Truck engine wont rev up

There is a very simple check for a loose timing chain due to a broken tensioner, worn gears, or a stretched chain. Pull the distributor cap and observe the rotor position. Take a breaker bar and a appropriate socket and put it on the crankshaft damper pulley. If you have a degree wheel put it on the damper pulley else you can just chalk mark the timing mark position at the appropriate time. Now - slowly turn the crankshaft pulley in a clockwise direction. Watch the distributor and observe that the rotor is moving. Stop turning. Now - mark the damper pulley position with the chalk or observe the degree wheel. Very carefully turn the crankshaft in the other direction and VERY carefully observe the rotor in the distributor. The instant it begins to move STOP turning and mark the crankshaft position again. Measure the number of degrees of rotation of the crankshaft. If there is a lot of slop in the chain then you will have moved the crankshaft ten or fifteen degrees (or more) before taking the slop out of the chain after the reversal before the camshaft began to turn. Get the picture?? If all is well and there is no slack in the timing chain then you will see about three to five degrees of "reverse motion" before the distributor begins to turn. If you are not sure how many degrees it turned during the procedure there is a simple way to calculate that based on the spacing between the chalk marks. Take a string and wrap it around the crankshaft damper where you made the chalk marks to measure the circumference of the damper. Let's say it was 18 inches. If there is one inch between the chalk marks then divide 1 by 18 and multiply the result by 360 (the number of degrees in a circle). In this case the answer is 20 degrees and it is time to replace the gears!!! If the valve timing is off then the engine will run poorly. There are many reasons for that but one main one is that the compression will be low on all cylinders. If the chain slop is not excessive it is still possible that you have jumped a tooth especially if it is a rubber timing belt. To check the valve/crank timing just pull the valve cover and observe the valves for the #1 cylinder. Pull the spark plugs, hand crank the engine over with a finger covering the spark plug hole for #1, and wait until it starts up on the compression stroke. Make sure you are rotating the engine in the same direction that the starter motor turns it. You will feel the pressure escaping past your finger. That means that the piston is on the way up towards top dead center (TDC). Now, watch the timing mark on the crankshaft damper pulley and continue rotating the engine until the timing mark is at TDC. Both valves must be completely closed at this time. There should be no doubt of it. You can watch the valve action as they close and make certain that they are closed. If either one is even slightly open you have a problem. A compression test will also reveal timing problems. From the above description it should be obvious that the compression on all cylinders will be low if a valve is still open at TDC. So all cylinders will show low compression values.