Do I need to tune/rebuild my carb?

[Secret Chimp]

For the last several tanks, my 67 Coronet (318, Carter 2bbl, pretty sure it's a BBD, 727 transmission) has only been getting around 12 mpg. That's with mostly byway driving with some highway thrown in. I never see any unburned fuel out of the pipes, but the ends of them are pretty sooty, and it's pretty stinky even for a non-smog car.

It always starts within a few cranks, usually without having to touch the gas, and it seems to have good power throughout. I'm not laying rubber or anything but gives a good shove from a standstill. The only odd behavior it's given me is sometimes I need to jab the gas to reset the choke again after driving it for awhile and then taking it out of gear (unless that's normal). The ignition has been upgraded to a Mopar orange box so I'm assuming timing is fine since it's not the original setup.

I'm new to carbureted cars in general and not sure what's typical for these motors, so I'm not sure if I'm doing fine or if I ought to throw some money at this area. It doesn't look like the carb has ever been touched. Is it worth a rebuild or some tweaking in search of another 2 or 3 mpg and better running or does 15 mpg just not happen even on a two barrel setup?

 

[Coronet 500]

You should get much better fuel mileage. I always liked these carbs, super simple and reliable. Hopefully you have the I.D. tag under one of the top screws, take that number when you go to get your rebuild kit.

 

[Meep-Meep]

So the choke stays on after driving it? The 318's were prone to the exhaust heat crossover in the intake and heads getting plugged up with carbon so it's possible you have that problem. When that happens the intake does not heat up soon enough to open the choke. It's possible your carb needs attention but before you tear it apart have a look at the choke function.

 

[Coronet 500]

Excellent point, had this happen to me twice. You can go in through #4+5 exhaust port with a wire to feel if it is clear. make sure that valve is closed before you start so no debris drops in the cylinder. Good call Meep!

 

[Secret Chimp]

Would that cause poor fuel economy or just the car idling too fast after being driven?

 

[Coronet 500]

Both, the colder manifold will reduce fuel atomization and if it's one of those single plane 2bbl manifolds, even distribution to cylinders too. Remove your plugs and see how they compare in colour.

 

[Patrick]

You should snap the gas pedal and release the choke before you aver put the car in gear to start with. If you have an owners manuel it will tell you this. All the gas we get now is 10% corn whiskey and we lose about 3 mpg. Its also denser so it runs richer than before. Turn your mixture screws in 1/4 turn each. Let me know if this helps before you go throwing money at the car.

 

[74sundance]

So the choke stays on after driving it? The 318's were prone to the exhaust heat crossover in the intake and heads getting plugged up with carbon so it's possible you have that problem. When that happens the intake does not heat up soon enough to open the choke. It's possible your carb needs attention but before you tear it apart have a look at the choke function.

Nice tip :blob1:

 

[Secret Chimp]

The choke seems to be working OK. It falls shut with the car off and opens after the engine catches. I'm not sure if the well choke is working right but it moves freely, and the vacuum actuator holds the choke open during idle. The heater valve in my exhaust manifold seems to be moving fine too.

It looks like this carb was installed later in the car's life, as it has no ID tags, and according to my service manual, BBDs were only used on 273s in 1967, and the 318 originally came with a Stromberg WW3 (though I'm not sure about that still).

Here are some photos of what everything looked like:

It looked like this after a cold start before I set it:

It looked like this after I set the choke after the same start when it had warmed up enough:

Notice the vacuum diaphragm is pulling on it:

Then here it is with the car shut off:

Not fully closed but pretty close to as closed as it gets:

 

[Secret Chimp]

I'm going to pull the carb and manifold at this point - it may just be a manifold temperature/carb vacuum load problem with how well the car runs otherwise.

 

[Meep-Meep]

From the picture the choke pull off is pulling the choke flap open too far. For a normally working choke you should have some spring tension with the engine cold and the choke pull off only should crack the blade open a little - against the spring tension. It's possible someone adjusted the choke so it stays open because the manifold never get's hot enough to heat the bi-metal coil that normally opens it.

With the engine running for about 10 minutes you should be able to check the temp of the choke well to verify if the heat crossover is blocked. If it's blocked it may be warm but not hot.

 

[Secret Chimp]

I got a funny rattle yesterday and today figured out that I had a piece of wire around the heater valve on the manifold - looks like somebody tried to wire it open.

I have some light tension on the choke but not a whole lot - it will kind of bobble back to its resting position with the engine off but it doesn't have the real springy snap-snap that the heater valve does. Is it supposed to be a pretty light tension or could my well choke thing be messed up?

 

[Secret Chimp]

Well I pulled the well choke out - here's what it's set to currently, how it holds the choke with the engine off, and the farthest I can close the choke by manually lifting it:

So this combined with the heater valve being wired open suggests I have a blocked heat crossover. Based on where it's set now, should I need to reset the well choke setting too (after I get the passage cleared)? What exactly is the spec for that? I haven't found it anywhere in my manual.

 

[wvmarv]

Dude you guys are way over thinking this problem. Put your choke back together. If it is wide open when hot and open about the width of a pencil or a little less on a cold morning it's working ok. You need to adjust the fuel mixture, unless the float is going bad, but if it doesn't stall going around corners I would say that's not a problem. She sounds like she's running way to rich (too much fuel to air ratio). Start by finding an rpm gauge. Hook it to the 2 coil leads. If it pegs the gauge to 0 rpm you have it hooked up backward. Find the mixture screws under the bowl on the front of the carb. they have springs on them. Start with either screw, slowly screw it in until engine starts to stumble. Then watching your rpm gauge very slowly turn the screw out until you reach the highest rpm you can. Then go to the other screw and do the same thing. I usually go back and forth a couple times just to get a feel for it and to get it as close as I can to highest rpm. once you have both done both sides set the idle to the specs on the emission sticker. If it still exists. If you don't have an rpm gauge you can do it by ear. But that takes more practice, because you don't want it too lean ( more air to fuel ratio). By the way the outboard screw on the throttle lever is the idle adjust screw. the inboard screw is for fast idle adjustment.

 

[daredevil]

hows the points. bad points effects mileage

 

[Secret Chimp]

I checked the idle and idle mixture tonight and something was out of whack.

For starters, the idle was set to closer to 600 rpm than 500 rpm (the initial drop to 500 rpm didn't make the motor terribly happy - it didn't threaten to die but it acted like half the cylinders weren't firing quite right)

I found that the right hand screw was really out of whack - the engine responded to changes on the left hand screw pretty quickly (I think I only got to maybe 1/3-1/2 of a turn before I got an idle droop at first go) but nothing happened on the right hand screw until I had it in a good turn plus.

The car seems to have more power now, both from initial step-off after I put it in drive and poked the gas to swing it around my cul de sac and from a rolling WOT run. I didn't encounter any weird engine behavior between cruising and light to moderate acceleration so the rest of the carb seems to be in sync with itself outside of the corrected idle mixtures.

I'll have to take it on the highway and really get it heated through so I can triple check my settings as right on point, but so far I seem to have fixed something. After I'm super certain I have it as close to spec as possible I'll drive a few dozen miles and fill it back up and see if I've made a noticeable difference in fuel consumption.

 

[Secret Chimp]

Well the car ran a little better after a retune of the idle mixtures but not enough to affect my economy much. I'm in the middle of rebuilding the carb and I'm going to pull the manifold to clean the heat crossover afterwards.

Someone who rebuilt the carb previously messed up a couple of things. For one, one of the jet tops was chipped to the point where I couldn't get it out (which is fine as the actual jet is unobstructed, but still), and one of the main vent tubes was completely unseated from the cluster; it was left behind down in its orifice in the main body when I pulled the venturi cluster. Evidently somebody rebuilt this without reading the part in all of the manuals that say "do not remove the tubes from the cluster."

I was able to reseat the tube in the cluster just fine but the inner idle tube was bent inwards a little so it contacts the inside of the main tube. I'm not going to attempt to re-remove that and reseat it for fear of snapping the tube.

The accelerator pump plunger's leather was pulled up off of the internal spring but that's likely just due to wear. I think I'm missing the spring seat, though. The spring didn't appear to have been clipped on the plunger arm, either (it just kind of sprang out separate from everything).
Edit: maybe I don't have a spring seat in the first place. Some later BBDs apparently just have the seat built into the bowl cover.

Still, what could a dropped tube have potentially been causing for me?

 

[Cazbah362]

Carb

Secret Chimp,
Hey I have one of these carbs off a 1972 Demon. Was rebuilt not too long ago and looks good. If you want it, I will pull it off and ship it to you, just reemburse me for the shipping -

If you want it - call me at 775-230-6984 during the day