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1970 Carter AVS tuning kit

F4R/T

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Guys I’m preparing to take my refreshed 70 440 for a run / break in/ tune on a dyno. The engine has been rebuilt for two years but is pickled so I don’t for see any issues. The carb is correct for 70 ac car and I’m wondering if a new carter avs tuning kit will work with the old? The only difference I recall is the metering rods and jets, the 70 has three step rods with matching jets and the new ones I believe are two step. I had the carb restored by Harms and it looks great but I just want to have my ducks in a row when I get to the dyno and can tune it one the spot.
 
Guys I’m preparing to take my refreshed 70 440 for a run / break in/ tune on a dyno. The engine has been rebuilt for two years but is pickled so I don’t for see any issues. The carb is correct for 70 ac car and I’m wondering if a new carter avs tuning kit will work with the old? The only difference I recall is the metering rods and jets, the 70 has three step rods with matching jets and the new ones I believe are two step. I had the carb restored by Harms and it looks great but I just want to have my ducks in a row when I get to the dyno and can tune it one the spot.

YES....you are correct....the original metering rods are 3 step design....and some of the origional Carter carbs used a special 2 component step up pistons to operate the 3 step metering rods. This design allowed the rods to move between the cruise (large diameter) step and the middle step (intermediate diameter) smoothly affording a more even fuel flow thru the main jet. And yet allow the step up pistons and rods to move to the power step (smallest diameter) quickly, providing a even mixture transition.
Can 2 step rods be used with a origional metering jet?.....yes....providing the location of the actual metering orifice in the jet is in the same position AND the overall length of the step up pistons are the same AND the overall length of the metering rod is equal.
REMEMBER...the AREA presented with the rod in the jet is the factor not simply the diameter difference. To accurately represent the effective area, one must compute the area presented by the jet orifice minus the area of the rod when in the rod diameter is in the jet and the difference converted to effective area. Most people won't do this and just compare diameters. Area varies as the square of the diameter...A=3.1416 x radius^2 (of the rod or jet diameter).
IF converting to two step up pistons and rods, make sure to use the flat step up piston cover plates to insure that the power step of the metering rod remains in the jet. The 2 step rods will not be as accurate in metering the fuel flow, as the 3 step system, but with experimenting with the rod diameters and power piston step up springs, this will minimize any inaccuracies in the fuel delivery curve. Realizing that the Edelbrock tuning kits are more readily available compared to trying to find Carter 3 sep metering rods and step up pistons and springs, your point is well taken. Just my opinion of course.
BOB RENTON
 
Although I'm a Holley guy, this a great post. I may have reason to work on a Carter/Edelbrock carb. Was not aware the metering rods etc, had changed.
 
His AVS should have domed metering rod access covers on the top.
I believe you need flat covers to use a 2-step rod and jet.
It was a long time ago but I think I made covers for the AFB on my 66.
If I can find my strip kit (somewhere) or if I take a look at my old carbeurator I'll verify.
If you've made any cam change you will likely need to work towards changing to the 2-step rods.
There is also the step up spring and accelerator cluster... but you a dyno wouldn't be a place to play with those.
 
I used an Edelbrock tuning kit to do my AVS on the dyno and it was fine.
I believe it was the kit for the 500 cfm carburetor.
I used the Edelbrock metering rods and the Edelbrock flat covers.
 
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