Front Hemi Carb

My 70 stock street hemi isn't running smoothly, like it used to. Does anyone know where the air/fuel mixture screws are supposed to be set on the front carb of a street hemi? I know there are supposed to be plugs in the openings, but my front carb didn't have them, and the small mixture screws that are there may have vibrated out of their correct position. Can you help?

An initial setting might be available but it really needs to be tuned to the combo, which would be unique to your setup.

Dual quad tuning

This!


The mixture screws on the secondary (front) carburetors are there to provide a little fuel to the forward cylinders to keep them from going lean at idle, this is why they were factory set and plugged off as an anti-tamper measure. Once set they won't usually vibrate out unless someone has modified or damaged the screws or holes making them overly loose.

Thanks to both of you for your input. The mixture screws on the front carb turn way too easy. At some point in their life, I'm sure they have been adjusted and probably multiple times. I'll follow the write-up on tuning dual quads, and also I'll try to address the looseness issue so that the screws will stay put. Thanks for your help!

Which screws are you actually referring to,the ones that go strait into the base that originally had plugs or the angled ones ?

If the mixture screws are loose you can remove them and replace them with conventional mixture screws and springs (which hold pressure on them to maintain the adjustment). The recessed secondary screws are basically the same as the primary carb screws (picture the same screw with the large head cut off and a slot added). Try to find a set of mixture screws that best match the pointed tip shape and length and they should work.

The Hemi AFB had angled mixture screws, not horizontal ones that later AVS had. Hidden screws only on the front starting in 68 with CAP pollution control.

Maybe semantics, but the screws are the idle mixture screws for getting air fuel ratio at idle and off idle correct for minimum emissions. And while yes the adjustment will effect idle, it is only to a point. Your really shooting for the correct air fuel ratio. Once you have both carbs dialed for idle mixture, you want to set the "idle screw" on the front carb so it is contributing 50 or so RPM to your idle set point. This is the way you get the primary blades of the front carb open some to contribute fuel and air to help prevent starving the front cylinders. Having the "idle mixture screws" correct but blades closed really won't contribute to helping get fuel to the front cylinders.

The 66-67 carbs did not have hidden screws, so both could be adjusted. But starting with 67 CAP only cars, and 68-69 is when the front went hidden and the rear had a lock screw to prevent over adjustment. 70 they went to the plastic cap on the mixture screw to limit adjustment. Must have been a Chrysler method for limiting, as other manufacturer Carter Carbs did not adopt this method. It only had a short run with Chrysler too.

As far as the screws, if they are long enough, you can use a Dremel blade and cut the slot longer. Then spread it some. This will act as a a self locking method to tighten up the idle screws. Or you can use the 30A-80 idle mixture screw which the 66-67 hemi used (and other period AFBs) and is similar to the hidden screw on later carbs. Even the AVS idle mixture screw version is very close. They are #10-32 screws at least for the 66-69 front. I have not actually seen a 70 screw to know for sure, but I assume it is probably the same.

What is your carb Number?

I just finished a set of unmolested 70 carbs two weeks ago,100 % as new other than the caps had been removed by the original owner which he included.He had a note with them asking why he could only adjust the screws a minimal amount even tho the caps were off and also could not completely remove them.After removing the plugs for the angled screws and screws,they came out as they should.The main screw has about a 5/16" portion with no threads about 2/3 the length of the screw from the front and recessed about 1/3 the screw diameter,the angled screw went into that recess limiting travel and making the idle screw unremovable unless the angled screw was/is removed.

...And if you are still stumped, simply pack them up and send them to Mr. Katter (Dragonslayer) Mr. Overcash (62Maxwgn) or myself, Scott Smith (Scott Smith_Harms).

I'm 100% positive that any one of us can take care of your carburetor needs

Scott, I am not Dick Katter. Just a hobbyist.

Bill, What number where those carbs? I did not think 70 carbs had locking idle mixture screws. I have never seen a front hemi with locked idle mixture screw?

Here are some pictures. From what I have seen, FRONT carbs do not have locked idle mixture screws; just hidden starting with 67CAP, but primarily 68 and up Hemi AFBs. 67 CAP AFB, and 68-69 REAR carbs had locked idle mixture screws. I did not think 70-71 carbs did.

The carb on left is 66 4139 Front but C8 date code, this was over counter carb. One on right is 69 4619 with F8 date code. Production carb.

The idle mixture screws are from Left or top the 66 non hidden 30A-80, next is the hidden 69 Front, for comparison one I made from a bent AVS screw 30A-199 or 200 (don't remember which), next is a brass hidden screw from a 68 AVS and last is the hour shaped original REAR 4620 69 hemi screw that is locked with a hex head screw from the side. They are 1/4"-36 thread. While all the others are #10-32 thread.

Picture of the hex lock screw on an original 4620 carb dated J8.

The hidden lock and hidden idle mixture screws would have a lead ball pushed in to close the hole. After adjustment at the factory.

I would like to see a picture of an original 70 and 71 idle mixture screw out of the carb if any one has one?

Dick,they were 70 auto,4742 and 4746.

Sorry Dragonslayer, thought you were Dick....Why cant we all just use our real names? Also, you might want to clean your camera lens, your pics look blurrier than a Liz Taylor White Diamonds commercial.

It is clean, just too old and scratched. Hopefully they can help the original poster. Bill, not dick. So did that front carb have a locked idle mixture screw or just hidden? The original poster is talking about a front carb. Interesting to hear the rear does use a locked idle mixture screw. Seen a few ebay examples that did not have the lock screw.

My front carb has the small mixture screws like the ones in dragonslayer's pictures. They turn too easy. That's why I think they may have moved out of adjustment. My front carb is #4742, and it never had the plugs to hide the small mixture screws. I started adjusting the front/rear carbs Air/fuel mixture by turning all four screws out 2 full turns, as stated in the dual four carb setup article mentioned earlier. I hooked up my vacuum gauge to the rear carb to show me the highest vacuum setting, 2 turns is very close to giving me the most vacuum while idling at 900 rpm's. I didn't see where I could hook up the vacuum gauge to the front carb, so I adjusted it to what sounded like the smoothest and best idle. What I have left to do is to make sure that I have 50-100 rpm's from the front carb and then readjust the rear carb idle.

Meanwhile, back to the small front idle screws, I think I will try to find regular mixture screws and use springs on the screws to offer some resistance against movement, at least for the time being.

By the way, I purchased the carbs 17 - 18 years ago from R. D. Katter. The front carb has never had plugs to cover the front mixture screws. Once I know the adjustments are correct, I would like to cover the mixture screws like they are supposed to be, if I can find the correct plugs.

Know where there may be a few !!

Does your rear carb have the lock screw for the idle mixture screw? Could you post a picture of your idle mixture screw of the rear? The plug is a lead ball that is pushed in to cover it.

Great, simple info. 3 fat thumbs up. >.

Wondering how you made out on your issue? Any progress?

As far as that tuning guide for an original Hemi set up, it really has you equal balance both carbs to support idle. Most experience builders I have talked too probably would not do that for a street hemi with OEM carbs. They recommend just adding 50-75 RPM from the front.

The 68 and up Hemi rear carb is unique as far as carter go in that there is a second idle circuit (fixed) in the secondary venturi's of the rear carb. With progressive linkage you won't even open those front primaries until your at much higher rpm.

Well, as it turns out, the idle screws on the front carb is just one of two problems that I had. I think I have their looseness fixed by applying a tiny bit of permatex form-a-gasket to the top of the threads, thereby creating enough resistance that they stay in place.
My second problem is that the front carb is not working properly. When I tried to increase the idle on the front carb, it wanted to kill the engine. So I find that the accelerator pump is not pumping gas. So, I decided to go through the carb. I ordered and received a rebuild kit and have cleaned and replaced parts and measured float levels, etc. My only obstacle now is that the accelerator pump from the kit is not the same length as the one I want to replace. So, I have to find the correct one. That's where I'm at now. I'll keep you posted as to whether my efforts pay off or not.

If you adjust the pump stroke as to specifications,the minimal difference in length really doesn't matter.If it really bothers you,on some but not all,you can increase the over all length of the pump.Put the top barrel in a vise and use a small pair if diagonals to pry the shaft out,you may or may not get to the length you need but doesn't cost anything to try.

I am confused why trying to raise idle with front carb kills engine. Pump is a separate issue with accelerating, and should not have any effect on idle. Frankly, getting a good idle is mostly the rear carb. Your just trying to ensure you have mixture right on the front, and then open throttle blades with idle speed screw to gain last 50 or so rpm you want. If opening front primary throttle blades cause the car to stall, some thing not right, and I think would have to be way off.

Dragon Slayer, I am confused too. What controls the idle? I was getting no gas coming out of the two nozzles inside the carb when I activated the throttle. I assumed it was the acc. pump. The rear carb is providing 95% of the idle and I was trying to get the front carb to contribute the other 5% when I increased the idle screw.

Lets be clear. The idle mixture screws, which are the hidden screws on the front carb control fuel to the idle circuit that already has some fuel and air coming in through an idle port on the carb body. Screwing in the idle mixture reduces extra fuel and leans out the idle mixture. Screwing out richens mixture. Your adjusting the screws front and rear carb to get best idle and vacuum. Then turn the actual idle speed screw which pushes on the throttle shaft lever to open or close the primary throttle blades. Normally probably coming out to get idle back down around 650 to 700 or what ever your manual calls for and what you want it to idle at. Then do the idle mixture again to refine mixture. Then adjust idle speed back down with lever screw. Normally this is done with progressive linkage detached. And front carb idle screw backed off and throttle blades closed. The rear carb should idle the motor. If you want 650 idle and the mixture screws are all good. Get 600 on back carb, then adjust front idle speed screw to get 650.

Reattach progressive linkage and test.

Important point here. If those screws are really loose, and if you have lock screws they have to be in place though not tightened down. You need to ensure no air leaks coming in around mixture screws or locks that can be upsetting calibration.

Also make sure good ignition set up and timing first.

Now if your idling car on rear, and just open front with rear at idle, maybe it will stall. Never tried that on car. The front does not open until the rear is well past mid throttle with the progressive linkage.

Gas flows through idle ports, a full depression of gas pedal would pump fuel out of nozzles, but that would assist with starting, and is really the acceleration circuit to shoot fuel as the carb transitions between low speed and high speed circuit. Since you don't seem to have any problem starting car, it should idle and accelerate to moderate speeds in neutral without the pump shot.