Question about 1981 Honda CX 500 B

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Hi, This is probably a simple question but it has me baffled. I bought a cx500 1981 custom last week and I have had a couple of teething problems. The main one being that she looses power - at full throttle she only goes at about 80kph. One of the spark plugs had black deposits, as I understand, a problem with air/fuel mix. I have changed the spark plugs but still have the same problem. I have been stripping down the twin carb to clean today, and have noticed that one of the needle jets(piston needle jet?) is loose and the other is rigid. There was some dirt in the carb, but the spark plug that was black matches the same side as the loose needle. Could this be the problem? what would be the solution? I have tried doing the needle set screw up but it won't tighten any more. Is this problem normal? Is it a problem? Any help would be gratefully recieved.

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  • sacha_marles Sep 26, 2009

    Hi, what about the needle is this normal?


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You definately have a fuel problem .
clean your tank first as deposits in the tank might have made way to the carbi .
the fuel/air mixture is one thing but your bike may have also oil burning issues .
try first by making sure the carbies are clean , fuel tank clean , the air mixture adjustment is tricky but usually screw the air mixture screw on the carbi all the way in and undo it 1.5 turn back and no more .
try that first and see how you go . the vaccum menbrane in the carbi may be defective too and may have to be changed .
have a look at all that and see how you go

Posted on Sep 26, 2009

  • chris Sep 26, 2009

    the needle will always move slightly and i would not worry about it .

    try all the previous ideas that i gave you as the needle will travel into a seat and will not affect the performance of the bike .


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1981 honda cx500 idle problems I have a 1981 Honda CX500 Custom. It putters real bad almost to the point of stalling but only when its warmed up. This sucks because it putters at every stop sign or red light I hit. I have the haynes manual so i checked everything it suggested including cleaning the carbs twice, checking the valves, adjusting air fuel ratio, renewing spark plugs, checking for air leaks (I have cracks on my intake boot, but don't go through to the interior). I tried the carb cleaner approach but didn't get any results. At one point it was so bad to the point that it would stall every time I would start out in first gear. I then found a loose wire connection on a yellow wire which led to the ignition coil. Tightening that helped drastically until this current problem. What has me puzzled is that it ran fine in the early spring, even after sitting for four years. Whenever i don't want to be embarassed, I will put the choke on just slightly and it will idle high, but smooth. I don't ride with it on for more than 10 seconds. There is a small oil leak from the clutch cover but I don't think that would cause an issue. I changed the oil at the beginning of spring but I plan on doing it again soon since I have a new filter on the way. I am all out of options and need someone else's advice! The bike has 34k miles and am not looking forward to rebuilding it if that is the problem. I am going to look more closely into the electrical side tomorrow to be positive there are no loose wires going from the computer to the ignition coil. Any ideas will be appreciated.

Jun 22, 2012 | 1981 Honda CX 500 B

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