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Fleet Admiral
- Joined
- Mar 25, 2001
- Messages
- 8,217
This post is meant to be a simple primer to carbs. I won't go into particulars, like manufacturer, jetting, or other high-performance theory here. And I might not have the answer if we did. I just want to explain some of how they work, what goes bad on them most frequently, and how to get you going.Carburators look very complicated all blown-up in those "Exploded views" in the shop manuals. But actually, they are as complicated as assmembling a plastic airplane model, without having to wait for the glue to set before moving-on to "step-35".<br /><br />HOW A CARB WORKS:<br /><br />The carbeurator takes in fuel and mixes it with air, making a fuel-mist. This mist is much more explosive than pure fuel. It then sprays this mist<br />into the combustion top of the cylinder,where the plug detonates the mist.<br />BOOM!<br /><br />DETERMINING IF THE CARB IS THE CULPRIT:<br /><br />First-off avoid adjustemnts. If adjusting a carb is needed,it is needed because something isn't the way it was inside the carb before the problem developed.More on adjustement later.In order for a carb to perform it's vital function , it must be supplied with raw fuel from the tank. No supply ,no end product.In order to supply fuel ,the engine needs a good fuel pump. Most outbaords use a vacumm-powered fuel pump, so it needs compression.So run a compression test.<br />The primer bulb on the fuel hose should firm-up,and no fuel should exit the carb anywhere. The inlet to the carb must be getting fuel, suppllied from the fuel pump. To make sure it is, remove the fuel line where it enters the carb.Some carbs have the fuel pump mounted on them,but even these should have a hose that goes to the carb's bowl. Place this hose into a can and turn-over the engine a few turns..BE CAREFULL that this fuel can't spill ,as if it comes into contact with the ignition or an electric wire, it could become a desaster real fast. Anyway, the fuel should pump into the can in pulses. It's no garauntee all is well with the fuel pump if it does,but we can elliminate it for now as the culprit if you get fuel into the can.No gas in the can means "not a carb problem".<br />If you get fuel,re-attach the hose for now.Pump the primer bulb. Does it firm-up without fuel spilling-out of the carb anywhere? If not, the carb is getting fuel, but is not shutting-ff the supply when the bowl is full..So it overflows, and exits out the overflow hole. This job is suposed to be done by the needle valve being pushed-up by a floating float,and shutting-off the incomming fuel. The needle valve(sometimes called a float valve,samey-same), can be stuck open, or the float has sunk,or is out of adjustment. Take a screwdriver handle,and gently tap the side of the bowl while someone pumps the primer bulb. If the fuel stops, it was the needle valve, and you jarred it loose.<br />At that point, you may be done.But if the needle valve stuck open once, it can stick open again, and at the worst of times! So plan on pulling the carb off anyway, and cleaning it.<br /><br />Turn the engine over, while holding your hand over the throat (the large opening in front). You should feel a slight suction.If you don't, go back to compression,or suspect a stuck or broken reed (RARE).<br />Now pull the plugs.If they are dry, it's a good bet the carb is blocked with trash.<br />It doesn't take much to completely shut off the fuel inside a carb, The fuel must be metered inside a carb so that the engine gets the right amount,and has the right amount of air mixed-in.<br />This is done by jets (small fittings with a precise hole in thier center, to allow only a certain amount of fuel to go thru them), pickup tubes, mixture needles (in two words,adjustable jets) , venturis (small holes like jets) and other tunnels and orafices.These must all be clean and open for the carb to work.
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