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Re: only running on 3 cylinders, nrew plug and clean
Bike has probably been sitting for awhile. Carb jets probably plugged up. This is usually whats wrong. I have a 1999 and had a similar problem. It required removing the carbs and cleaning them. This is NOT a fun job.
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Gotta love a puzzle.
The black smoke is an indication of too rich a mixture.
What did the plug from the back cylinder look like? If fouled and wet then too rich. If dry then no fuel.
You're close to solving it. The plugs should tell you what you need to know. You could also install new plugs for a "fresh start" to the diagnostic.
Reply as a COMMENT to this post for more input.
Your #1 cylinder ( the front cylinder) is not firing. It could be that the spark plug is fouled or improperly gapped, or the carburetor is not delivering gas to the cylinder. pull both spark plugs Clean the plugs and gap them. The gap should be about 0.035 In. (thirty five thousandths). Reconnect the wires to the spark plugs. Lay the spark plugs against the outside of the cylinders with the metal part of the spark plug body touching the cylinder (creating the ground part of the circuit). push the starter and look for spark at both spark plugs. If you have spark at both plugs, put them back in and start the bike and check the exhaust pipes to see if they are both hot. If one is still cold, the carburetor for that cylinder is not delivering gas. Could be a kinked or clogged fuel line, disconnect the fuel line from that carburetor and see if gasoline flows out. If gas flows out, the carburetor needs to be tuned or removed and cleaned.
Check the pipe right where they come out of the heads. It depends on what type of exhaust system you have on your Road King. If the stock exhaust is on the bike, the rear cylinder exhaust is directed to both mufflers. With the exhaust from the front and part of the exhaust from the rear cylinders going to the right side muffler, it gets more exhaust than the left side and runs hotter. Like I said, check the temp of the exhaust pipes at the cylinder heads. If one of them is cold, that cylinder is not firing. As long as both of the pipes are hot there, you're fine. Just ride the bike and have fun.
I am not sure if the bike is a 650 or 1100 but in either case it sounds like a fuel issue or and air leak. When the carbs were off for re-jetting did they get cleaned? The slow circuit controls to about 2000 rpm and it sounds similar to your problem.
The first thing to look at is the carb synchronization. If the front cylinder is lagging, it can give the same symptoms. I do not think that the exhaust is a factor because the front is cold when running.
Spray some WD 40 at all the intake manifold area when the bike is running and see if the idle changes. the idle will change and push some smoke out of the exhast if the intake is leaking.
If both of these check good I would pull the carbs back off and clean the slow circuits out completely. This is delicate work and shoud be done by and experienced tech.
cylinder head is getting red hot because it's running lean. Check to see if there is an aftermarket fuel filter in the fuel line somewhere, also the float hight in the carbs are set right. Also drain the fuel tank and apply fresh fuel. Check to see if the tank is rusted, also if it has a device at the bottom of the gas tank that turns on and shuts off the fuel (petcock) it's possible it has a fuel flilter in there as well.
*Note if you take the petcock off you will more than likely have to replace agasket for it. Given the age of the bike it will be hard to find a gasket.
fuel flow to cold pipe cylinder may be stopped with trash or sticky float.Resistor in plug wire for the one not firing could be bad. Try changing coil from one that is firing to the one thats not,see if that pipe gets hot or does the same thing still happen. coil ,plug wire, or fuel would be my primary focus.