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In a message dated 6/5/03 2:21:35 PM Eastern Daylight Time, [EMAIL PROTECTED] writes: > runs fine at 900 > rpm or better but seems to want to stall below this mark unless I put the > carb heat on. I have a C75-12F with a stromberg carb The fact that carb heat helps indicates the engine is running lean at low power settings. (It is supposed to be running rich, in fact idle mixture is the richest mixture the engine should ever see.) The most likely cause of this behavior is some sort of air leak in the induction system. Induction system leaks are most noticable when the engine airflow and manifold pressure are both low. This is when the air leaking in (without passing through the carburetor) is the greatest percentage of all the air entering the engine. The Stromberg idle system is active up to about 1200 RPM, and the fact that the engine runs pretty well at 900 or 1000 would seem to exonerate the idle circuit as the cause. Induction air leaks can be located by pressurizing the system with a vacuum cleaner. Control the pressure by restricting the airflow out the tailpipe. Be sure not to blow dirty air in! (Use a clean bag...) Locate the leak by spraying soapy water on the suspected spots. Deteriorated or improperly installed induction tube couplings are high on the list of usual suspects. Each end of the induction tubes should have a rubber spacer inside the rubber sleeve. They could be missing. I've also seen a very difficult to detect problem caused by a bad carburetor parting gasket. > My AME (A&P) has messed a bit with the main mixture setting with varying > results I'm assuming you mean the idle mixture, as the Stromberg does not have any "main mixture adjustment". As Greg mentioned, your problem could be rooted in the idle circuit itself. One way to (sometimes) find out is to remove the idle mixture screw and blast a shot of carb cleaner in the hole. It should go straight through, out the idle fuel nozzle into the carb throat. Be sure to allow time for it to evaporate or get it out of the air box before running the engine again. (Screw the mixture adjustment in until it bottoms lightly to establish the setting prior to removing the screw.) Someone made reference to "idle cutoff" and the Stromberg. The Stromberg does not have an idle cutoff. The mixture control works by venting venturi suction into the float chamber thereby varying the pressure in the float chamber and reducing the pressure differential between the float bowl and the main metering jet. At idle there is essentially no venturi suction and no fuel being metered by the main metering jet, so the mixture control is completely ineffective. The engine can be killed using the mixture control, however. Simply pull the mixture to the full lean position and then open the throttle completely. If the engine doesn't die then the mixture control is not functioning correctly. This does not have the same effect as idle cutoff, however, as the metering circuits are not deprived of fuel. Problems with the mixture control do not generally affect idle performance. John ========================================================================== ==== To leave this forum go to: http://ercoupers.com/lists.htm Search the archives on http://escribe.com/aviation/coupers-tech/
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