Rosiesdad

Carter idle speed problem

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Hi all, My latest problem is that I get a rough idle as I open the throttle plates any more than one screw turn from fully closed. The idle is nice and smooth up to 350 RPM. Beyond that throttle opening, one of the main jets begins to kick in and it goes rough before I get to normal idle speed. I just installed a carb rebuild kit. It appears I need to keep the metering rods from lifting but the vacuum is too little to keep them on their jet seats. What do I need to do here? I set the timing and dwell already. Any Carter WCD experts out there?

THanks!

Phil

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The idle mixture control screws (2 of them) should each be out approximately 1 to 1 1/4 turns from lightly seated.

The curb idle screw (only one, which rotates the throttle plates) should be out only enough to give the proper idle RPM. This screw out more than one turn would have the throttle plates sufficiently open to effectively remove the idle circuit, and force the carburetor to operate on the main metering circuit.

Jon.

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I agree with Jim, look for a vacuum leak: do you have a gauge? If you don't, get one, it will become your bestest friend when you work on your car.

Your manual will show you how to read it too; if I remember correctly, it should be pulling about 18-inches of vacuum?

Do you know the trick of spraying the carburetor base with water or light oil while it's running to see if you have a gasket leak? It should smooth out while you're spraying and become erratic when you stop if you have a bad carb gasket. Also, disconnect ALL vacuum sources to the carb and manifold and replace them one at a time to try to figure it out.

Jaybird

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Rather than worry about the metering rod adjustment (which should be "in the ball park" UNLESS somebody's already been messing with them!!!), first get the idle mixture and speed screws adjusted as carbking stated. ONLY when you get that base adjustments successfully completed AND the engine's got normal vacuum at idle, only THEN do you mess with the metering rod adjustments (which are for the MAIN system rather than the IDLE system).

Did you make sure the air bleeds in the venturi cluster are clean and open? If one of them is clogged, it'll make that side of the carb go "full rich" without affecting the other side of the primaries. Even if the air bleed diameter is reduced (from non-removed deposits), it'll make that side richer than it needs to be. IF there might be "hard deposits", the carb cleaner soak will probably not remove them, which means "manual removal" with a suitable reamer device . . . WITHOUT removing any metal. There should be one bleed for the idle system and one bleed for the main system . . . on each side.

Just some thoughts . . .

NTX5467

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All good info guys. Thanks! I do have a vacuum gauge but have not used it yet. If I had a vacuum leak my ldle speed would be high rather than too low... Correct? How else can I increase the idle speed? I did not remove any plugs in the carb or make any adjustments. Just soaked cleaned and blew out the passages best I could. I did note that one venturi main begins dribbling gas before the other as I open the throttle plates. My rebuild kit did not come with metering rods or the jets they seat in. I wonder if my timing chain is stretched? What else could make it idle at 300RPM???

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Vacuum leaks can be tricky, and make your can run differently, but they usually make the engine run much slower than usual, thunkata-thunkata-thunk... What's your tail pipe doing? Get it started and put your hand about an inch away: is it smooth and strong, or erratic like your playing bongos? Bongos is pretty indicative of a leak, and that unburned fuel smell is also a good indicator too; it's a really bad smell, you know headache casing, stomach sickening, yuck!

Jaybird

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If you've got fuel dribbling from a venturi as the air flow increases past what the idle circuit will supply with higher rpms, that's a sure sign you've got clogged/restricted air bleed on THAT side of the venturi cluster. It doesn't take much, as I found out when I was experimenting on a Carter BBD several years ago. Rebuild kits will NOT have jets or metering rods in them, just . . . gaskets, ball checks, accel pump, needle and seat for the float(s), and probably a measuring gauge to set the float adjusment with.

I can't imagine a smoooth idle at 300rpm, much less an engine idling that slow anyway. 600rpm, much more likely . . . possibly a "situation" with your dwell tach setting?

If a timing chain stretches too much, it'll fall off of the sprockets. If the timing chain was stretched, it'd run really flaky, be hard to start, and possibly backfire somewhere along the line. Manifold vacuum would be significantly lower once it did start and run.

Thanks for the update!

NTX5467

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Hmmm. I assume the "Air bleeds" are passages that are plugged with lead or whatever is used. Where would I get that stuff to re-seal them after I drill out and clean the bleeds?

It's possible my tach is off. I will try another. Idle is VERY smooth. Also will adjust the timing for maximum vacuum and then back off a bit.

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Do you have an air compressor? You may want to pull the carb and carefully disassemble it, then blow out every orifice there is with compressed air. After which, put something like carburetor cleaner in them, and probe around with a piece of copper wire, or a nylon bristle an then blow it out again.

Didn't you say the ball in the check valve was stuck? There could be lime/calcium deposit in there, from water.

Again, just a thought,

Jaybird

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The air bleeds I'm referring to are drilled orifices in the venturi area which are not open on one end and sealed on the other, but should be open "straight through".

In my "normal" carb rebuild process, I discovered that I could get good results with using spray carb cleaner rather than the "dunk and soak" method. Usually, the carbs on my cars didn't really require that much cleaning as there was usually just a thin coating of "deposits/residue", if even that much. What might have been in the bottom of the float bowl would just wash out, then I'd spray through the various passageways to ensure they were clean and open. At least that's what worked for me until I had an issue with a Carter BBD 2-bbl on my then-recently-purchased 1980 Chrysler.

I took the carb off and did the spray routine on it, then reassembled it. Everything should work as it was all cleaned and such. The performance issue it had was that it would not idle, but ran fine on fast idle or when it was on the "main system". But if you backed out of the throttle, even at highway speed, the engine would die as the idle system was not flowing enough fuel.

In that venturi cluster (which is one piece and removeable -- with finesse -- from the topside of the carb on the engine), each side has a smaller brass tube and a larger diameter brass tube with holes drilled in the side of it. The smaller tube is the idle feed tube, with a corresponding small air bleed on the top surface of the venturi cluster. The top hole was open, but there was a significant obstruction in the fuel feed section of the tube . . . at the bottom.

In the carb spec section of my Chrysler service manuals, I'd seen a "low speed jet" spec, but didn't know where it was. I knew about main jets and such, but not "low speed jet". I noticed that it could vary a great bit in size, from engine to engine and emissions calibration. I finally found an illustration which designated where it was . . . in the bottom of the small, brass idle feed tube.

So, I carefully removed the venturi cluster again and started to probe the lower end of the idle feed tubes. On one side, as I probed it with a bent-wire spark plug gap gauge, the hole got bigger with several progressively larger wires. These same tubes had seemed to flow freely with the carb spray, so it appeared they were fine, but the car still would not idle after the residual carb cleaner was pulled out of the tubes at idle. Now that I knew what the max size of gap gauge wire that would not fit the hole, I went to a hobby store and found a hand twist drill set with assorted sizes of drill bits in the range that I needed. When I got back to my shop, for good measure, I used the smaller drill bits to ensure the tube's internal diameter was cleaned of any hard deposits . . . until I got to the size that just "got brass". Then I stopped and again flushed the tubes with spray carb cleaner, from each end.

After carefully reinstalling the venturi cluster, I anxiously started the engine and it idled on its on . . . at base idle. I fine-tuned the idle mixture and speed and called it a successs!

In the process of checking things out, I also got some small diameter wire and inserted it (with a right angle bend at the top so it would not be permanently installed into the air bleed hole's tube!) into one of the air bleeds. As soon as I restarted the engine, fuel dribbled from that side's venturi. Less air bleed diameter results in a richer mixture in that particular side's fuel circuit . . . larger air bleed diameter does just the opposite, makes it leaner.

There are drilled orifices and passageways in the carb's fuel circuits, some with staked balls or cup plugs to seal the machined passageways, which are the main calibration devices for the carb. To further fine-tune the carb, the air bleeds are used as a means to get fuel to flow where it needs to, but also as a means to slightly richen or lean the mixture for a better operational "fit".

In my case, the car came from an area which had a prevalent brand of fuel which had issues with hard, white carb deposits in the early 1980s. We saw some of these in our shop--only fix was a new or reman carb as the carb cleaner would not faze the accumulated deposits.

NOW . . . for some additional things about the "staked ball plugs" in the main fuel supply passageways of carburetors. It has been observed that with the increased ethanol content of many of the modern gasolines, the additional sealing compounds used when the carb was originally built, for the ball/cup plug seals CAN be sufficiently degraded to allow these balls/cup plugs to siginficantly leak and cause serious underhood problems AND damage! It happened to one of our chapter members on his 1958 Super (a "The Bugle" article resulted, as I recall), plus he'd already seen the same thing happen on a middle-'50s Cadillac at a weekend cruise event! Luckily, it happened with the Cadillac as the owner turned into a Saturday night cruise event and many fire extinguishers were suddenly available. SO, if you see any fuel "patterns" on these areas of your carb, clean and seal these with a good fuel and ethanol-resistant epoxy sealer, probably the same sealer you'd use to seal the secondary well seals on Rochester spreadbore QuadraJet carbs. Check out the prior article in "The Bugle" for more information on this subject!

Just some thoughts . . .

NTX5467

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Wow. Thanks guys. I do have a compressor and will re-visit the air bleeds this weekend to see if I can balance the beginning of the main jet flow. And I will look at the vacuum with a gauge to see if there is a leak somewhere.

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Results.... One of my tachs is bad. Now I can adjust idle to 450 on the idle circuit. Manifold vacuum is 20". There is a little flat spot just off idle. My vacuum advance is not moving much and there appears to be a slight leak through the diaphragm. I don't know if this is normal or not. One thing that is definitely not normal is that one side of the carb in not really responding to the idle screw much and I can close that side and not kill the engine unlike the other side. Looks like I need to re-clean the idle circuit and air bleeds. Hope this will eliminate the off idle stumble.

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If the idle speed is set too slow, it closes the primary throttle blades too much. It will idle fine, but have an off-idle flat spot. When the carb is at idle, it will get fuel from the idle circuit, for the most part (i.e., through the round hole the idle mixture screw sticks into and modulates). Right above the idle mixture hole is a vertical slot, which is the "transition fuel" supply. If it's closed off too much, there will not be any fuel "ready and waiting" there. It helps cover the time between when the idle circuit can't flow enough fuel for higher rpms and when the main system starts working and flowing fuel during normal off-idle operation.

Seems like the bottom .020" of the transition slot should be BELOW where the throttle plates are when the carb is at base idle? Or is it '040"? This dimension comes into greater play when long-duration camshafts are installed and the stock carburetors are used with them. When you have to drill a hole in each of the primary throttle plates (for additional air supply) so that the relationship between the primary throttle plates and the transition ports are maintained. If you try to do it without drilling the holes, you can't ever set the idle mixture and get the carb to act right (not to mention the excessive hydrocarbons from this situation!!!) when either idling in gear or coming off-idle cleanly. I mention this to point out how critical some of these "little things" might be, knowing that you have "stock everything" in your motor.

My "shade tree" method of setting idle speed goes like this . . . or at least back when we had decent gasoline. I would baseline the idle speed and mixture settings (after checking the point dwell reading and base ignition timing!). Then I'd put it in gear, after firmly planting my foot and depressing the brake pedal, to see how smooooth the engine would idle. If there was any roughness, then with the parking brake firmly applied and check to make sure the vehicle would not move, I'd go back and put my hand up to the exhaust pipe. If I felt individual pulses, that meant the idle speed was a little too slow. IF I smelled hydrocarbons on my hand, that meant the mixture was too rich. Then I'd tweak the idle speed to the point that the exhaust pulses just smoooothed out and also re-tweak the idle mixture adjustments. The end result goal was a smooooth continuous gas flow out of the tail pipe and minimal hydrocarbon smell on my hand (after I washed it after the prior check). I'd usually try to set the idle mixture to "lean best idle" or for a 20rpm drop from max rpm with each idle screw (I got that tip from the emissions decal on our 1969 Chevy C-10 350 V-8 pickup). It worked for me, but proceed at your own risk!

In the earlier times, vacuum gauges were much more common than electronic tachometers (other than in BIG Sunn Equipment "machines"), much less the smaller ones we have now. This is one reason that most of the service literature for the 1960s and prior was oriented toward vacuum gauge readings, but also had the electronic "shop" tachometers in the service literature, too. If you know what you're looking at, a vacuum gauge can tell you more about the internal condition of your engine, but so can noticing what it's doing and when . . . at least for me.

Just some thoughts,

NTX5467

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Great tips NTX. After I get the second idle screw to respond at ALL I will try them out. At this point there is unburned gas smell and I HEAR a sound change as I screw the mix screw but it doesn't affect idle. The first barrel works normally though. Must have a plug here somewhere!

Thanks !

Phil

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SUCCESS!!!!! I went back through the carb and found one of the two "Low speed jet" tubes was partially restricted. I cleaned them both out and adjusted the metering rods as per the instructions. She idles beautifully with not a hint of hesitation. Thanks to all for the advice!

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I know this is an old post, but I see lots of great advice, and some not so good in here. NONE of this stuff should even be attempted before ensuring that the PCV is working correctly. Since about 30% of idle air is via the PCV, anything at all wrong, plugged, stuck, or otherwise will kill proper idle. That's from Buick, not me :)

 

I had a helluva time getting idle setup and just off idle working well after rebuilding the AFB on my 425 Nailhead. Funny thing is, NAPA had provided me with a new PCV. Just would not settle down. Ordered a new PCV online from Rockauto, and though identical in appearance, it did the trick. Smooth, easily idled at spec (550 rpm with A/C on).

 

Hopefully this saves someone ...

 

 

IMG_8812.JPG

Edited by RivieraRon (see edit history)

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