What carb size mopar 340.

[rasmus westlin]

Recently bought a -71 challenger with a 340.

Specs are

340” drilled 0.30
Eagle rods
Keith black pistons
Edelbrock torker 340 intake
Edelbrock rpm heads that are matchported.
Xtreme Energy 236/242 Hydraulic Roller Cam (COMPS CAM) grind number xr286hr-10
Mopar performance distributor with plugged vacum. 18 advanced timing with 34 total.
The carb on right now is a holley street avenger 670 with vacum secondary.

its also a 4-speed car with 3.55 gear in the rear.

I cant get the car to run nice on light throttle, it just bogs and hops when trying to run in ordinary trafic. I have read alot of people having problem with this carb. Should i take it of and buy a quick brawler 650 or 750 DP? (Holley made). The problem is on the primary side and i read that you need to drill or change the metering block to get it to work good.

Hope i can get some good tips and help! Thanks.

 

[moparleo]

Hi, Did you do the engine work or was it already done when you bought it ?
If so has it ever run properly ?
Or have you replaced anything since you got it and now it has a drivability problem?
Street car, reconnect the vacuum advance.
That is a race track move.
Too much initial timing. What counts is not how much total timing you get but how fast you get it.
Initial closer to 12-14 ° Should idle less than 900 rpm. Check vacuum at idle as well. as close to 19 lbs as possible. No double pump unless you like to burn a of of gas. Another track item.

 

[Steve340]

Can you borrow a known good carb of a friend to try it ?
If the problem is gone you will eliminate the engine, ignition etc.

If you do replace the carb my advice is if mainly street driven go vacuum secondary.

 

[rasmus westlin]

Can you borrow a known good carb of a friend to try it ?
If the problem is gone you will eliminate the engine, ignition etc.

If you do replace the carb my advice is if mainly street driven go vacuum secondary.

I gonna try and see if i can borrow from a friend and see. Why is it better to have a vacuum secondary? I feel the car has a quite performance built engine, even if its for street. The carburator that is on now seems like it would be better on a more original built engine with less cam and so..

 

[pschlosser]

Why is it better to have a vacuum secondary?

Mechanical secondaries are more sensitive to timing, and linkage changes, and can open too fast for the engine, causing a slowness in acceleration. Better suited for the strip than the street, but once dialed in, they perform quite well.

Vacuum secondaries open slightly slower (milliseconds) than mechanical ones. But they are driven, more or less, by the demand of the engine. These are easier to adjust to ideal opening speed.

 

[pschlosser]

I gonna try and see if i can borrow from a friend and see. Why is it better to have a vacuum secondary? I feel the car has a quite performance built engine, even if its for street. The carburator that is on now seems like it would be better on a more original built engine with less cam and so..

The original carburetor for the 1970 (for example) 340 was rated for less than 400CFM. You're gonna have to take your engine over 7000 RPM before you outgrow the 670 CFM you have, now. BTW, your 670 is a 4150 clone, a really nice carburetor that many enjoy using on their mopars, even 440 engines.

We don't know how broken or modified your current carburetor is, so borrowing another for testing has some really good value.

I think ANY carburetor you buy new, will need some adjusting and possibly a change to the jetting to be ideal. So new or used, you may still be looking at troubleshooting and adjustment. Any way you see it, you're looking at figuring out the problems.

In the past, when I wanted a simple "bolt-on solution" with minimal adjusting or tinkering, I've always reverted back to factory OEM hardware. But those parts are getting harder and harder to fine in "working order."

While you don't have to use something new, I recommend using something that is already working on another engine as your primary goal.

If you don't have the time or motivation to figure this out, then you have to pay a shop to do it for you.

 

[gzig5]

Modifying the carb to work the way you need it shouldn't be too hard, and it's a lot cheaper than a new one. If going with a new carb, I'd put on an AVS2 650 or maybe even the 800cfm if you are going to wind it out. I put a 650 AVS2 on my Cuda's 340 and it ran pretty darn well right out of the box. If your are willing to tinker and tune, then a Brawler or Proform 750 DP with changeable bleeds would be more flexible. The Holley 3310 750 with vacuum secondary gets a lot of recommendations for a mild 340 like yours too. If changing the carb, I'd put a Performer RPM or RPM Air Gap on it too. The original Torker intake wasn't that well regarded, and a dual plane will give you a better lower end torque on the street than that single plane.
Nice looking car.

 

[rasmus westlin]

Modifying the carb to work the way you need it shouldn't be too hard, and it's a lot cheaper than a new one. If going with a new carb, I'd put on an AVS2 650 or maybe even the 800cfm if you are going to wind it out. I put a 650 AVS2 on my Cuda's 340 and it ran pretty darn well right out of the box. If your are willing to tinker and tune, then a Brawler or Proform 750 DP with changeable bleeds would be more flexible. The Holley 3310 750 with vacuum secondary gets a lot of recommendations for a mild 340 like yours too. If changing the carb, I'd put a Performer RPM or RPM Air Gap on it too. The original Torker intake wasn't that well regarded, and a dual plane will give you a better lower end torque on the street than that single plane.
Nice looking car.

Thanks for your tips! I would like to get this carb to work good, but i read about it and it seems like its known for having stumble issuses and hesitation when giving it light throttle. Some people wrote that you need to drill and change the whole metering block to get it fixed.

Thats why i lean more to try and change to another carb, this carb is atleast 10-12 years old. I know that all carbs needs tuning. But if i have to drill in the metering block to get the hesitstion and stumble to go away, then i rather buy something else.

Is all edelbrock torker 340 manifolds single plane? My friend told me that i have a dual plane? I have also thinked about to change it to a performer RPM. The car has no torque at low rpm, it starts to come alive only on higher RPMs. I wish it ran as good as it looks

 

[moparleo]

Just remember that the farther away from stock that you go with engine mods, the more tinkering you will have to do.

You always need to research the entire engine combo that you have chosen to be sure you will get the results you are looking for. Carb intake, cam, distributor timing, heads, compression etc... all need to work together.

Tuning is the term used to get everything working together. Like tuning a guitar. You need the proper strings in the correct places and the right tension or it will always sound bad. The wrong engine combo is the same.

Change parts and you have less references to tune it properly.
Once you start changing a lot of parts trying to fix it, you generally make it worse and very costly.
Try to get back to the original specs to get a baseline and go from there.
Never make more than one change at a time or if there is a change you wouldn't know which part made a difference.

The engine is a complete, matched package from the factory and millions of dollars in engineering and testing have gone into the factory package.
Street/strip doesn't always give the results you want.
The drag engine doesn't use vacuum advance because the pedal is on the floor. You want the advance to come in very quickly and the carbs demand to match the timing curve of the distributor.

When driving on the street you are almost always driving at part throttle, not to the floor.

 

[moparleo]

Used carbs unless you bought it new are a crap shoot.
If you want used, go to a carb rebuilding specialist for that type of carb who can do everything including flow testing it.
Lots of used carb can have warped base plates from improper installation, worn throttle shafts etc.
The top rebuilders and address all these problems. In the end try to go new.
Dual plane manifolds always for street use. You are looking for low end torque.
Air Gap, RPM, Performer etc..

For street applications that have stock engines an Edelbrock Performer/Performer EPS intake manifold would be a great choice. Edelbrock Performer intakes also work well with applications that are running a mild camshaft. For modified engines running larger camshafts, it would be best to match up the intake manifold with the operating range of the camshaft your running or refer to cam card. Speak to the cam manufacturer if you need to find out the operating range of the cam. Edelbrock offers intake manifolds with matching camshafts to help eliminate the guess work.

 

[540HemiCuda]

With a 4 speed and 3.55s, you don't need vacuum secondaries. Remember that originally most of Mopar's performance engines got mechanical secondary carbs whether they had an auto or a stick. If you do have a carb with vacuum secondaries you will likely need to change the spring for a quicker opening.

The best advise so far is for trying a different, known good carb. And the bigger the cam, the more 'tuning' the carb and ignition will need to accommodate it.

Intakes are designed to work in a specific RPM range. A dual plane is typically a higher torque, lower RPM range geared intake for use with milder and closer to stock engines. If however, you RPM range is higher than stock, then the open plenum would be preferred. To use a dual plane on a hotter engine would usually be a mismatch of parts. You need to match all components so that they compliment each other to the extent possible.

Again, your stick and 3.55s will be a lot more tolerant of a hotter engine combination that most autos and higher gearing.

 

[pschlosser]

Remember that originally most of Mopar's performance engines got mechanical secondary carbs whether they had an auto or a stick.

Umm, yea. I support this forum where everyone's opinion can be heard. We all deserve that. But not everything people post, and not everything ChatGPT (AI) says is true, is based on reality or factual data. In some circles, AI = almost informed.

A review of the Chrysler Dealer Parts books for 1968-1971 years will show the only cars coming with mechanical secondaries (the AFB carburetor) was the 426 Hemi, and for good reason. That was the most performant engine in their product line, and still carries a prestigious reputation. This applies to both standard and auto transmissions.

With all this posted, I acknowledge when "making a car streetable" we give back, or compromise some of the power we might see from an ideal (for the track) setup. Unlike the track, we don't want all our power at the top end of the RPM scale. For the street, making the car more practical to drive on said street, we want some of that upper-end RPM in the mid and lower ranges, too.

The best advise so far is for trying a different, known good carb.

This may be a pain in the butt if you don't have access to a working carb for testing, but if you did, most of us may agree it could be a big time saver to confirm if the problem(s) lies solely in the carburetor. If you don't have access to a working carburetor, don't worry about it too much. It just may take a little more time and effort to diagnose and solve the issue(s).

 

[540HemiCuda]

I don't know what parts manual you are using, but it is INCORRECT.

And I am not going by anything other than 55+ years of personal experience. Starting in 1972, I have had over two dozen Mopars, sometimes as many as 4 at once. Some stock, some modified. I can list them off for you if you would like. I am now down to just one, a '70 HemiCuda with a 540" Hemi which I built myself like all my engines. And just to throw one last point on my resume; I built a '71 GTX in 2000 (which still had it's original Carter AVS with mechanical secondaries by the way) and it was awarded 95 points out of 100 at the 2001 Mopar Nationals which earned it O.E. Certifed status. So, don't accuse me of using anything other than personal experience especially when you so obviously lack that yourself.

Have you never heard of the Carter AVS? Or the Carter Thermoquad? Both are mechanical secondary carbs, and were standard equipment on just all high performance Chrysler single four barrel applications. And before those, the standard four barrel carbs were the Carter WCFB and the AFB both of which are also, mechanical secondary carbs.

And since you brought up AI, even AI knows that you are incorrect. Google up the carb application for a hi performance 340, 383, or 440. Or you can go to a source like Mopar Muscle magazine where this excerpt on Mopar carbs came from:

'The most common four-barrel carb found on Chryslers during the '60s era were manufactured by Carter and came in two specific designs: the AFB (Aluminum Four Barrel) and the AVS (Air Valve Secondary). From a distance, the two look virtually identical; both are box-like in shape and use a quadrant of evenly or almost evenly sized barrels when viewed from the bottom.

After the WCFB was discontinued, the single AFB was the standard carb on Chrysler performance models until the debut of the AVS in 1968. The specific date code and four-digit part number on both the AFB and the AVS models are easily visible in front of the passenger-side mounting flange.'

Chrysler OEM Carburetors - Treasure Hunt

We search through Chrysler OEM carburetors to create this guide to identifying the one originally installed on your classic Mopar. - Mopar Muscle Magazine

www.hotrod.com

Seems EVERYONE (except you) knows what carbs came on most of Mopar's hi performance engines.

.

 

[pschlosser]

Have you never heard of the Carter AVS? Or the Carter Thermoquad?

I agree. If you're calling the AVS a mechanical secondary, then you are indeed correct. Now I'm not sure what that whole "air valve" thing really means.

 

[540HemiCuda]

Another correction; I am not calling the AVS a mechanical secondary carb. They ARE a mechanical secondary carb. What other category would YOU put them in? We only have 2 basic designs: mechanical secondaries and vacuum secondaries. Does direct, independent mechanical action operate the secondaries or does vacuum? It is the simple.

If you are not sure about the AVS being a mechanical secondary carb, then how about the AFB? Same basic carb as the AVS. Only real design difference is the secondary air valve on the AVS which plays no role in the secondary butterfly operation.

The 'air valve' is an enrichment device. It does the same job as the secondary accelerator pump on the double pumper Holleys. It momentarily inhibits air flow so that there is more pull on the boosters thereby giving an extra shot of fuel. Think choke. Same principle. It's a little more complicated than that, but that is the basic premise. When the secondaries are mechanically slammed open and the engine isn't ready for it, it will need a burst of extra fuel. That is why they are adjustable. But the air valve in no way controls the opening of the secondary butterflies.

With vacuum secondaries, you can slow their opening to the point that they don't need the extra shot of fuel.

At any rate, when the secondaries are opened MECHANICALLY, the carb is a mechanical secondary carb.

I am glad to discuss these topics. But you would be wise to ask questions rather than make statements that reveal your relative....................inexperience.

.

 

[Steve340]

The Carter AFB does have mechanically operated throttle valves/butterflies in the base of the carb.
But mounted above that is a counterweighted air valve which operates like a vacuum secondary dashpot and how far it is open determines the amount of air/fuel able to pass through the secondary venturis/butterflies.
The AVS uses a spring loaded air door to achieve this.
The Carter AFB and AVS operation wise have far more in common with a vacuum secondary Holley than a mechanical double pumper. They have a way of regulating the amount of air that can go through the secondary venturies and no secondary accelerator pump that covers up the bog when the throttle butterflies are slammed open.
The "air valve" is not an enrichment device it is an air regulation device.
"As engine speed increases, the forces exerted by the velocity of intake air down through the venturis of the carburetor increases and tends to overcome the counterweight attached to the velocity shaft permitting the offset velocity valves to position themselves according to engine requirements."
That last paragraph is word for word from the factory service manual on the AFB.

 

[Chryco Psycho]

My suggestion is always Proform , they work great !
2X the Ci or a bit less, 670 is right on target .

 

[540HemiCuda]

The Carter AFB does have mechanically operated throttle valves/butterflies in the base of the carb.
But mounted above that is a counterweighted air valve which operates like a vacuum secondary dashpot and how far it is open determines the amount of air/fuel able to pass through the secondary venturis/butterflies.
The AVS uses a spring loaded air door to achieve this.
The Carter AFB and AVS operation wise have far more in common with a vacuum secondary Holley than a mechanical double pumper. They have a way of regulating the amount of air that can go through the secondary venturies and no secondary accelerator pump that covers up the bog when the throttle butterflies are slammed open.
The "air valve" is not an enrichment device it is an air regulation device.
"As engine speed increases, the forces exerted by the velocity of intake air down through the venturis of the carburetor increases and tends to overcome the counterweight attached to the velocity shaft permitting the offset velocity valves to position themselves according to engine requirements."
That last paragraph is word for word from the factory service manual on the AFB.

So you're saying that the air door's resistance to air flow does not help pull fuel up and out the boosters as well as enrich the mixture at the same time? If so, I do not agree with that assessment.

And why are you quoting an AFB service manual? The original AFBs on Mopars did not have the air valves that the current crop of CarterBrocks have. Anyway, the air valve does reduce the amount of air that goes through the secondaries, but the choke does the same thing on the primaries. Do we say that the choke's job is to regulate air flow? No. It is an enrichment device. By interfering with the free flow of air, it causes more pull on the boosters and provides more fuel than would flow without that interference to the air flow.

When the secondaries open, it takes a fraction of a second for the fuel to be pulled up from the bowl and into the boosters so that it can be distributed into the engine. Without that boost, there would be an inevitable lean spike at least until the fuel got from the bowl up to the boosters and into the engine. The air valves resist air flow so that there is more pull on the boosters, both pulling the fuel up and out quicker and enriching the mix, too. It momentarily reduces air flow which pulls on the boosters harder than would otherwise happen. Both actions are to get the engine the fuel it needs at the moment.

The basic purpose of the choke blade and the air door is to pull harder on the boosters by resisting air flow, not to regulate the air flow like the butterflies. That is why they are both ABOVE the boosters, not below. If your theory was correct, it would be prudent to have air valves on the double pumpers, too. But that's something I haven't seen. Have you?

.

 

[Chryco Psycho]

True because the secondaries do not have squirters to add fuel as the throttle is opened

 

[Steve340]

No I don't believe the secondary air flaps resistance to air flow draws the fuel out of the discharge tube similar to the choke. The difference is the choke flap largely seals the primary venturies the secondary air valve flap does not - there are notches cut in it and it is bent to encourage the flap to be opened by the air flow - does not even come close to sealing the secondary venturies. Even with the air flap closed some air must flow through the secondaries for the secondary system to start operation. If it closed tight like the choke flap when the mechanical throttle valves opened below it how would the extra air get in to the engine. The secondary system would be sealed by the closed flap and no air could flow.
The way they work is as the secondary butterflies are opened up the RPM of the engine increases meaning the engine requires more air so the air valve opens up when the air flow increases enough to counteract the weight, when this happens more air flows through the venturi drawing more fuel out of the main discharge tube.
More air through the venturi the greater the depression at the venturi so more fuel is pulled through the jets from the float bowl. Close the throttle butterflies the air flow stops at the venturi, the depression is now stopped so no fuel is drawn from the discharge tube.
This is a basic operating principal of any carburettor.
I was quoting the factory Mopar service manual in particular the section on the 426 motor AFB carbs.
So I'm confident they knew exactly how the AFB worked.