There are so many causes why a car engine runs rough in idle or in accelerating.
1. when you have just replaced your spark plug, there is a possibility that its GAP between and electrode and its tip is either too near or small or too far or too big. your car will still run but it will run a little bit roughly. adjust that spark plug to the specification of your engine. If you do not know the correct gap it is somewhere at 0.04 inch in your feeler gauge.
2. by the age of time, the spark plug cable or hi tension wire of your spark plug deteriorates. it that would be the case , the conductor of electricity inside that plug cable ( a wire or a carbon type ) could not 100% deliver the spark produced by your ignition coil thru the rotor to the spark plug cable, hence the rough idle.
3. if you have accidentally or intentionally remove the spark cable from the spark plug but it were not replace or return to the right spark plug then of course , your car engine will run roughly or even has a misfire. though , remote that would happen because that plug is exactly made to reach its corresponding spark plug, it will be difficult to have it interchanged.
4. the same as no. 3, but by this time , what was detached was those cable attached to the port of the distributor. Rough running does manifest in this case because when the piston reaches the TOP DEAD CENTER ON FIRING MODE a corresponding spark that came from the spark plug cable should be in the spark plug of that piston in the firing mode to meet exactly or on time with the piston at firing mode but since spark goes to the other spark plug, the engine will not run smoothly. ( too long to discuss how to put back it correctly) maybe can search to internet. Nest time , it would be better that you put a number in every port of the distributor that correspond to the sequence of the spark plug. example, put no. 1 in the port where the number one spark plug is placed , so on so forth. the number one spark plug is the one first in line near the drive belts.
5. if your intake or exhaust valve has some leakage , the engine also will not run smoothly. you need to remove the cylinder head to grind that valve. what is happening is , there is no complete compression in that combustion chamber because the valve is not closing fully.
6. please also examine your distributor cap, look for any cracked on it. you might as well cleaned the four metal post inside that distributor.if it has a lot of accumulated white residue.
7. if your car has undergone repair on its engine or you yourself have removed or just loosened the distributor assembly, it may have been erroneously return to the proper position. take note that the holding nut is place where there is wide space in it so that you can move or slide that distributor left to right. Try loosening again that nut or two nuts, while the engine is running move that distributor left slowly and see if the engine would run finely , if not move it to the right slowly and see if the engine runs smoothly. Next time, put n a white mark on the holding nut and also on the cylinder head , that two marks should be on line with each other.
8. maybe there is no power reaching the terminal socket of the injector. first you pull out the socket of the injector ( press it ) there are two terminals, one of which is supplied by positive, get a voltmeter, SET TO 50 VOLTS DC, insert the red test prod to any of the two terminals of the socket, the other test prod to a good ground, switch on the ignition key to ON , IF IT DEFLECTS TO 12 VOLTS, there is power reaching the terminal where your positive test prod is inserted.,if the tester needle did not move , transfer your red test prod to the other terminal, if it deflects to 12 volts then the terminal where your red test prod is inserted is the positive supply of that injector, if it did not deflects then no power reaching that terminal of that socket.
If there power reaching that socket , IS THERE A SIGNAL REACHING THE OTHER TERMINAL, then proceed by getting a LED LIGHT 12 VOLTS, insert the positive terminal to the positive term of that socket, and the negative terminal of LED to the other side of the socket , start the engine , if that LED blinks, then there is power being received by your injector.
9. maybe your injector is the one defective. how to check. remove the spark plug for easy rotation of the crankshaft pulley. switch on the ignition key, rotate the pulley, you must hear a clicking sound coming from the injector every time the piston reaches FIRING MODE on every piston. that means the rotor tip touches the metal terminal inside the distributor.
10-. maybe the manifold position sensor is defective or its wire has some loose connection. It is a socket type sensor screwed somewhere near the where the throttle valve is location. So this is on top of intake manifold
11. the hose attached to the HYDROVAC to the INTAKE MANIFOLD may have some leakage .
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