Misfiring after Carter 2bbl install

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73SlantSwinger

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Need some advice on what direction to go with this thing. 71 Swinger 225 40K miles car was changed over to 2bbl setup before I bought it but it had a cheap aftermarket 2bbl on it. Previous owner supplied the Carter BBD but it needed a rebuild. I had it rebuilt and put on but now the motor shakes and misses. It idled a lot better with the other carb on it, although the idle was a little low. Weird thing is it drives down the road nice and pulls good, don't feel any missing until it almost comes to a stop slowing down or idling still. Throttling up you can hear the miss and the motor shakes really bad. Pulled and checked all the plugs (NGK ZFR5N) all were a tan color. Cap, rotor, plugs, coil, wires are all new. Timing checked, valve lash adjustment done, vacuum lines all look brand new. I even pulled a wire off each plug when running to see if there was a difference in each cylinder, strange that the back two cylinders by the firewall didn't change sound at all when pulled off. Should I check compression next?
 
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Idle circuits and idle richness jets are plugged yet.

Overhauling a carb is one thing, but running the carb through an Ultrasonic cleaner is a good way to clean out those small passages and jet runs.

Make sure everything is cleaned out well and that air will blow through all the small passages.

New replacement carbs online can be a good solution, without the hassle of trying to find the plug.

Screenshot_20230116-105341_Gallery.jpg
 
If you feel lucky you can pull out the 2 lower idle jet screws, spray some carb cleaner in the holes and blow them out with compressed air.

Polish up clean the ends of the idle jet screws and give it a go again.

A small parts tag wire can assist in cleaning out the plugged idle ports.

Readjust your idle speed richness when done.

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Also, don’t forget to check the base gasket as a leak there can cause the same issues.
While at it- check for an intake leak as well. I realize you did not change the intake and it ran fine but it will eliminate that possibility of a head to intake leak. Maybe the other carb was so rich it was compensated for the leak.
 
Have you put a vacuum gauge on it?
 
Try adjusting the idle mixture screws. One at a time. Screw IN until rpm drops, then OUT until you have max rpm/smoothness. If nothing changes, then that indicates a blockage/malfunction with that side of the carb.
 
I played with the air/fuel mixture screws yesterday afternoon with no luck. They were already dialed in where they should of been. I reached out to the carb rebuild shop and asked them how often they have an issue with something coming back on them as they have great reviews, etc. He said it happens every once in a great while but said if I brought it back to them they would take care of it.
 
RustyRatRod said:
Are you actually gonna make me ASK what the vacuum readings were. Geez.
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No. Don't. Nevermind. I shouldn't have even had to ask that. This is another stupid tooth pulling thread. I'm done.
 
Sounds like it is idling lean. My 1969 slant w/ Holley 1920 did that for decades. Didn't suspect the carb since tried 2 rebuilds, so spent years searching for a vacuum leak, even replacing the intake manifold. Finally, the 4th carb made it purr like a kitten at idle. As said, the idle circuit in the carb can clog, especially in a 1920 with sealed metering block and not all rebuilders address that. Try blocking off vacuum consumers like the PCV valve, cabin controls, and brake booster, then suspect the manifold and carb gaskets. These days, I install an O2 sensor in the exhaust of my old Mopars so no longer flying blind.
 
Just cherry pick the best parts of all and build one "super" BBD
 
I ended up dropping it off with a local guy that works on my truck to check some things out, I can't remember what the vacuum readings were, it's been there since Monday, I'll post hopefully soon with what he finds out, I know they're really busy for a small shop....
 
What BBD do you have? Can't really tell on these carbs if they don't have the metal tag from the factory. I call all of the old ones "toilet bowl," and the newer style "tin tops." The metering rods and jets are totally different. The old ones are really thin rods. More like tiny sewing needles. Pretty fragile. The older style are "air metered" IIRC. I have no idea what that means. :p

The Super6 has a different base plate for the idle circuits. They also take a different carb to intake gasket. The easy way to tell if you have a Super6 is the PCV tube coming out of the base plate. Small blocks are straight, the 6 has a sweeping 90 elbow. Side by side the 6 has a thicker wall on the front side of the choke tower. The venturi is smaller for them.
 
73SlantSwinger said:
It's got the PCV elbow coming out of the base plate I remember that from when I checked the PCV valve itself....
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Yeah they all do.
The difference in whether it was originally a /6 or a V8 model is whether that tube is short straight and stubby or longer and curved.
I have a V8 BBD on my /6 and so far runs great.
I only had to switch the lever on the choke shaft where the electric choke rod connects, those are different between the 6 and v8
 
Yeah but it didn't seem like the other guy "got" it like he thought that some BBDs didn't have a pcv nipple at all
 
If the intake Manifold is a factory dicast aluminum unit make sure it is completely sealed with paint because they have porosity issues that make tuning them a headache. I chased lean issues for a while until I figured that out.
 
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