There is no doubt that a properly chosen Holley carb will out perform a majority of Carters. But, here are some ways to get the most out your Carter and a few strengths that they have for a daily driver.
First the strengths. The float bowls are common to both primary and secondary, so the fuel is always fresh in all circuits. They have no power valves to blow if you get a sneeze. Another, there are no gaskets below the float bowl. Everything you do to it can be done with it on the car. The bodies are thick and stable, but a phenolic gasket is a good idea, just like any other carb. They are a very simple carb to tune, and they stay that way for a long time. Also, rebuilding one is as simple as a lawn mower carb. One gasket, an accelerator pump, maybe a float (never found one I couldn't solder up though) and possibly needle and seats. In my case I have gone as long as 7 years without even touching the carb.
Weaknesses. You have to pull off the top of the carb to set the floats, but it isn't bad. They lack some venturi differential that the Holley has and are much more dependent on the transition circuits. This has one ramification that cannot be denied. If you have less than 10 inches of vacuum and your fuel pressure drops to less than 4 lbs or rises more than 5.5, these carbs are out of their range. They want 5 lbs for fuel constant and work best on 13 hg of manifold vacuum and up. You can tune some things to make them more forgiving, but a Holley is just a better choice there because they have more little things to tune for low vacuum engines and are better vented for bowl pressure fluctuations.
Now, throw the hype out the window. These, like any other carb are not bolt ons. Pull it out of the box and pull the top off. Flip the top over and use a 5/16" drill bit under the floats. They say 3/8" I know but trust me, it likes 5/16" better. Don't pry around adjusting it on the needle and seats, pull the pin and adjust it off of the top. Get them square and even. Flip the top back over and measure the drop. I use a caliper and set them to 1.25". The tang at the needle and seat are where you adjust this, again square and even. If the float bowl gets that far down you need the needles nearly jumping out to fill it back up. If you are getting fuel delivery issues, it is better to increase the volume of fuel pumped than to go to a bigger set of needle and seats. The smallest you can get away with is good, it prevents hot start problems because it can resist standing fuel pressure and percolation better.
Next put the carb top back on, put the accelerator pump linkage on in the middle hole. Back out the idle stop screw (counting the turns) until it is no longer engaging. Bend the link rod until you can measure from the tip of the accelerator pump plunger to the carb top at 3/8". Turn the idle screw back in, and it should be right at 11/32" It needs to be here because if you get the pump plunger too high, and the float level too low, the pump will sip air. This is what causes many stumbles. The relationship of float height and accelerator pump are key elements. If you go with the 3/8" float height, drop the plunger height to 1/4"-5/16" with the carb screw in. You may still get an occasional stumble. There are bigger and smaller squirters available, and they can bring into line fine tuning of both volume and timing of the shot. They can put more fuel volume in for the plunger height setting and throw in bigger or smaller drops which equates to time perceived in duration by the intake signal and velocity. The quickness of the shot is in the lever. The closest to the body is the quickest, and vice versa. Between the squirters, pump height, and lever hole placement, you should get a clean signal with no stumbles, and with the right float height and drop, with steady fuel pressure, you should eliminate the "second pump strike" stumble these carbs are known to have. There is a check ball in the pump system that may need optimizing to the passage, but with all of these other things dialed in that would be more the exception than the rule.
The big transition circuit is the power springs, and they have to be tailored to your vacuum. A lot is made of jetting and rods, but the calibrations of jet and rod in most cases are pretty darn close, when you have the right carb for your displacement. One thing though, that 5hg orange spring is really only good if you are down at 10"-12" vacuum at idle or care about gas maileage. I have always just stuck the pink 7" spring in there in any of them, and its like grabbing a moving bus. This is confusing sometimes, but the vacuum pulls the needles down into the jet, the stronger spring lifts them sooner as the vacuum drops. The pink springs are long, but also more supple than the others in the pack, so they seem to dither the rods more sensitively. The rods by the way are two step and not tapered like a Qjet (Thanks BarryR for pointing that out to me) so they don't work much differently than a Holley power valve. So, expect the mileage to drop, the more aggressive you get with the springs.
Next, the secondaries. The AVS being sold today is not the same AVS of yesteryear. This one has boosters instead of dump tubes. The top can be interchanged with an AFB, but you have to pull the boosters off and remove the counterweighted air door and put the boosters back. It is nice to adjust them with a screw, but it isn't really too adjustable like say a Qjet is. Its OK but I have done just as much doing the following. If you are running an AFB as is you can adjust the secondary engagement by removing the counterweighted air door drilling holes or adding weight to the counterweight If it bogs, add weight, if its flat, drill a little in it. If you overdo the drilling or need to add weight, a lead ball fishing sinker can be hammered in there and you can try it again. It is a process to do this, because it will likely need more than one try to get it right.
Another small issue. There is a secondary lockout rod on the drivers side. If this link over centers and you hit the secondaries, it will close the choke, so check this before you install it, open the choke wide open, open the secondaries wide open and be sure the rod doesn't bind. You wouldn't believe how many of these carbs I have laid hands on that this was the only problem. Well that and having someone just bolting them on out of the box :D
So is it worth it? It has been for me. I have tuned these carbs since they were OEM and I was little kid. I have had great luck with them through the years. I hope if you have one, that the above info helps. If you want more, Dave Emmanuel has a really good book on all Carter carbs, and I have a good friend who was an R&D engineer at Carter who developed the Competition Series, and was there back when they were developing the Thermoquad for the OEMs. I would be glad to ask him anything that I might not have run across through the years.
First the strengths. The float bowls are common to both primary and secondary, so the fuel is always fresh in all circuits. They have no power valves to blow if you get a sneeze. Another, there are no gaskets below the float bowl. Everything you do to it can be done with it on the car. The bodies are thick and stable, but a phenolic gasket is a good idea, just like any other carb. They are a very simple carb to tune, and they stay that way for a long time. Also, rebuilding one is as simple as a lawn mower carb. One gasket, an accelerator pump, maybe a float (never found one I couldn't solder up though) and possibly needle and seats. In my case I have gone as long as 7 years without even touching the carb.
Weaknesses. You have to pull off the top of the carb to set the floats, but it isn't bad. They lack some venturi differential that the Holley has and are much more dependent on the transition circuits. This has one ramification that cannot be denied. If you have less than 10 inches of vacuum and your fuel pressure drops to less than 4 lbs or rises more than 5.5, these carbs are out of their range. They want 5 lbs for fuel constant and work best on 13 hg of manifold vacuum and up. You can tune some things to make them more forgiving, but a Holley is just a better choice there because they have more little things to tune for low vacuum engines and are better vented for bowl pressure fluctuations.
Now, throw the hype out the window. These, like any other carb are not bolt ons. Pull it out of the box and pull the top off. Flip the top over and use a 5/16" drill bit under the floats. They say 3/8" I know but trust me, it likes 5/16" better. Don't pry around adjusting it on the needle and seats, pull the pin and adjust it off of the top. Get them square and even. Flip the top back over and measure the drop. I use a caliper and set them to 1.25". The tang at the needle and seat are where you adjust this, again square and even. If the float bowl gets that far down you need the needles nearly jumping out to fill it back up. If you are getting fuel delivery issues, it is better to increase the volume of fuel pumped than to go to a bigger set of needle and seats. The smallest you can get away with is good, it prevents hot start problems because it can resist standing fuel pressure and percolation better.
Next put the carb top back on, put the accelerator pump linkage on in the middle hole. Back out the idle stop screw (counting the turns) until it is no longer engaging. Bend the link rod until you can measure from the tip of the accelerator pump plunger to the carb top at 3/8". Turn the idle screw back in, and it should be right at 11/32" It needs to be here because if you get the pump plunger too high, and the float level too low, the pump will sip air. This is what causes many stumbles. The relationship of float height and accelerator pump are key elements. If you go with the 3/8" float height, drop the plunger height to 1/4"-5/16" with the carb screw in. You may still get an occasional stumble. There are bigger and smaller squirters available, and they can bring into line fine tuning of both volume and timing of the shot. They can put more fuel volume in for the plunger height setting and throw in bigger or smaller drops which equates to time perceived in duration by the intake signal and velocity. The quickness of the shot is in the lever. The closest to the body is the quickest, and vice versa. Between the squirters, pump height, and lever hole placement, you should get a clean signal with no stumbles, and with the right float height and drop, with steady fuel pressure, you should eliminate the "second pump strike" stumble these carbs are known to have. There is a check ball in the pump system that may need optimizing to the passage, but with all of these other things dialed in that would be more the exception than the rule.
The big transition circuit is the power springs, and they have to be tailored to your vacuum. A lot is made of jetting and rods, but the calibrations of jet and rod in most cases are pretty darn close, when you have the right carb for your displacement. One thing though, that 5hg orange spring is really only good if you are down at 10"-12" vacuum at idle or care about gas maileage. I have always just stuck the pink 7" spring in there in any of them, and its like grabbing a moving bus. This is confusing sometimes, but the vacuum pulls the needles down into the jet, the stronger spring lifts them sooner as the vacuum drops. The pink springs are long, but also more supple than the others in the pack, so they seem to dither the rods more sensitively. The rods by the way are two step and not tapered like a Qjet (Thanks BarryR for pointing that out to me) so they don't work much differently than a Holley power valve. So, expect the mileage to drop, the more aggressive you get with the springs.
Next, the secondaries. The AVS being sold today is not the same AVS of yesteryear. This one has boosters instead of dump tubes. The top can be interchanged with an AFB, but you have to pull the boosters off and remove the counterweighted air door and put the boosters back. It is nice to adjust them with a screw, but it isn't really too adjustable like say a Qjet is. Its OK but I have done just as much doing the following. If you are running an AFB as is you can adjust the secondary engagement by removing the counterweighted air door drilling holes or adding weight to the counterweight If it bogs, add weight, if its flat, drill a little in it. If you overdo the drilling or need to add weight, a lead ball fishing sinker can be hammered in there and you can try it again. It is a process to do this, because it will likely need more than one try to get it right.
Another small issue. There is a secondary lockout rod on the drivers side. If this link over centers and you hit the secondaries, it will close the choke, so check this before you install it, open the choke wide open, open the secondaries wide open and be sure the rod doesn't bind. You wouldn't believe how many of these carbs I have laid hands on that this was the only problem. Well that and having someone just bolting them on out of the box :D
So is it worth it? It has been for me. I have tuned these carbs since they were OEM and I was little kid. I have had great luck with them through the years. I hope if you have one, that the above info helps. If you want more, Dave Emmanuel has a really good book on all Carter carbs, and I have a good friend who was an R&D engineer at Carter who developed the Competition Series, and was there back when they were developing the Thermoquad for the OEMs. I would be glad to ask him anything that I might not have run across through the years.
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