How do I know whether I am on number one cylinder compression? Is there a timing mark on crank shaft?

Which pulley is 180 degrees out? Crank or Cam?
The Crank shaft turns twice for every one turn of the camshaft so you could have positioned it halfway through the 4 stroke cycle so you are not at tdc on number 1.
If it is the Cam pulleys that are out of position try this...
Using a socket and ratchet, By Hand turn the CRANK Pulley CLOCKWISE One full rotation. This should bring the timing marks into the right position.

No different than setting one that turns clockwise. You set it in so the rotor on the distributor shaft lines up with the number 1 cylinder spark plug tower on the distributor cap, and then you follow the firing order around in a counter clockwise direction.
This will get the distributor set right, as long as you have the number one cylinder at Top Dead Center of the compression stroke before you drop the distributor in.
To find TDC of compression stroke, pull the number one spark plug. Turn the engine over by hand with a socket on the crank pulley-turn it clockwise-or you can have someone bump the starter over in short bursts. With your finger over the spark plug hole, turn the crank over until you feel air pressure build up in the cylinder. Now look at the timing pointer scale next to the crank pulley. Keep turning the crank until the notch or mark on the crank pulley lines up with the zero degrees mark on the pointer scale. The air pressure behind your finger tells you that you are on the compression stroke. When the crank pulley notch lines up, you are at TDC of the compression stroke. The piston will be at the top of its travel. Now you can drop in the distributor. Keep playing with it until you have the rotor pointing directly to the number 1 spark plug tower. The shaft will turn a little as it is dropped in, so you start with the rotor a little ways back from directly to number 1.

You have to retime it. You put number one cylinder at top dead center (TDC) of compression stroke, then drop in the distributor so the rotor is pointing to number one tower on the distributor cap.
To find TDC of compression stroke, remove number one spark plug. Use a socket and ratchet on the crank pulley and turn clockwise. Keep a finger over the number one spark plug hole. When turning the crankshaft and you feel pressure against your finger, the number one piston is rising on the compression stroke. Now use the ratchet and turn the crankshaft on around until the crank pulley notch lines up with the zero mark on the pointer scale. Piston will now be at TDC. You can now drop in the distributor. When seated, the rotor must be pointing to the number one cylinder on the dist. cap tower. Dist. shaft will turn as it is seating-if you get it wrong, pull up distributor and reposition the shaft until it drops down into the exact right position. When done, start the engine and use timing light to set the timing to specs.

Right, so you have two TDC's-the exhaust stroke and the compression stroke Top Dead Center-and both occurr when the crank hits the zero mark on the timing scale.
Always set timing with the number one cylinder at TDC on the compression stroke. How do you know which is which? If you have a distributor, the rotor will be pointing to number one cylinder tower, when at TDC of the compression stroke. If you are on the exhaust stroke's TDC, the rotor will be pointing just opposite of number one .
If you don't have a distributor, look at the valves for cylinder one under the valve cover. If at TDC of compression stroke, both valves will be closed. At TDC of exhaust stroke, the exhaust valve will be open.
You can also find the compression stroke on number one by pulling out the spark plug for number one, and either put a wrench or socket on the crank pulley and turn it clockwise, or use the starter and bump the engine over in short bumps. Put your finger or thumb over the spark plug hole and feel for the pressure to build up. As soon as you feel pressure on your finger, (if turning by hand, pressure will be slight, but you can feel it) you are on the compression stroke. Stop bumping the starter and turn the crank on around by hand until the zero mark lines up on the timing scale-TDC of compression stroke! The exhaust stroke will have no pressure build-up as the piston comes to top-because the exhaust valve is open. On the compression stroke, both valves are closed as the piston travels up the cylinder.

Make shore valve timing is correct compression test all cylinders same new spark plugs set ignition timing check ignition leads and vacuum hoses adjust tappets .make sure crank shaft on TDC cam shaft on number one cylinder valves close and cam pulley on marks belt tide fit distributor firing number one cylinder line up rotor and distributor cup lead to that .

Hi Robert, It would help to know what engine is in your vehicle? A rule of thumb way to set up the valve timing is as follows. Rotate the crank shaft until the crank shaft mark is about 90 degrees from TDC. This will protect the valves from damage when rotating the cams to align them. With the tappet cover removed rotate the cam shaft or shafts so that the number one cylinder is on compression (both inlet and exhaust valves fully closed) and the last cylinder in line on overlap, Look for adjacent timing marks on the cam gears and somewhere close to those look for the alignment marks (maybe notch marks, holes or indented match marks). (Some manufactures use the machined gasket surface where the tappet cover closes onto the cylinder head). Look closely for the static mark on the cylinder head and align the cam(s) to it or them. Once completed, set the crank at Top Dead Center and look carefully for a timing mark which aligns with the gear and set them. Fit the belt so the marks align when the tensioner bearing is released and presses the belt under load. (follow the manufactures recommended tightening instructions.) . (Remember never to crimp a timing belt, if it has been, throw it away and get another, even if it is new!) If the belt snapped and that is the reason for replacement, internal damage may have resulted inside the engine. A snapped belt may cause damage to the valves, the camshafts, the cam shaft caps and to the pistons. If the engine turns much faster than expected and sounds as if there is no compression, you have problems! (Those I've just mentioned. I hope not for your sake. best of luck Regards John

Gear on crank has a dot stamped on it, gear for cam also has a dot stamped on it. when all is put together, the two dots must align with each other at the closest point of rotation.

350, 305, 400. Crank shaft gear mark is straight up to the camshaft gear and the camshft gear is straight down to the crank shaft gear. If your looking for the rotor button / distributor. The button should be pointing to the number one cylinder (drivers side front) On the compression stroke. To find the compression stroke pull the number one plug , put you finger over the hole, crank the engine untill the pressure pushes your finger off, then put a stiff wire in the hole and turn the engine by hand (wrench) untill the wire comes up and just starts to drop. Move it back and forth to get top dead center. On the distributor Number one and number eight should be horizontal with the radiator.

It is possible to set the timing wrong....if the belt broke well running I believe this is an interference motor..meaning if the timing is off the cylinder can move up and smack a Valve being exhaust or intake. which means a valve can break or bend cause lack of seal thus no compression. When setting the timing you want to align your marks with Cylinder # 1 it needs to be on the compression stroke..you can do this by removing cyl 1 spark plug and rotating the crankshaft with your finger over the spark plug hole you will feel air press against your finger meaning all valves are closed and your in compression stroke. Then line up your marks and you should have it...I would consider purchasing a compression tester to make sure no damage to the cylinder head....good luck hope this helps

the crank shaft mark is 180 degrees of. It has to be set on the compression stoke. You set yours on the exhaust stroke. Sorry