No start , fuel present , spark present but appears weak

My thought would be immediately sheared flywheel key, it is easily done to not enter the key in the flywheel to the recess in the crank, the key is alloy and part of the flywheel itself.

If vehicle has spark ( even weak spark ) with a fuel supply it should start or at least try to start. Possible fuel pump problem. You need to determine if there is a fuel supply. You can try banging on the fuel tank with a blunt object ( hammer ) while trying to start the vehicle. If it starts or tries to start, suspect a fuel pump failure.If no luck using this procedure, you will need to tap into a fuel line to determine if fuel supply is present and at the proper pressure and volume

You say that but something is wrong!! If the engine is catching then the fault may not be major. I am assuming the vehicle is gas rather than diesel... Check the sparl plugs are clean and undamaged and correctly adjusted. Check they all give a good spark. Uou say that fuel is present, it has to coincide with the introduction of the spark in the combustion chamber...if it does not the vehicle will not work.
If all plugs give a very good spark check the fuel supply is adequate and then the timing.
If the plugs give a weak spark at the electrode replace the wires also check that the distributor cap is in god condition. A weak spark might also be associated with a failing coil/coils


Ensure your battery is full of electrolyte and is fully charged

GOOD LUCK

have you checked for spark? it has been my experience that most chevys have a problem with electrical stuff when cold. i would check for spark, it may be present but weak. i have a 95 k1500 suburban, and things like window motors and seat motors lag alot when its cold outside. if spark is weak, u may need a new coil

might have unseated a valve check the compression it is good at 100 lbs min to run is 90 lbs thats bad

sounds like your coil might be bad

Is the fire and fuel actually reaching the combustion chambers, or just to the coil and fuel regulator. If fuel and spark and compression are all present, and the timing is correct, there is nothing to stop it from starting. Something has to be missing from the equation. Try to start the engine, then pull a spark plug. If it's getting fuel it will be wet with gasoline. Now, with the spark plug still attached to the plug wire, lay the spark plug where you can see it while cranking the starter. There should be a bright bluish white spark at the tip of the plug. If the spark is weak and yellow looking, it may not be strong enough to fire the air fuel mixture.

Recheck your spark and make sure you are getting spark at each cylinder. Check the fuel pressure by popping off one of the gas line and see how strong the flow is, if you have a weak flow your fuel pump isnt operating properly or you have a clogged fuel filter. Give that a try and let me know if that works if not I will take you to the next step.

Remove rubber connection from carby inlet (between carby and air filter) enough to get a piece of thin card to cover inlet to carby. Turn motor over with throttle half open for a few seconds. Remove card and see if any fuel is on the card. If there is, repeat process (fresh card) without throttle, fuel should still appear. If there is no fuel present, you have a fuel blockage. If engine still not starting with fuel present, you have an electrical problem, check for spark by resting attached plug to engine metal and turning engine over.

Possible weak spark from ignition coil or low fuel pressure due to weak pump. Try spraying carb clean into intake throttle body when problem is present. If vehicle will start and run with a little spray then fuel pump is failing.