HEY! I have that exact same cement mixer! I have almost worn it out with all the small projects I have done with it. I too have all the front sheet metal off so I can get at the engine better. I used the steel exhaust gaskets. The trick is to really clean the surfaces on the manifolds and heads so they are as flat and smooth as you can get them. That paint is good but, yeah, it may smoke a bit until it is cured. OK I will post the carb stuff in a little while. I am kind of busy today. Stay tuned and happy Memorial Day. As an Army vet this day is special to me.
OK first off, I know you know stuff, but I will word this like you don't know anything. You never know who is reading this and wants to learn. You have the base timing set but let's expand on that. The engine needs a small amount of timing advance even at idle. Some engines use 0° and others use up to 16°. Every engine and situation is different so we go with what the factory wants. Your 1970 350 is 9° BTDC. At idle with no vacuum or centrifugal advance that is what will show up on the balancer with a timing light. The distributor has two more timing advance mechanisms. Under the rotor there is a centrifugal advance using weights and springs to hold them in. As the rpm increases, centrifugal force overcomes the springs and throws out the weights which advance the timing. This timing advance curve is there for the entire rpm range. These weights and springs are able to be changed to get different timing curves but we will stick with what you have. Then on the side of the distributor is a vacuum advance canister. Vacuum applied to this will also advance the timing. A rubber hose connects the canister with a port on the carburetor. Here is where it gets tricky. Some engines like ported vacuum, and some like manifold vacuum. Manifold vacuum is the high vacuum that exists UNDER the closed throttle plates while the engine is running. A normal stock engine should have between 18 and 20 inches or more of manifold vacuum at idle. As the throttle opens, this vacuum drops. Ported vacuum is taken from ABOVE the throttle plates. So at idle there is NO ported vacuum. As the throttle plates open the air velocity increases and ported vacuum increases. Older, high compression engines liked manifold vacuum. It advanced the timing at idle and smoothed it out. As rpm increases, all that timing advance would be bad and cause detonation. But as the throttle opens, manifold vacuum drops, and the extra timing goes away. Eventually at higher rpm the manifold vacuum increases but at that point the engine is spinning fast enough to use more timing. Engines with a big lumpy cam suffer from low manifold vacuum and the idle can be very rough. By using manifold vacuum hooked to the vacuum advance, the extra timing speeds the engine up and builds more manifold vacuum and smooths out the idle. Ported vacuum is usually used on a stock low compression engine. This type engine has good manifold vacuum and a smooth idle. As the throttle is opened the ported vacuum rises and adds timing which the engine can use to make power. Your 1970 engine has 8.8 to 1 compression. While considered low compression in 1970, later smog engines would go as low as 7.6. So you are actually in the middle. I would test out both ported and manifold vacuum and see which one worked best for your particular engine. If you have a nice smooth idle I would expect ported vacuum to work well. On a Quadrajet carburetor, the ported vacuum port is usually on the front, down low, on the driver side of the fuel line. Sometimes it is on the passenger side. Check with a vacuum gauge to be sure. Almost all the other vacuum ports on the carb and intake manifold will be full manifold vacuum. Next we will address the carburetor.
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.
So with the engine timed, and sitting cold, we can adjust the carb. The air cleaner can be off for this. Plug all open vacuum ports. The two idle mixture screws on the front of the carb should be screwed in lightly, then screwed out 4 or 5 turns. This is a very rich setting for now. When you first press the gas pedal down, it closes the spring loaded choke flap. It also sets the fast idle cam on the pass side of the carb so the engine will run fast until it warms up. The choke flap closes completely and that forces extra fuel to be added. This helps the engine start and once it does, the vacuum break cracks the choke flap open a little to provide more air. That is the little plastic vacuum canister on the pass side of the carb with a hose going right into the port on the carb. There is a spec for how much it opens. Leave it alone unless the engine runs badly and then we can fix that. The fast idle cam has a screw that can be adjusted for the fast idle. You need the tachometer for this. Start the COLD engine and see what the fast idle is. Adjust it so it is around 1200 to 1500 rpm. No more than that. If it starts going way too fast, shut the engine off or it will start to warm up too soon. Adjust it down and re-start until it is right. As the engine warms up, the choke will open and the fast idle cam will drop down lowering the idle speed. But don't expect it to do that by itself. This is where you tap the gas pedal once or twice to take the screw off the cam and the idle will drop. Once at operating temp the choke will be totally open and fast idle cam will be totally released. Now the curb idle screw on the driver side is adjusted to around 700 rpm. To adjust the mixture screws you need the vacuum gauge hooked to manifold vacuum and tach. You slowly screw in each mixture screw alternately and watch the vacuum gauge. As the mixture is leaned out the vacuum will rise. The engine speed will also increase so use the tach and adjust the idle speed screw back down. Keep doing this until you get the highest vacuum reading you can, while keeping the idle speed between 650 and 800 rpm. As you get past the ideal lean setting the vacuum will drop so back the screw out until it goes back up. A healthy stock engine will show 20" or more vacuum and the needle will be steady. If you have the THERMAC air cleaner do the final fine adjustment with it in place. As stated above, the vacuum advance you choose will affect the idle characteristics. Try both ways. It is very rewarding to do this operation yourself and get the maximum smoothest possible idle.
This is a tool that is indispensable for adjusting the Q-jet. It comes with several different sockets for the various types of mixture screws you will come across. This baby gets right in there around the hoses and stuff that will be in your way. Blue Point, Snap-On, and others make it. I also discovered this neat item. On the 301 engines they used a ported vacuum advance valve that used a combination of manifold and ported vacuum. First there is a thermal vacuum valve on the head that denies any vacuum advance until the engine warms up. Once it is warm, the valve opens and the vacuum advance valve operates. First it gives around 10° of timing advance using manifold vacuum. Then as rpm rises, ported vacuum is added and even more advance is added. This results in a smoother idle, and more power across the rpm curve. I tried using this valve on my 1977 TA 400 and I was pleasantly surprised at what a difference it made. But remember, these were low compression engines that had EGR in place. You can get away with a lot more timing on engines like that. I never tried it on a high compression engine.
Brian I just realized you are working on a 55 year old car that is probably all original. Have you checked out the suspension? Brakes? Driveshaft U-joints? The sub-frame bushings are probably shot. Control arm bushings, ball joints, springs, shocks, tie rods, all important stuff before you take off on a joy ride. Being a 400 car did it come with the front disc brakes? Because 4 wheel power drum brakes is a nightmare. I had a 1968 Buick with 4 wheel power drums. You could put your head though the windshield if you weren't careful.
