1982 CB900C, getting good compression on all 4 cylinders, getting fuel, and getting spark on all 4 plugs. The bike was running fine and with in a month it was a slow progression to what we are dealing with now.
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This is a simple engine to work on. Check for compression in the dead cylinder. If low or none, you found your problem....likely burnt exhaust valve.
If compression good, check injector for fuel spay or fuel delivery head for fuel delivery. With engine idling, crack each line nut at the fuel distributor and see if any fuel squirts out.
If so, check spark delivery by pulling lead to dead cylinder and install used/spare spark plug into plug lead. lay this against engine and run engine watching for spark at plug tip. Do not touch the plug or wire while engine running.
If no spark, check distributor cap for cracks inside. The electronics are primative on these so the above should get you close to the reason cylinder is not firing.
How the computer determines a misfire is through a lagging effect in the crankshaft turning-when one cylinder is not producing power, the crankshaft doesn't turn as fast, and the computer senses it through the crankshaft sensor.
Misfires are usually a fuel problem, spark problem, or a low compression problem. Compare the spark at #4 with the other cylinders-all should have a blue, snapping spark. If good spark, check the spark plug -it might be defective. Compare the spark plug in #4 with the others. If it has a black sooty appearance, it is running rich-injector may be leaking. If the lower insulator on #4 is whiter than the other plugs, it is running lean, and the injector may be partially clogged, or not pulsing well.
If gas and spark is good, do an engine compression test. If #4 has low compression, it could be a valve problem, or worn piston rings, or a head gasket failure.
what an awesome bike. You'll to eliminate problems one at a time, spark, fuel, compression. I made a dandy home made tester with a spark plug, hose clamp and a battery charger alligator clamp. With goo clean grounds attach the plug to the alligator clamp with the hose clamp, open the gap to around .040 if your ignition is good it should jump that gap, as if it's under compression. Attach to the wire, turn over the engine and see if you have a nice blue consistent spark. If you do, be sure you have good fuel flow, remove the fuel line to the carb (kinda tricky as it's hard to get to) and be sure you have fuel. What I do is with the engine running I shoot a very meager shot of starting fluid into the suspect carb with the engine running, if it perks up momentarily odds are something's wrong inside the carb. Lastly compression. Test the two running cylinders, average your readings and then test the bad cylinder, if it's 10lbs lower than the other two, internal engine damage trouble. Try these steps and report back, we'll figure it out.
Not likely valve timing if it will run, cams are one piece so if one is right all are right. Have you checked compression with a gauge, or just by feel? Try swapping the sparkplug wire with the #1 cylinder and see if the dead cylinder moves with the wire. Also try moving the spark plug to another cylinder and see if the dead cylinder moves with it. If the compression is indeed good, the problem will be with the spark plug/ wire, or in the carb, these are the only things that can kill a single cylinder. The coils run a pair of cylinders so a bad coil would put out 2 cylinders at the same time. Start with the spark plug, I have seen a perfectly good looking plug, that would fire whun held to the head, but would not fire in the cylinder.
the truck has to have the basics . Spark fuel and compression in that order. Remove a plug wire from a plug either remove the plug or install a different one in the end on the wire and crank the engine with the plug laying on engine ground in good view and see if there's spark. Being you did'nt mention what motor is in the truck, desiel. 10 cylinder gas 8 cylinder gas 6 cylinder gas 4 cylinder gas I can only guess at what kind of engine were working on. The gas engines have spark plugs as the desiel has glow plug heating system and direct fuel injection. Silveradoo is a trim level and theres 1/2 ton 3/4 ton 1 ton and 1 1/2 ton versions. They all have fuel pumps and if they have more than on tank or a desiel they can have more than one. The gas engine v8 has troble with the ingtion and the module is the most likly for the cause of no spark but the mileage wasn't stated so I have no idea where to start. if it still is'nt getting fuel with the new fuel pump it's still possible that is'nt running or if it is its wired backwards and is pumping backwards. If the fuel pressure checks OK (above 50 lbs. that part shold work. Below 45 lbs they might run but they won't start. Compression has to be good 100 lbs or more on any engine. most chevy engines are much more in good shape.150 or even as much as 220. low compression and it was running OK the last time it ran sort of says the timing chain skipped as the compression normally does'nt go away quickly. The exception to this is overheating and racecar type blowups. some engines after overheating will have little or no compression and if started with added fuel and revved up they can unstick rings and or valves. and go back to normal. the reason for the overheating needs to be found as this happens again the motor might not come back to life.
FIRST OF ALL YOU NEED TO RUN ENGINE FEW MINUTES DONT GET IT TOO HOT. TURN OFF ENGINE. THEN PUT ON SAFETY GLASSES. USE COMPRESS AIR BLOW DEBRI AWAY FROM SPARKS PLUGS.TO KEEP FROM GETTING INTO CYLINDERS.DISABLE YOUR SPARK IGNITION.AND DISABLE FUEL PUMP.BY REMOVING FUSE.NUMBER AND TAG SPARK PLUG WIRES.REMOVE ALL PLUG WIRES AND SPARK PLUGS.BESURE YOU NUMBER THEM AND TAG THEM SO THEY GO BACK IN THE ORIGINAL PLACE.THEN GET COMPRESSION GAUGE TESTER SCREW IT IN SPARK PLUG HOLE TO CYLINDER 1 FIRST.TAKE YOUR FOOT PRESS ACCELERATOR PEDAL TO FLOOR.PUT KEY IN IGNITION SWITCH CRANK ENGINE AROUND A FIVE TURNS.THEN CHECK COMPRESSION ON GAUGE.IT SHOULD BE NO LOWER THAN 100 PSI.IF SO SQUIRT SOME OIL IN THAT CYLINDER. IF COMPRESSION INCREASES.THAT CYLINDER HAS BAD RINGS IF OIL SQUIRT DONT INCREASE COMPRESSION. VALVES IS LEAKING. YOU DO ALL CYLINDER THE SAME.SCREW COMPRESSION IN PLUG HOLES.TURN ENGINE OVER 5 ROUNDS. WRITE DOWN ALL READINGS.
Are you saying that 2 & 4 got gas with external tank but not with gas tank? If so something else has happened ,if gas is getting to the carb it cannot by-pass 2 to get to 1. Take the tank back off ,connect to external, again. See what happens.
The cylinder is not getting fuel, spark or compression. You have already checked for spark on that cable and it appears to be good. Pull the spark plug and see if it is damaged. You can also switch spark plug and cable to another cylinder and see if the miss travels with the plug and or cable. If so, you'll know its the spark plug/cable that is bad.
You can check for fuel by listening to the fuel injector to see if it clicks. Use an automotive stethoscope or a long screwdriver--put the metal end of the screwdriver on the injector and the other end against your ear and listen to the injector while the engine is cranking or running. You should hear it click for each time it pulses fuel.
If both fuel and spark are good, then you are probably missing compression in that cylinder. You (or a mechanic) will need to pull all the spark plugs and do a engine compression test with a compression test/gage set. Lots of things can lead to poor or zero compression including: broken piston rings, burned/stuck/broken valves, cracked cylinder head or engine block, cracked pistons,etc.
Because one cylinder is "dead", the engine has to work extra hard to start and run against a cylinder that is dragging it down. If you find low/no compression in that cylinder, you will probably have to remove the cylinder head to correct it. A big expensive job. It may be cost effective to replace the whole engine with a rebuilt one.
If all the rest are fine then it's not timing, and if you took care for the carbs it's not fuel
Last two things to check is spark and compression. A quick compression test of all your cylnders will give you a good idea of the condition of the engine and if thats the problem.
The spark could be missing for several reasons. First replace the spark plug, local auto parts shops should have it, should be the same as most honda cars. then test the coils for the proper ohms, a service manual will have the specs for your bike, or google it, and then make sure the coils have juice running to them.
Check for spark at the cylinders you mention. Unplug the wires from the spark plugs, put a spark plug in the end of the wires, and attach the body of the plugs to earth, or an exposed metal part of the engine. With the engine running, you should have spark at the plugs. If no spark, get the ignition system tested. If there is spark present, and you think the fuel is ok, then the compression must be at fault. The things that are required for combustion are: spark, fuel, and compression. Get a compression test done on the engine, but first check for any vacuum leaks in the inlet manifold around the cyl 5 and 6 area.