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This is a discussion on Don't shoot the messenger! within the Technical forums, part of the Caferacer.net Forums category; OK, I'm doing this as a favour to catboy, who has no internet at home. His new cb750f (1976) seems to be having a rich ...

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  1. #1
    Senior Member Judeyramone's Avatar
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    Don't shoot the messenger!

    OK, I'm doing this as a favour to catboy, who has no internet at home.

    His new cb750f (1976) seems to be having a rich condition. Had trouble starting it, and when we changed the plugs, they were very dark & sooty, or dark & wet, on the 2 cylinders that weren't firing. New plugs solved the problem, but of course, that's only treating the symptom. So, he checked the air filter, and it was apparently completely plugged up & black & oily. However, the same richness issue is still occurring, running without any filter at all. Jetting issue, I assume. Since they're not cb350 cv carbs carbs, I know very little about them, but I assume we want to adjust the needle jet? He has a Clymer manual, which came with the bike, but it apparently doesn't tell him much about tuning. I know I'm breaking the rules by not searching, but as I said he has no internet & I'll be damned if I'm going to spend my day off looking for info about his bike.

    Any quick help would be appreciated, since we were planning to get in a ride tomorrow (unless we can fix this, it looks like he will be borrowing on Baby Sister - again).
    Many a man who falls in love with a dimple makes the mistake of marrying the whole girl. - Evan Esar

  2. #2
    Senior Member Judeyramone's Avatar
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    Let me add, before the scolding...

    Stock airbox, stock exhaust (with many rust holes in muffler), stock ignition.
    Choke seems to be working properly. With new plugs, bike starts & runs strong. Starts no trouble, even without choke.
    Many a man who falls in love with a dimple makes the mistake of marrying the whole girl. - Evan Esar

  3. #3
    Senior Member catboy's Avatar
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    BUMP

    ...i think it's weak ignition, but help me out bitches!!!
    iphone generated fag

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  5. #4
    Senior Member HackAsaw's Avatar
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    well cough up something useful to go on.........

    this surely doesn't help one bit

    "With new plugs, bike starts & runs strong. Starts no trouble, even without choke"

    sounds to me like you don't need any help.....

    have you any idea how to test for 'weak ignition'?
    Accidental inventor of the worlds fastest wearing rocker arms and other edible engine parts! Try my yummy blue cheese camshafts!

  6. #5
    Senior Member bmartin's Avatar
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    I would do a compression test of the motor - it's looking like either bad rings or bad valves or both. The sooty plugs are coming from oil being burned in the cylinder. Its an easy enough fix, pull the engine, re-ring the pistons and redo the valves. Sounds like this motor was beat before he got it - maybe it had been sitting for a few years before it was sold.. What ever the case, start with a motor compression test...
    Bob - Palmyra NY
    2 - 69 CB750, 1 Turbo
    1 - 71 CB750

  7. #6
    Senior Member Judeyramone's Avatar
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    Bike shows 15,000 miles on the odometer. And it's in such good shape, I'm inclined to believe it. Doesn't seem like it should need rings. I was under the impression black & sooty meant rich carburetion.

    Hack - weak ignition was suggested by a friend, and no, I don't know how to test for weak ignition (then again, it's not my bike - mine are running with no problems). Same friend said hotter plugs & premium gas would cure it, but I'm skeptical. It starts & runs well with new plugs, but fouls them out in 15 - 20 miles, so I'm told.
    Many a man who falls in love with a dimple makes the mistake of marrying the whole girl. - Evan Esar

  8. #7
    Senior Member HackAsaw's Avatar
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    shiny black fouled is oil

    dull black is fuel

    what do you have?
    Accidental inventor of the worlds fastest wearing rocker arms and other edible engine parts! Try my yummy blue cheese camshafts!

  9. #8
    Senior Member kenessex's Avatar
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    Catboy,
    I believe the crankcase vent went to the bottom of the airbox on those. If so, then they will trash out the airfilter with oily gunk and leave a layer in the bottom of the box. So make sure the whole air box is clean. Have you checked the carbs? I would go for float levels being too high. Have you checked the points on cyl 2-3? It could be an ignition issue. Finally, have you checked the valve clearances?

    Ken
    WERA 119
    CCS 119
    CRA 119
    AHRMA not anymore
    \"Think twice before you speak, and then you may be able to say something more insulting than if you spoke right out at once.\"
    Evan Esar
    Newbies, Geeto and Tex (Bye Tex)hate me!!

  10. #9
    Senior Member HackAsaw's Avatar
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    the tell tale thing I am able to extract is that the plugs are likely dull black (fuel fouled) and the only thing I am going on is the fact you mention it starts easily (I'll presume from cold) without the choke

    no mention is made of any symptoms that manifest themselves as the engine warms up or even right before it fouls plugs

    no mention is even made of which plugs the bike has

    too much missing info to do anything but indulge you in a game of blind folded dart throwing
    Accidental inventor of the worlds fastest wearing rocker arms and other edible engine parts! Try my yummy blue cheese camshafts!

  11. #10
    Senior Member Judeyramone's Avatar
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    Can't remember the plug #, but they are the correct plugs recommended in the Clymer manual.

    Dull, sooty black fouled plugs. I can read a plug, and it's fuel fouled. Fouling even when run without an air filter.

    I am not aware of any symptoms the bike exhibits as it warms up, as I haven't ridden it, except around the block once, right after we put new plugs in. I'm told it just loses power, due, I assume, to the fouling plugs.

    Ken, my vote has been carbs from the get-go. Matt says ignition. He doesn't want to fuck around with carbs yet. Points on cyl 2-3... It's fouling all 4 plugs. Perhaps all points are bad? That's something we haven't checked.
    Many a man who falls in love with a dimple makes the mistake of marrying the whole girl. - Evan Esar

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