Learned some good info about fixing Carter BBD Carbs
An old man at the local carb shop had offered to build a friend of mine an almost 'bulletproof' version of a Carter BBD for the Jeep we sold him. While hanging around his shop we learned a few things that I thought I would pass on for anyone who might not know about all of these fixes;
1. He said he'd permanently fix the problem with the upper and lower throttle shafts where they sometimes wear out the holes in the body allowing air to be sucked in creating a vacuum leak. He said you can drill out the holes in the carb to a specific size and tap in hardened bushings on both the top and bottom shaft holes which would eliminate that problem from ever happening. So anyone with carbs they think are junk because the holes are warbled out and sucking air around the shafts, that can actually be fixed. He said the throttle shaft is the same size as a Quadrajet carb for which bushings are readily available, then you just drill out the hole in the body to the same size as the outside diameter of the bushing and tap the bushing in with a light coating of thread lock on the outside of the bushing and wipe off any excess once the bushing is in place and centered.
2. He also said he'd replace Scotts yellow upper front upper vent with one of the new white ones. I have read where someone on the forum here said any carb with a white upper vent is a Chinese knock-off and not a real Carter. I mentioned that to the old man and he said not true. The top plates with the white plastic vents may or may not be made in China like so many parts now days but that has nothing to do with the rest of the carb and most of the remanufacture companies now use those if the original yellow one needs replacement. He said where most of the yellow ones are just a vent hole the white ones have a check valve built into them so vapor can be drawn out of the carb bowl but vacuum (and debris) can not be drawn back into the carb if the carb is something goes wrong or the hose is left unplugged for some reason. So just because you see a white vent on top of a carb does not mean it's a Chinese knock off carb and it's actually a better designed vent and the color is how you tell if you have the newer style, or you can test by trying to blow/suck through one as long as it's not got gas in the carb (unless you want a lung full of gas vapor when you suck in).
3. He said from 45 years working with Carter BBD's he recommends 0.039, not 0.032 for the size if your going to drill out your idle tubes. He said 0.032 can still get clogged sometimes but 0.039 will rarely get clogged but is still small enough it does not affect your idle. Go much larger and you may not be able to idle it all the way down properly though some of the remanufacture companies are now going as large as 0.042 but he's found that sometimes they are hard to get to idle down at that size.
4. If you have ever had an idle tube clogged up and cleaned it, or drilled them out larger (per #3) only to find that it did not totally fix your idle; Did you check the venturi cluster cap that the idle tubes plug into? Those have very tiny passages in them that run from the base of the idle tube over into the passage way the screws that hold the venturi cluster in place pass through and they are often known to clog or even get aluminum corrosion around where the brass idle tube sticks into the aluminum venturi cluster cap, particularly on one side. You can remove the idle tube and scratch around with the tip of a small drill bit (but do NOT drill into them) or use a straightened paper clip while keeping it wet with carb cleaner to clean out the cavity where the idle tube pushes in, then bend the last 1/4th inch of a small paper clip so you can stick it into the hole that passes sideways from that opening into the openings where the screws that hold the venturi assembly in place go thru because that's where they come out and with the 1/4th inch long piece bent over at the end of a paper clip you can slip it into that hole from both directions and work it around. You can check it by blowing through it once you put the idle tubes back in. Both sides should blow easily and evenly (one side not having more resistance than the other). He said this is a common problem even with remanufactured carbs, the companies do not take time to check this and he has seen newly purchased reman carbs that would not idle correctly because one of these passages was still clogged and did not get cleaned when they remanufactured the carb. Just dipping the carb and it's parts usually does NOT clean these out no matter if a reman place does it or the home builder does it. You almost always need to clean out the idle tubes AND the venturi cluster cap holes by hand even after the carb has been dipped.
5. Also be sure to check the two screws that hold the venturi cluster in place, they have holes in them both lengthwise and sideways that need to be cleaned though are much larger so clog less often. You can also fine tune your idle and running somewhat by changing these screws, they are available with 3 difference size holes in them and he has also seen remanufacture places get them mixed up and have a screw or even idle tube with one size hole on one side and another size on the other side because some hourly wage employee just grabbed two out of a bin and didn't check.
He even knows how to juice up a Carter BBD for people who have hopped up their engines but we didn't get much into that. Something about the Jets and Rods being interchangeable with the Carter AFB series so you can get a set of jets out of one of the 4BBL carbs if you have done a lot of performance enhancements to your engine and need to bring the Carter BBD up to match.
Hope maybe that info will help someone with Carter BBD problems so I thought I'd pass on a little knowledge that I learned from a very very old man who's been rebuilding carbs since the late 1950's and owns a very reputable local carb shop with his son and grandson. One of those old timers that it's not only entertaining to sit and talk with but you just can't help but to learn a lot from them. Hope maybe it will help someone with their Carter BBD problems.
~ SouthernGypsy's Jeeps ~
2004 Jeep TJ Rubicon 4.0L/Automatic
1983 Jeep CJ-7 Limited 4.2L/5 Speed