Carter BBD issue on 1966 dodge Monaco


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Jun 10, 2023
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Hey guys, new to the forum I still gotta post some pictures.
but I have a question on my grandpas Monaco, Inherited it last year and have been trying to get it running good and only 2 things have been standing in my way, brake issues and carb issues.
I think I have the brakes figured but this carb is weird.
its a factory BBD but has a port on it I haven't seen anywhere online, there was a tube fitting from the drivers exhaust manifold that went into a port on the baseplate of the carb right underneath the fuel bowl.
that port fed a chamber that is sealed off and only has 2 much smaller ports that go directly into the Venturi and underneath the butterfly's.
best I can guess its some sort of Canadian option for ether warming it up or emissions?
I rebuilt the carb and plugged the port(the tube had broken off and it was plugged at the manifold anyways) and couldn't get it to run without setting the blades open a bunch or it wouldn't idle.
so today I figured I would pull the plug and just let it suck atmosphere it fired up good but when I put and engine vacuum gauge on it, it read erratically between 16 and 18 inches.
if I plug the port it raises to 16inches but is a more stable needle.

any info would be appreciated.
It might be Canadian, but that hose hook-up sounds "different". Where is the pcv valve hose hooked to? Is this on a 318 or 383?

In general, the pcv hose should go into the throttle body, usually on the front but could be on the back sometimes. Once in the throttle body, that passage will divide into two passages, one to each throttle bore.

The air for the crankcase ventilation system entered through a mesh-equipped ("hogshair") breather cap. On 383s, it was on the lh side and usually had some small drips of oil which would leave their presence on the top of the lh exhaust manifold. That might have been what somebody was trying to "fix"?

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That shows where the exhaust tube went in, the chamber and the two journals that went to the venturi below the butterflies.
The gasket blocks that chamber off from anything else it is its own specific circuit
What is the carb tag number? What I'm seeing does not match what's in the '65 or '66 Polara servcie manual (Chry FSM).
4124S is the correct carb for your application, but that carb was used on 318 engines until '73, I suspect that this carb has the additional port for emission controls on the later engines. There were no EGR valves or other elaborate emission controls on a Canadian built Poly engines. Photo of the 4124S available at 4124S Carburetor Info Page. A photo of the lead coming off the manifold would be helpful. Suspect it is a "service" modification. The port in question was not likely used on your '67 Poly, plug it off. If this is a later carb, it might be jetted too lean for the '67 poly. Carb idle settings are the two slotted screws on the the throttle plate. Set these to 2.5 turns out from a locked position and it should idle, you can do fine adjustments from there.

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Thanks for the pictures! Just wanted to make sure you had a Carter BBD 2bbl. On other carbs, that center port (which is plugged) is where the pcv port is, but on the BBD, it's on the back side and angled instead. So that mystery (to me) has been solved.

The pcv system should be hooked to a larger-diameter manifold vac port, as the orifice in the pcv valve is spring-modulated, so its not like a pure vacuum leak.

As to the vac readings, be concerned more with how steady the needle is rather than the amount the needle points to. If the needle is steady at 16, that's good. Manifold vac will be affected by altitude, too. Like about 1" Hg for each 1000ft of altitude above sea level. Which is one reason you will not find any reference to idle vac amounts in the FSM. And the readings apparently can be affected by how good the gauge is, too. BTAIM

From my experiences, using a dwell tach is much more accurate in setting idle mixture. If you set for the max rpm at idle, then the manifold vac is "max" anyway. Once at the "max" level, you can then lean each mixture screw until you get a 20rpm drop, then ease it back to where it was . . . on each screw. That is the check that you have the optimum mixture/speed adjustment at the spec hot base idle speed. "Lean best idle", it's termed.

Others, by observation, like to use vac gauges, but whenever I tried doing that (back in about 1969 or so), I never could get the readings others did on the gauge I'd bought. Using a manual printed back in the 1950s. That's when I determined that looking at how the needle moves is more important than the numbers it points to. In that "how the needle acts" CAN make the vac gauge valuable for diagnostics, too.

Thanks for the additional pictures,
when I got the car off my grandpas farm it has a stainless tube that went from that port to a port on the drivers side exhaust manifold, but the tube had broken off just over the drivers valve cover and was completely plugged at the manifold but the tube was clear so it was sucking air in to the carb. it was wrapped in heat wrap so I was assuming it was channeling exhaust into the carb.
when I rebuilt the carb last fall I blocked that port off but had a hard time getting it to idle which is why I have been running it open and so far was running good.
I will plug it off today and see if I can get the idle tuned up.
I think part of my eratic vacuum reading is I figured out the pct valve is shot last night. I had a sound coming from passenger side valve cover I couldn't figure out but figured out the pct valve is clattering.
In the middle-'60s time frame, some GM cars had a tube coming out of the rh exhaust manifold, BUT it was a tube which sent through the manifold, to bring heated air into the automatic choke mechanism (pulled through the tube by a vac bleed which pulled additional heat through the pipe, to the choke thermostat, and then into the venturi area). That might have been what was trying to be done, but in the wrong way.

The pcv valve is supposed to jiggle internally, as that's when you know it can work. Sounds like some vac pulsation harmonic was at play in your situation. Such pulsation and vac level variations (sharply defined pulsations rather than a "drifting" situation) usually come from a burnt or sticking valve. Might need to do a cylinder balance test to find the weak cyl (which can be quicker and easier to do than a compression test, by observation.

When you rebuilt the carb, was the spring under the power piston which runs the metering rods up and down? Just curious.

Please keep us posted on your progress.

its funny you mention something wrong with a valve, I wasn't Sure it was related but cylinder 6 has low compression. I tested them all and they were 160-170 but cylinder 6 was 125ish. I dumped a bit of oil in the cylinder and rolled it over to see if It was the rings and it didn't change at all so it has to be a valve issue. I suspect and seat issue after running nothing but regular unleaded(probably farmer fuel) since the 1970s.
before I found the pct snapping a bunch I thought it was a noise under the valve cover so I checked the lash and everything and couldn't find anything.
As CBODY67 mentioned that tube was for hot air. Ford. back then, used to move hot air from the exhaust to operate their chokes. The picture shows the back of an FE exhaust manifold where you can see the two outlets Ford used along with the chamber.