Your YU engine was originally a two barrel Rochester 2GC carburetor. It is a good carb, but the current Quadrajet is better. Do you have a stock 4 barrel air cleaner? I ask because it is important to use the stock THERMAC air cleaner on a stock engine. It has a snorkle tube that draws hot air from the exhaust manifold and a sensor in the air cleaner regulates the temperature so 100° air goes into the carb. The stock carb is calibrated to use the 100° air and will run best when it is in place. Not using it WILL affect operation so if it is missing try and find one. I know, a shiny chrome air cleaner looks better, and if you only use the car on sunny warm dry days you will be fine. But crummy cold weather will be a different story. The same with the heat riser valve on the pass side exhaust manifold. It is a weighted valve that closes when cold. That forces all the pass side exhaust to go through the exhaust crossover in the intake manifold. That heats up the intake so the fuel atomizes better and the engine runs much smoother until it get to operating temp. It also heats up the choke stove faster. When it gets hot, the valve opens, and the exhaust goes out both head pipes as usual. Older weighted valves often rust shut and that causes the intake manifold to get all the exhaust all the time and the fuel in the carb will actually boil. If you remove the valve or wire it open, it will take much longer for the engine to warm up and smooth out. Not a problem for a casual drive, but on a cold wet crappy day and you are late for work, it WILL be an issue. The other item on the stock air filter is the hose that goes from the pass side valve cover to the cleaner. It is the second part of the two-part PCV system. Engine blow-by gets into the crankcase and will push oil out of any weak spot. The pushrod cover mounted PCV valve goes to the big port on the carb and draws the crankcase fumes out via manifold vacuum. But since manifold vacuum goes away as the throttle is opened, and airflow increases through the carb, the hose that goes from the valve cover to the air cleaner continues to draw the fumes at high engine speed. Most aftermarket chrome air cleaners may have a knockout on the bottom plate. You can hook the valve cover hose to that and continue to have PCV at all speeds. Just use an in-line filter to catch oil. Oil oozing from any and all gaskets is a sure sign the PCV is not working.
