66 Hemi jetting

[Dragon Slayer]

Fran, Can be minor calibration differences on air and fuel bleeds to significant mod to venturi length, emulsion, and such. Are they 802/803 or 902 and 903?
The 678 is missing a secondary discharge transition port.

Your rear looks fine. I can swap a 347 for your 678 you pay shipping. I also have the correct primaries for your front. If you have 902/903 we could probably do a swap of some sort, they are CAP venturies for some of the other AFBs. That would get your front correct. Can contact me via message if you are interested.

 

[AR67GTX]

Thanks - I think the carbs are squared away. Hopefully I’ll have the other work I’m doing finished in a week or so and can start it up again.

 

[nitrohemi]

I have a stock 67 hemi. it has always run a tad lean until it really warms up. I am sure headers dont help the stock jetting. I have been trying to figure out if stepping down the metering rods is or stepping up all the jets is the best move. The metering rods are only on the primary circuits and with the carbs net being jetted square I am leaning towards changing jets. Its all stock except headers and electronic ignition. I am sure a little more fuel is what it needs. Any opinions whether to change jetes or rods?

 

[RJRENTON]

I have a stock 67 hemi. it has always run a tad lean until it really warms up. I am sure headers dont help the stock jetting. I have been trying to figure out if stepping down the metering rods is or stepping up all the jets is the best move. The metering rods are only on the primary circuits and with the carbs net being jetted square I am leaning towards changing jets. Its all stock except headers and electronic ignition. I am sure a little more fuel is what it needs. Any opinions whether to change jetes or rods?

IMO....I like Carter carbs....easy to adjudt. Since your carbs are basically stock, do you know if it still has tgw 3 step primary meteting rods or have they been changed to the Edelbrock 2 step rods. The step up pistons are different to accomodate. The step up piston covers are slightly different as well. Bear in mind that the metering jets, both primary and secondary positions CAN BE VERY DIFFICULT TO REMOVE, ESPECIALLY THE PRIMARY JETS as they are in a little "nitch" and you have to have a hollow ground blade screw driver that fits EXACTLY in the slot....maybe even an impact screwdriver. If you "boogger" up the slot, the jet is VERY DIFFICULT TO REMOVE the jet,.....be very careful.
My preference is to change the primary step up rods (and pistons if needed) FIRST. Consider also different step up springs to start lifting sooner to lessen the lean condition you noted. The Mopar 3 step rods alliw a smoother mixture transition from lean to rich but are difficult but not impossible to find....
Lean idle mixture and off idle fuel transition is controlled by the primary booster venturii not the primary jets and step up rods, but thru factory drilled orifices in the booster assemblies that are very difficult to change. Just a few additional thoughts......
BOB RENTON

 

[Dragon Slayer]

Hemi AFB are not 3 step. They are 2 step. Pistons are mostly the same regardless of 2 or 3 step. Yes some AFB used an odd piston. Covers is the difference on most AFB/AVS 3 step rod carburetors especially the later years.

As far as changes, is the carb lean at cruise or WOT? Also how is the set up balanced? Most of your driving is on the rear carb only. Do you have an idle contribution from the front carb?

 

[nitrohemi]

I ment 3 steps up in rod size not 3 step rods. anyway it lean off idle when going to half throttle. I have it on the hole to give it more squirter and I have tried heavy springs in the cups. I may try a different rod in the rear carb to just see what happens. Its been like this for a long time. its fine for the most prat. A little embarrasing when you het the gas and it stumbles at a red light. Totally warmed up 180 -190 degrees and its fine.

 

[Fran Blacker]

I ment 3 steps up in rod size not 3 step rods. anyway it lean off idle when going to half throttle. I have it on the hole to give it more squirter and I have tried heavy springs in the cups. I may try a different rod in the rear carb to just see what happens. Its been like this for a long time. its fine for the most prat. A little embarrasing when you het the gas and it stumbles at a red light. Totally warmed up 180 -190 degrees and its fine.

Have same issue, off idle stumble. If you solve it sure would like to know how. Some have said plunger isn't down in the well so it doesn't supply gas fast enough.

 

[Dragon Slayer]

Even the race manual sticks with the stock jetting on a street hemi. So I guess I would validate you have the correct venturis in the carb, what jets on primary, should be .089 both carbs front jets. Then the rods. Is choke working correctly, do you have manifold heat. My car hard to start cold, but warm I barely turn the key and it starts. A cold manifold is probably pooling some fuel and you have a lean mixture going to the cylinder. Also do you let the front carb have 50 to 100 rpm of idle contribution?

 

[AR67GTX]

I had an off idle stumble in my GTX with one of the Edelbrock 800 cfm AFBs. A new accelerator pump didn’t help (bought one specific to the 800 carb) so I started measuring the depth of the v-notch in the accelerator pump well, the height of the stock accelerator pump setting, the position of the bottom of the pump cup. Put it all together and discovered at the Edelbock pump height setting, the bottom of the cup was well below the bottom of the v-notch and the well was not refilling when the accelerator was released. So I ended up having to raise the pump height setting - a lot - probably over a 1/4” from memory. After that I could mat the throttle hard from a stop and it just churned the tires and charges off with a strong, stumble-free pull. So not sure if there is some issue with the Edelbrock, specs, pump shaft length, or a casting defect in the pump well notch.

 

[nitrohemi]

Even the race manual sticks with the stock jetting on a street hemi. So I guess I would validate you have the correct venturis in the carb, what jets on primary, should be .089 both carbs front jets. Then the rods. Is choke working correctly, do you have manifold heat. My car hard to start cold, but warm I barely turn the key and it starts. A cold manifold is probably pooling some fuel and you have a lean mixture going to the cylinder. Also do you let the front carb have 50 to 100 rpm of idle contribution?

 

[nitrohemi]

jets are all stock. carbs havent been toched in 30 years. Headers so I do not have heat going to the intake. I am sure that a cold intake has something to do with it. My dad bought the car brand new. says its did it since day one. Although I assume it has headers since then also as I have the stock exhaust manifolds that ook life they have about 100 miles on them. 160 thermostat dosent help either. I know these things like to run when warmed up and there is a lot of iron to heat up.

 

[Dragon Slayer]

So how do you operate the choke? Manual, electric? Seems if you change jets and such to have a better cold response, you will be rich once warmed up. Easier to foul plugs.

 

[nitrohemi]

I wouldnt think steppingup a jet size or too will foul the plugs when warmed up. maybe if I go up 5 or 10 sizes there would be a problem. With hearders there is no way for the choke to warm up. It does work somewhat but not as the factory intended

 

[AR67GTX]

You could probably hook up a manual choke pull for use in cooler temps or if needed to slightly richen it up when cold. I have mostly just run my 66 in the driveway while I’m trying to get everything in the car sorted out. It does have heat to the intake but the choke has been deactivated in the past. I got an internal baffle plate for the choke that was missing just yesterday so I will have it back in service shortly. But, similar to my aluminum head and intake 427 Corvette, it runs rough until it warms up thoroughly. The Vette stutters when cold in lower gears until it gets somewhat up to temp and then it’s fine. It’s pretty wild but well tuned with a reworked Holley with a stock choke. The Hemi idles fairly well but noticeably shakes off idle and stutters a bit (somewhat like the Vette) which seems to go away once warmed up. I’m a bit doubtful the choke will affect this any but hoping. It just seems to take a little bit to build heat in these big lumps of iron and aluminum to where combustion smooths out.

 

[nitrohemi]

I think we get spoiled with the way our new fuel injected cars idle and run in cold weather we forget how these old cars run. I have a 5.2 magnum in an old truck with an air gap intake and n cold wether that think stumbles going up a hill until the intake is warmed up.

 

[64BEL]

My Hemi ran well when I first bought it, but eventually it ran a little rough - especially when cold. I rebuilt both carbs and what a difference that made. It ran great after that. The choke is disabled, but I really don't need it since it's not a 4 season car around here. The AFBs are really simple and easy to rebuild.

 

[AR67GTX]

I remember back in the late 60s and 70s, I would rebuild my carbs (at first an AFB on my Sport Fury and later my GTO’s Quadrajet and my Firebird TA Quadrajet) about every two years. Especially the Quadrajets, it seemed like after a year or two it would always seem to be falling off. But then I changed points and plugs and cap/rotors about yearly. I think we tend to forget all the regular maintenance that used to go into these cars, especially when they were driven 10 to 15 thousand miles a year. Grease jobs, coolant flushes, oil & filter ever 2,000 miles, gas filters, air cleaners, brake jobs, etc. I didn’t relax my 2,000 mile oil and filter changes until sometime in the 80s. I was doing that all the time. And younger members probably don’t have any idea.

 

[Dragon Slayer]

Interest that we come full circle on trying to richen this up when it seems it is actually jetted right. Easy to add an electric choke or manual right on to the hemi choke body. I would also say if it is not attached, not sure how the choke blade opens, though that would tend towards rich, unless it has been locked open. Then it definitely is lean when cold. Distribution of fuel on the hemi manifold is pretty important, hence the jet differences, distribution tabs, and even lack of secondary discharge on one venturi. Go rich at idle, especially with a cold plug (stock hemi plug) with today's fuel and you really risk loading it up and fouling plugs at any prolonged idle. My opinion/experience and as told by experienced hemi racer and motor builder. It is also interesting that the carter manual for some of the early hemis 66-67 gave 2 rod for going leaner 1 or 2 steps, but never gave a rod for richer. These were pre-emission carbs.

 

[AR67GTX]

Got my choke adjusted and put back together and bolted the carbs back down. Installed the carb linkage and checked it out - no surprise, it was set way too short by whoever built this car and when the front carb is wide open, the rear carb secondaries were about half open. Had to lengthen the linkage about 3/8”. Now fixed. Still have to get my wife to get in and mat the pedal to make sure it opens everything up.

It’s amazing what some people do to these cars. I remember the throttle cable on my 67 GTX would only open the carb up about 3/4 when I bought it. Kick down was out of adjustment. Took me a year to get all the loose bolts in the car tightened up.

 

[Richard Cranium]

I just stumbled upon this thread and all of the useful info that @Dragon Slayer has given. My 1966 Hemi (automatic) has an issue where if I'm slowing down and taking a turn onto another street, the engine runs rough & wants to stall, so I have to two foot it until I complete my turn. After a couple seconds, it clears up. To me, it seems like it may be flooding out.

The car had a big cam I it when I bought it, but I got rid of that and installed a stock street Hemi cam. My thoughts are that the jets were changed when the big cam was installed, or the floats are too high. Being that it's winter now and having nothing to do today, I took the carbs off this morning and took out all the jets. The metering rods check out to be stock as do all the jet sizes except for one. See below .......

Is this 380 jet the same as the .080?

I checked the float heights and they were at 8/32" & the float drop is 3/4". My 1969 service manual says the float height should be 9/32". Would 1/32" high be enough to create create that condition?

Also, where is the low speed jet located?

Thanks in advance.