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Larry, The spring just puts tension on the screw to keep it from vibrating out of adjustment. You have afterfire right, up the stack as opposed to backfire out the carb? You could have either but the former is much more likely in this situation. Afterfire occurs when you are going down a steep grade because the governor closes the throttle. When the throttle plate is closed it uncovers the idle circuit orifice in most carbs. So now you have the idle mixture screw adjustment as well as the power screw adjustment to content with. When you close down on the power screw, it reduces the amount of fuel delivery but has less effect than normal because the throttle plate is closed. You are providing most of the fuel to the engine via the idle circuit. Idle mixture adjustment screw can either control the air or the fuel in the idle circuit depending on your type & model carb. Also remember that your engine vacuum is higher than normal. A small air leak will have more affect on the combustion mixture. The bottom line is the combustion mixture is too lean or too rich causing a slow burn. Joe
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