CB450 too rich at idle
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Thread: CB450 too rich at idle

  1. #1
    Member Mickeyluv's Avatar
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    CB450 too rich at idle

    I've just got my CB450 back together after a rebore and top-end overhaul. It starts and idles just fine but as it warms up it start to get rough - this is with no filters fitted for the time being. The mixture screws are turned right in as far as they'll go and both sides are still too rich. The bike was like this before I dismantled it. I have new float valves and the float height has been set on the bench with a credit-card gauge and fuel flowing. When I go the bike the carbs were exceptionally clean but I stripped and checked them anyhow and replaced the gaskets. One of the main jets was originally 145 but I've borrowed a 130 so both carbs have 130 mains. I checked the rest of the components and they're all the right parts.

    Off-idle the engine runs cleanly with a crisp throttle response. I've been very careful to set the ignition timing as well as valve clearances.

    I wonder if anyone has any specific pointers to assist in getting this sorted out? I've spent a good deal of time researching this and reading through the factory manual, but nothing I've tried has made any difference. The air and fuel passageways look good but someone with more experience of these bikes may suggest something I've overlooked.
    CB450 K4 rolling chassis fitted with CB450 K2 engine.

  2. #2
    Junior Member dgjessing's Avatar
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    What makes you think it's too rich at idle?

    There should be a starting setting for those idle mixture screws in the manual - put them there and see how it runs. They certainly should not be closed.

  3. #3
    Senior Member Chippe's Avatar
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    It may need the air restriction that the filter give so get a new one.
    Honda CM400 1979

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  5. #4
    Member Mickeyluv's Avatar
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    Quote Originally Posted by dgjessing View Post
    What makes you think it's too rich at idle?
    It soots up the plugs very quickly at the factory default setting and a sign of a too-rich idle is the bike starting and running well when cold and then progressively worsening as it warms up. The running begins to improve when the screws are fully home (they're NOS screws). Exhaust smells of unburnt fuel. If I open it up to 3000 rpm it clears. It also bogs down when warm when the throttle is opened slightly. I could set it up with my gas analyzer but it would soot the filters up in seconds the way it is. Maybe it's pulling fuel from somewhere else other than the pilot circuit, or there isn't enough air being fed in via the emulsion tube.

    I have new air filters and they make no difference, so I left them off for the time-being.
    CB450 K4 rolling chassis fitted with CB450 K2 engine.

  6. #5
    Senior Member Rscottp's Avatar
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    I think the stock jets are 145 or 150's, why are you using 130's? No idea if that could be the source of your problems though.

    Edit, I'm all wrong. FSM shows jet is a 130!

    BTW manual says air mix screw should be 1-1 1/4 turn out.
    Last edited by Rscottp; 03-25-2019 at 05:12 PM.
    1981 Honda CX500D
    1974 Honda CB450
    1986 Yamaha Radian 600

    Martha's Vineyard Ma

  7. #6
    Sensei 66Sprint's Avatar
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    Weird question.... During the rebuild did you remove the throttle butterfly plates?

    I ask because they aren't simply flat discs, and the beveled edge MUST be correctly oriented
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."

  8. #7
    Senior Member tbpmusic's Avatar
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    Quote Originally Posted by Rscottp View Post
    I think the stock jets are 145 or 150's, why are you using 130's? No idea if that could be the source of your problems though.

    Edit, I'm all wrong. FSM shows jet is a 130!

    BTW manual says air mix screw should be 1-1 1/4 turn out.
    Stock main jets on 14H carbs are 130.
    Rscottp likes this.
    Bill Lane
    Honda Twins Founding Member

    No advice, just info

    '75 CB200T, '71 CB450 K4 Half-Breed, '72 CL350 (Sold), '81 CM200T, '72 C70M
    and a little red Chineeze scooter

    http://www.bikeexif.com/honda-cb450-restoration

  9. #8
    Senior Member WintrSol's Avatar
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    Hmmm... Over-filled carbs, wrong type idle mixture screws, loose pilot jets, blocked pilot air jets spring to mind.
    CB450 too rich at idle-carb_operation.jpg
    Rick

    Mine is a mostly 1970 CB450K3

  10. #9
    Senior Member Rscottp's Avatar
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    Quote Originally Posted by tbpmusic View Post
    Stock main jets on 14H carbs are 130.
    I swear my 450 came with a 145 in one carb and a 150 in another, ended up getting another 150 so they matched and bike seems to run well. Mine is a 74', I remember reading somewhere that they came with 145 or 150 but perhaps I was hallucinating.
    1981 Honda CX500D
    1974 Honda CB450
    1986 Yamaha Radian 600

    Martha's Vineyard Ma

  11. #10
    Senior Member tbpmusic's Avatar
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    Quote Originally Posted by Rscottp View Post
    I swear my 450 came with a 145 in one carb and a 150 in another, ended up getting another 150 so they matched and bike seems to run well. Mine is a 74', I remember reading somewhere that they came with 145 or 150 but perhaps I was hallucinating.
    That info is correct.......
    Bill Lane
    Honda Twins Founding Member

    No advice, just info

    '75 CB200T, '71 CB450 K4 Half-Breed, '72 CL350 (Sold), '81 CM200T, '72 C70M
    and a little red Chineeze scooter

    http://www.bikeexif.com/honda-cb450-restoration

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