Sudden rough running, out of ideas

[5lugnuts]

"If fuel shut-off valve is left open while the car is parked, it will leak gas, slowly. That's been true for a while. Is that a carb problem?"


Sounds like you found your carb problem (too much gas). If the gas is leaking from the carb and not the line to the carb that is your issue. When the float bowl is overly full, the gas flows into the intake manifold and usually out the mouth of the carb into the filter if you have one installed. Not a problem if the engine is not running, but if the engine is idling the extra gas gets drawn into the intake manifold and cylinders causing your overly rich mixture which does not burn completely and creates black smoke which leaves soot on the spark plugs and in the exhaust manifold. Either the float valve is stuck or dirty or the fuel level in the carb bowl is too high which could cause the overflow into the intake manifold.

There should be a round gasket on the float valve between the valve and where it screws into the upper carb housing. If that is missing it could be causing a leak which is overflowing the bowl and going into the engine. If you find one gasket on the float valve it may need another one (2). The float level (gas in the bowl) is reduced by adding those float valve gaskets which lowers the float level and reduces the level of gas in the carb.

You get the gaskets with a rebuild kit. It would be a bad idea to try and bend the float. You could also have a leak in the float causing it to sink and let more gas in (probably not unless someone tried to bend the float). The float valve gasket looks like a thin reddish plastic ring between the valve and the top of the carb.

There is a measurement to check the float position with the carb apart by turning the top half over and measuring but I can't remember it now. Somebody will give that information soon. I'm just an old beginner.

Good Luck