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Car Forum / Oldsmobile Cars / February 20041986 Cutlass Ciera - Carburetor Problems/questions (ADJUSTMENT SCREW)
Question: Where is the Primary Metering Screw? I have an 86 Cutlass Ciera with a 2.8L V6 w/Carburetor (NO throttle body). I need to adjust the settings on my carburetor to allow it to run smoother. The car currently stalls, hesitates, and runs at a low rpm on idle. I have removed the plug for the idle mixture to be set, and I have found and adjusted the idle set screw. I cannot find the primary metering screw on the carburetor to set the fuel mixture. I have a service manual which shows a blown-apart schematic of a similar carburetor but I cannot find the screw anywhere on it. Any help on identifying it would be greatly appreciated! I could even get a digital picture to attach if necessary as I have a digital camera at my disposal. Thanks! Steve Not sure there is such a beast. Off idle mixture is controlled by fixed oriface jets. The mechanical stop screw is the adjustment for the idle speed. The idle mixture screws will smooth out the idle if you have a rough idle due to lean mixture, but they are intended to run pretty lean at idle. You can back them out all the way and you can't get an over rich condition. If you've got a hesitation I would be looking at a few other things first. What's the outside temp where you are? If you're in a colder clime an inoperative fresh air pre-heat system will give you a pretty severe hesitation. This is that flex hose and it's valving that runs from your air cleaner housing down to a heat stove on the exhaust manifold. The thermostat in the bottom of the housing is set to maintain about 100 degree charge air, so if you look in the snorkel and see the flap open to the cold air you've probably got a problem there. If all that is working okay you want to check the accelerater pump circuit in the carb. Looking down the throat you should see a nice even steady stream of fuel from the nozzles as you open the throttle. If you get some throttle movement before seeing fuel spray, that's probably your problem. You should also be checking for vacuum leaks and ensure that ignition timing and advance is working correctly. There are very few maintenance type carb adjustments. Float level, all of your accelerater pump linkages, power valve etc are either not adjustable or set at carb build or overhaul. They never change so there's no need for any kind of handy screw type adjustment. If they start to not meter fuel correctly it's because some component is malfunctioning and needs cleaning or replacing. Steve too > Question: Where is the Primary Metering Screw? > [quoted text clipped - 14 lines] > Thanks! > Steve Hi Steve 2, thanks for the detailed reply. I do find that the rough idle and stalling is more severe in colder/wetter weather, so I'll check on that thermostat on the snorkle next. I recently read on the net that the idle mixture screw sets the air/fuel mixture for the first 3000 RPM, so having this set properly will make a difference to the performance in most of your city driving. Now b/c I read this on some website, I'm a bit scheptical. Do you know if this is true, b/c I would think the idle mixture would only make a difference to your actual idle.....(?) Steve > Not sure there is such a beast. Off idle mixture is controlled by fixed > oriface jets. The mechanical stop screw is the adjustment for the idle [quoted text clipped - 43 lines] > > Thanks! > > Steve Definately not in effect to 3000 rpm. It's not tied to engine rpm, it's dependant on throttle position. As the butterflies just start to open and airflow through the venturi increases the main system takes over. Idle mixture screws do nothing once the that happens. If the engine is still rough off idle I would want to be sure you're firing on all cylinders. Usually if you've got an engine that is noticeably uneven at say 1500 - 2000 rpm and it's fuel mixture related it's caused by a vacuum leak and is so severe that it won't idle at all. The air charge temp also won't cause a rough running. It'll run perfectly smooth but when you try to accelerate it will hesitate severely. Are you sure you don't have one cyl out? Set the idle quite high, 1800 let's say. Momentarily pull the plug wires off one at a time. If you encounter one that makes little or no difference when pulled off (or shorted out) you've got a problem with that cyl. Longer message needed for that diagnosis. Steve2 > Hi Steve 2, thanks for the detailed reply. I do find that the rough idle > and stalling is more severe in colder/wetter weather, so I'll check on that [quoted text clipped - 71 lines] > > > Thanks! > > > Steve Sorry for the late reply, been away for a few days. I'm pretty positive I dont have a a nonfiring cylinder because it doesn't 'run rough' per say. If i put my foot on the accelerator to raise the rpm a bit its fine and perfectly even. And there's typically not a problem of hesitation coming off of idle once its up to full operating temperature now that I've upped the idle rpm with the adjuster screw. Lately I"ve found the problem occurs almost based on the weather (humidity, temp, etc). Some days its perfect, other days its terraible.... if i see its a foggy or cold/humid day, i know i'm in for some shotty engine performance (when heating up the engine, and power when up to temp) I dont' know if this is related or not, but when crusiing any speed under 75km/h (expecially when going 60km/h) the engine jerks and hesitates....so its necessary to put my foot into it for a moment and let the car drift, then put my foot into it again and repeat. Thats why my first instinct was to check the primary metering jet (if one did exist) incase it was running lean and not getting the proper amount of fuel. This happens when up to temp or not, but oddly not all the time (again based on the weather). Also, I checked the flap in the snorkle when warming up the engine and it was fully closed. So that looks good. And it opened up to allow air-flow once up to temperature as well. Steve > Definately not in effect to 3000 rpm. It's not tied to engine rpm, it's > dependant on throttle position. As the butterflies just start to open and [quoted text clipped - 96 lines] > > > > Thanks! > > > > Steve Check your vacuum at idle. Does it flutter @idle then even out under heavy throttle? Your comment about putting your foot into it, then drifting sounds similar. You may have a broken valve spring. Which one? Well pull the valve cover(s) and look or do a compression test. -PapaRick ------------------------------------------------------------------------ > Sorry for the late reply, been away for a few days. > [quoted text clipped - 224 lines] >>>>>Thanks! >>>>>Steve |
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