k? I know the single plane with small carb isn't the optimal setup but when I can get a 650 ultra hp for less than 300 I thought that if i went with that it would be more out of the box ready?
Also the 750 on it right now is vacuum secondaries and I thought the mechanical secondary 650 would be better for the manual trans?
Any suggestions on a good place to start with trying to tune the 750(jet sizes, power valves, etc). This is the first carb I've learned on and i feel like im messing things up more than fixing problems.

 

The 650 will help some. "Crutch" it - ie help it. It is a racy piece , from the '70's.

 

Buy one of the good Cartech, or other brand, Holley tuning and modifying books, or buy two.

 

Start here and understand what he's saying before you do any actual tuning.

http://members.tccoa.com/392bird/tuning.htm

 

I think the carb is only one of your problems.

While the Scorpion may look like a "cool piece" it really isn't the best manifold for a stock/street engine. It's great if you spend all of your time between 5000-8000rpm, otherwise you would get better street performance with a dual plane manifold.

If you ran your car on a dyno with the Scorpion/Holley and then swapped with a dual plane manifold/quadrajet you would most likely see a hp/tq improvement across the entire rpm band with the latter.

This isn't directed at you, but it's amazing how many people just cobble mis-matched engine parts together for a rebuild because they can save a $ or have the parts laying around.

 

Get rid of the manifold, switch to an Edelbrock RPM Air Gap. Install a 6.5 power valve in the back along with number 76 jets.

 

Althought the Airgap may have fitment issues, get a dual plane intake. An original GM/Winters would look the part and fit perfectly. I would also suggests to take the primary jets down to ~68....

On a side note I have a 750 Ultra DP carb. The ultra series are great carbs!

 

Hi 70L46CONV,

I know how you feel with regards to selecting and tuning your carb. I spent most of my summer tuning my QF 750SS to try and get it to run better.

What I found is that there can be a lot of trial and error when it comes to carb tuning. What works for one engine maybe totally incorrect for another engine.

Things such as altitude, temperature, etc. also affect the tune of the carb.

After much thought I decided to approach it from another angle, I decided to install an air/fuel gauge into the car. I also purchased an air/fuel monitoring system (a LM-1 from Innovate).

This approach helped me tune my carb to operate much better. I admit it is probably not as good as taking it to a dyno and having a professional tune but it is also better than guessing.

Besides, the dyno tuners around here charge large sums of money and getting them to do the work is like pulling teeth, they are never available.

With the air/fuel systems I have, I can tune my engine indefinitely and won't have to continually pay the dyno tuner.

This is just something to think about before purchasing another carb that you will have to tune and may not be happy with. I think you will be able to make your Holley 750 work just fine.

Anyway, this is just a suggestion to try to save you some wasted time and frustration.

Hope that helps.

 

The Scorpion and 750 are way too much for an otherwise stock 350. The motor can't keep up with what you are feeding it.

RPM performer with a Holley 600 or 650 stock from the box will work great.

You can recoup a lot of your money selling your parts on Ebay.

 

I agree with the guys who are telling you to get a good dual plane intake. The carb you have is fine. Yes its a little big, but get it set up properly and there isn't enough difference between that and the 650 to justify the money. Spend your money on the manifold. I also agree you're jetted way too rich. Find out what the carb had from the factory and start there. Probably about 71 primary 73 secondary. I have a 770 Street Avenger on my 383 and its jetted 67/69 and its still a little fat.

 

Tons and tons of info on how to tune Holley and specs.

750 should work fine with a stocker, but that big open manifold will not serve you well on the street.


To find out what your carb had stock just check the chart:

http://www.hotrodreference.com/951/h...valve-ratings/

 

I concur with those who say a dual plane intake is in order here. Although, I would say you don't need to go overboard with the Edelbrock RPM Airgap, a Performer EPS would work nicely, I think. You have to try to have all your parts optimized for the same rpm range. That's the reason the big, open plenum intake is wrong for your engine. It is optimized for something like 3000-7000 rpm. Putting on an Edelbrock RPM, which is optimized for 2500-6500 rpm will help, but not as much as an intake that is optimized for idle-5500 rpm, like a Performer EPS. It may not make as high a horsepower number, but it will give you a more usable power range, with better throttle response at lower rpms. In other words, it'll be fun to drive.

Scott

 

Check here for stock specs.

http://www.hotrodreference.com/951/h...valve-ratings/

Keith is dead on. When you rebuilt it, make sure you didn't get any dirt into it.