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Discussion Starter #1
Hello all,

I've got a L46 1970 vette with its original rochester quadrajet carb.

My car does not works very well, very, very high gasoline consumption (30L/100km) and have a big hole when I start to open the second stage throttle, and some times backfires.

My engine is OK, 11-1 minimum compression ratio for each cylinder, most of the ignitions parts are new except the cables.

I think that my carb is too old, and I don't know if repairing it would be a good idea, so I want to change, and with this opportunity increase my cars performances ... I read that rochester were very good carbs so I'm hesitating beetween severals options ...

1. find a new Rochester carb, a good quality recontionned one, the same as the actual .. or a bigger one ?

2. My engine is the little brother of the LT1, so I had the following idea, find a LT1 intake (or equivalent because original ones are very expensive) and put a 780 cfm holley carb.

3. by a combo ?

What do you advice to me ? ideally it would be great to keep the original choke system and have a stock look (keeping the possibility of using my original air filtering system)

I like also RPM, driving my car over than 5000rpm is just ... :partyon:

Thanks in advance for your advices and sorry for my english (I'm french).
 

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Carburetor sizing is fairly simple. Most people put way to big a carb on there engine.

This formula will give you the size you need.

CFM = (RPM times CID) / 3456

So if you have a 350 ci engine and you want to rev it to 6500 RPM.

6500X350/3456= 650CFM carb.

Now this is figuring your engine is running 100% volumetric efficiency. This is the rate your engine can draw air in. Unless you are running a blower with ported heads you will never reach this efficiency.

Most stock engines run less than 85% volumetric efficiency. so that means at best your stock 350 engine will not gain any thing from carb's bigger than 550 CFM.

650CFMX85%=552CFM

The back fire you are getting is the caused by the lean condition caused at WOT causing the large Q-jet carb to draw in to much air and not enough fuel because the vacuum in the carb goes to zero and will not draw fuel.

You will also experience a bog in acceleration with to big a carburetor.

Another consideration is your cruse speed. If you want to cruse at 2000 RPM your primary side needs to be 172CFM.

Calculating for cruise CFM = 350 * 2000 *0.85 / 3456 = 172CFM.

Check this link for more information:

http://www.thecarburetorshop.com/Carburetorsizeselection.htm

A better intake manifold is more important than a bigger carb. Along with the intake you need a better exhaust also.

So any square bore 650 Carb(325cfm primary-325cfm secondarys...650CFM WOT) will work fine for you. I will probably catch hell from all the Spread bore fans out there.

one old mans opinion...
 

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Discussion Starter #3
Thank you iburke for this very concentrated and instructive information. It starts to be more clear for me.

I've been looking for LT-1 spec, engine upgrading and several web site which sells hi performance parts all of my evening, and the more I discover the more question I have ... :)

Concerning the carb I read that a rochester is a very good carb and a holley too. Holley seem to be nicer because of the double pump system which are available on some models and provides more reactive behavior of the car. One thing strange is why my rochester is so oversized .. 750cfm, and same question with the LT1 vette whith its 780cfm holley ..?

Concerning the intake manifold you've said that this is a really important point and I've got some questions, there is the LT1 intake, but it's very rare and expensive, can I find some copies of this intake ? I've seen a GM performance cast iron one, is it the equivalent ? (I would prefere an aluminium one for the shipping price) I can find a lot of similar intake, and don't know really how to make a choice ...

For example, do I need to take a single plan or dual ? and beetween several single plane on which criteria do I will make the choice ?

other question, on my car the L46 intake is clearly design to perfectly match with the rochester, we can see the 4 holes of the four barrels, if I look on the aftermarker part, most of them don't match whith the carb' shape, is it critical ? and make turbulences problems ?

Last point concerning the intake : is a hi performance intake compatible with the correct hood clearance ? (want to keep my original air filtering system)

For the engine cruse speed I need to check the engine speed when driving around 60mph (my transmission seems to be short)
 

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The intake to look for is an Edelbrock Performer 2101. The difference in carburetor shape is known as "square" and "spread" bore. Square is obvious. Spread bore is the original quadrajet. The primary and secondaries have different bore diameters, they're "spread". It's important to match the carburetor style to the intake.

Backfiring when the secondaries open isn't usually the carburetor when you're talking about a rochester quadrajet. Possible causes are

1. Distributor could be sticking inside, causing timing to be wrong.

2. Camshaft could be very worn, backfiring under load is a normal symptom.

3. Vacuum leak, a big one.

All of these problems also cause high fuel consumption, and won't be fixed with a new carburetor.

Your carburetor is big because the secondaries are applied very slowly, a vacuum diaphragm controls the top air doors and won't give the engine more air than it can handle. So the exact same carburetor works on a 7.4L or a 4.3L, GM manufactured them by the millions. If a person tries to convert a Qjet to manual secondaries, all it does is backfire when you open them up.

good luck with the car.
 

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A Quadrajet DOES have mechanical secondary butterflies - its the air valve above the butterflies that is pulled open by airflow. Most likely, the spring which controls the tension on that air valve is too low. It can be adjusted easily without even removing the carburetor, but it helps to get a good book on the Quadrajet carb to learn all you can first.

On my '78 with a 406 SBC, the Quadrajet on an Edelbrock Performer RPM Q-jet manifold (pn 7104) runs mid 11s in the quarter and gets 17 mpg on the highway with air conditioning on. With my best Holley, it runs neither as fast, nor as efficiently as with the Quadrajet!

Cheers!
 

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A lot of people dump their old Q jets for other brands (such as Holly or Edelbrock) because the replacement carburetor looks good and is easy to tune.
I believe that for most applications it is hard to beat a correctly tuned Q jet. However they are relatively complex and require detailed knowledge to set up properly.
Due to their small primaries; the throttle response and fuel efficiency is exceptional.
They are however prone to warping with age and some older units had leaking bowl problems.

I original had a Q jet that eventually warped so I changed it for an Edelbrock. Big mistake, as It came full of metal filings from the factory, the throttle response at low RPMs was poor and it stalled going around corners also at low RPM. I lost track of the times I adjusted the float level to try and fix the problem. It was not the float level; it is a design default.

I have now gone back to a Q jet that I purchased second-hand and rebuilt it.
It is absolutely fantastic.

There are a number of good books on Q jets and there are also some good people that rebuild them.
Good luck
 

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Discussion Starter #8
Dear colleagues, it's still a please to read your answers :)

I'm really convinced that Q-jet carb is a great carb ;) when I bought my car there were only the first stage throttle which was operational, and when I found and fixed the problem the power became awesome !!! .. but I think my car does not work constantly because of the carb which seems to be very used...

All the gaskets are very old ... the 1st stage throttle axis has, I think, 0,5 mm of free movement, so the air can pass through the axis hole ...

Concerning the fuel pump this one seems not to be ok .. see the picture, on the pump mechanism theres is little a metal wire part, and this, I think theoretically lead to close the hole with a little rubber cap. All of these parts are used. The metal wire part seems to be broken / incomplete, because when I turn the gaz it does not close the hole, as it should do. New Q-jet does not have this mecanism ... :/

Same problem on all of the mechanical connections around the carb, the seems to be very used... and this was the reason of my second stage problem, the spring which links first and second stage was broken ...













So I think I will buy the book and a rebuilt kit to see if I can resurect my carb, but I'm not really convinced.

I'm hesitating beetween the holley double pumpers which has very good feedback and a reconditionned Q-jet.

Concerning this second option is the big block Q jet different ? are they all 750cfm sized ? and is it possible to keep the mechanical choke ?

still about the choke topic, larrywalk, was it possible to keep the original choke system ? with your high performance intake (seems to be very interesting part) because the choke spring base looks different...

To go back to the intake choice I saw that some of them have ni exhaust gaz circulation in it ? in this case I think that choke need to be forgotten, and is this kind of intake better for performances ?

Last question, PCV valve, mine is old, and I don't really understand how it work, can it makes troubles ?
 

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Dear colleagues, it's still a please to read your answers :)

I'm really convinced that Q-jet carb is a great carb ;) when I bought my car there were only the first stage throttle which was operational, and when I found and fixed the problem the power became awesome !!! .. but I think my car does not work constantly because of the carb which seems to be very used...

All the gaskets are very old ... the 1st stage throttle axis has, I think, 0,5 mm of free movement, so the air can pass through the axis hole ...

Concerning the fuel pump this one seems not to be ok .. see the picture, on the pump mechanism theres is little a metal wire part, and this, I think theoretically lead to close the hole with a little rubber cap. All of these parts are used. The metal wire part seems to be broken / incomplete, because when I turn the gaz it does not close the hole, as it should do. New Q-jet does not have this mecanism ... :/

Same problem on all of the mechanical connections around the carb, the seems to be very used... and this was the reason of my second stage problem, the spring which links first and second stage was broken ...


So I think I will buy the book and a rebuilt kit to see if I can resurect my carb, but I'm not really convinced.

I'm hesitating beetween the holley double pumpers which has very good feedback and a reconditionned Q-jet.

Concerning this second option is the big block Q jet different ? are they all 750cfm sized ? and is it possible to keep the mechanical choke ?

still about the choke topic, larrywalk, was it possible to keep the original choke system ? with your high performance intake (seems to be very interesting part) because the choke spring base looks different...

To go back to the intake choice I saw that some of them have ni exhaust gaz circulation in it ? in this case I think that choke need to be forgotten, and is this kind of intake better for performances ?

Last question, PCV valve, mine is old, and I don't really understand how it work, can it makes troubles ?
No matter what carb you end up using save the original for a future rebuild.

Buy both the books by Doug Roe and Cliff Ruggles.
You will be able to completely rebuild and mod any qjet.
They were put on 6 cyl as well as big cube Cadillacs.
The secondary air valve setup only gives the fuel that the engine requires, so it's an easy match.
The later 800cfm versions are great because of the APT adjustments. You can also put on an electic choke cup for an simple choke setup with no modification to your original intake manifold.
It's easy to fix any well leaks, throttle shaft bushings and other mods to make it easy to tune.
77 and up 800cfm carbs are a good choice.

And the pcv valves do need to be repaced from time to time, they can cause trouble. Lots of info on those in a forum search.
 
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