1984 CB450SC Right Carb Pop and Black Sparkplug
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  1. #1
    Junior Member JPHamilton's Avatar
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    1984 CB450SC Right Carb Pop and Black Sparkplug

    Hello all I'm new to this bike and new to riding in general. I have a 1984 CB450SC and I'm trying to figure out why my right carb is popping into the air filter box and the plug on that side is running a dry, sooty black. I've read it could be because it's running rich on that side, but I'm not sure how to actually adjust the carb to run leaner. There is a single brass looking screw that hangs down from the front of the carb, but other than that there are no visible adjusting screws on the carb. I'm guessing they're all inside? How do I adjust to make it run leaner?

    This "pops" only on idle and runs just fine at higher speeds. When I pull up to a light or stop sign the "pop" threatens to stall the engine (and has).

    I've changed to hotter plugs (DPR7EA-9) from the ones that came with it (DPR8EA-9) but I don't think it changed anything. Any help would be appreciated. Pictures with arrows a definite plus. Thanks!

  2. #2
    Senior Member Polish's Avatar
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    Hello and welcome. Picture is always welcome as much as the intro. Lol

    To properly help you, we need to know if your bike (meaning carbs, engine bore, exhaust, air box, pods or not) is stock. Also, do you have a Factory Shop Manual. If not, there is a post on tha matter, check it:

    https://www.hondatwins.net/#/topics/41985
    JL
    1979 CB 400 T (Cafe Racer)

  3. #3
    Senior Member Polish's Avatar
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    FSM sent via PM
    JL
    1979 CB 400 T (Cafe Racer)

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  5. #4
    Senior Member Polish's Avatar
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    Black spark plugs could be cause by dirty air filter, weak spark or too rich fuel/air mixture. Check in you FSM your pilot screw set up, could be that simple.
    JL
    1979 CB 400 T (Cafe Racer)

  6. #5
    Junior Member JPHamilton's Avatar
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    Thanks for your help, including the FSM. Is this the pilot screw you're referring to? (circled in red below). Thanks.1984 CB450SC Right Carb Pop and Black Sparkplug-20190520_141944.jpg

  7. #6
    Junior Member JPHamilton's Avatar
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    Oh and everything is stock except the exhaust. But everything has been working fine up until now.

  8. #7
    Senior Member chaindrivecharlie's Avatar
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    That is the low speed air pilot jet screw, yes!
    Charlie

    1972 CL 175 K6
    1972 CL 350 K4
    1981 CM 400 C

    Joshua 24:15

  9. #8
    Super Moderator J-T's Avatar
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    The mixture screw adjusts the mixture at idle. It has little impact on mixture at higher throttle settings. I don't think your floats are adjustable but one could have a leak causing a high fuel level in the carb. Crud in the float valve/seat area could prevent the float valve from sealing, also leading to high fuel level in the carb. Decel valve stuck open would cause a rich condition. Missing plug under the slow jet would cause it to run rich but then it should have been doing it since the last time the carbs were off.

    You might want to review the carb rebuilding tutorial just in case you end up taking that route:

    https://www.hondatwins.net/forums/63...tin-carbs.html
    JT
    81 CM400T Under re-construction
    82 GL500

  10. #9
    Junior Member JPHamilton's Avatar
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    Thanks I'll update this as I go. I tried adjusting the screw above but it had little effect on the problem. I'll move on to the other possibilities you mention.

  11. #10
    Junior Member JPHamilton's Avatar
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    So I took the carb apart and cleaned it. I think it might have helped the issue somewhat but not sure. I'm beginning to think it's related to the coil, but again not sure. I ordered a new coil from RMSTATOR but it died within a km. Yeah that was pretty annoying. Put the old coil back on and got myself home. I saw a discussion on here related to replacing the old coil with a GM one so I might try that, unless anyone has any recommendations for coils that are easier to come by than the original one. Thanks again.

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