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  1. #61
    CB750 Enthusiast mrtelec's Avatar
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    Is #4 still leaking? You said clear tube shows number 4 is at just above 1/4 inch from top of bowl, does it stay that way over time ? If it stays at that level and still leaks it could be a hairline crack in bowl by the drain screw from overtightening.

  2. #62
    CB750 Enthusiast Old Okie's Avatar
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    No more leaks, the drain screw was loose. I have the carbs off the bike again and will
    check actual fuel levels tomorrow. The floats are set with a gauge at 14.5 mm so we will
    see how that relates to the real fuel level. I had to get new clear plastic fuel line, what I had
    was not fuel line and the gasoline melted it. Jet sizes are so weird, no relationship between
    jet sizes from company to company. Still wondering about the size of the hole in the slow
    jet/emulsion tube. I can pass a high "E", .010 guitar string through them but the bike will not idle.

  3. #63
    CB750 Enthusiast mrtelec's Avatar
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    now that the accel rail is pumping and the low idle jets are clean have you set the air fuel mixture screws on the bottoms of the carbs? I believe mine are at 1 and a 1/2 turns out. have the carbs been synced yet?

  4. #64
    CB750 Enthusiast Old Okie's Avatar
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    The air screws are set at 1 3/4 turns out. I have not synched the carbs. I have not separated the linkage so they should be pretty close. I can not sync the carbs until I get it to run a little better. There is a tiny hole in front of the slides. Where does it go?? I have blown carb cleaner through every place I can find but nothing comes out of those tiny holes. My eyesight is really poor, I use a bright light and a mirror to see those holes.

  5. #65
    CB750 Enthusiast Old Okie's Avatar
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    I'm back. I have cleaned everything I can and tested the fuel level with the clear plastic tube method. All bowls check withing the 2mm to 4mm below the top of the float bowl, so that is good. I can see no reason that this bike will not idle. I did see one bit of info that may help me understand this problem. It seems these bikes are cold natures because of running so lean. I may be expecting the bike to idle before it is properly warmed up. I will put the carbs back on later today and see how it goes.

  6. #66
    CB750 Addict Dave's Avatar
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    Quote Originally Posted by Old Okie View Post
    It seems these bikes are cold natures because of running so lean. I may be expecting the bike to idle before it is properly warmed up.

    To clarify this, the bike will idle at any temperature but....

    …. the RPM at which the engine idles is directedly related to the temperature of the engine and the throttle position just like the (hypothetical) 2018 Chevy truck in your driveway. The only difference is the truck has a computer to work the throttle for you.

    Simply put, (Assuming everything else is good) when the engine is cold you have to hold the throttle open a little bit to keep it idling at just over 1,000 RPM and once it warms up you can let go of the throttle and it will idle between 700-800 RPM where you set it (When it was warmed up).

    Try this, sit on the bike, lean forward a bit and hold onto the bars with only your right hand and reach under the carbs with your left hand and find the large idle speed adjusting knob sticking down from the center of the four carbs.

    Notice how easy that is? It was purposely designed to be easy to adjust the idle while sitting on the bike, say in the driveway when you need to turn it up so you can do something else while it's warming up or at a stop light after then engine has warmed up and you want to turn the idle back down.

    This is the actual nature of the "reputation" you speak of.

    PS I only need to use the choke on my 74 when it's been sitting a long time (2-3 months) otherwise it starts right up when it's 60f outside. I just keep blipping the throttle to keep it from dying until it's warmed up.
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  7. #67
    CB750 Enthusiast Old Okie's Avatar
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    And the answer is, No change. The air box was a pain to install, it would not go forward enough to fit the support bolts. The rubbers are soft and I lubed them generously. and finally pushed had enough to get it on.
    What I have done so far.. cleaned the carbs twice, air screws 1 turn out, float bowl levels checked with the plastic tube method, acccel pump working, new spark plugs, Checked compression, 120, 120, 100, 120, Has electronic ignition, timing left as was, it ran at some time, checked valve clearances and probably more things that I do not remember. Checked for air leaks, none found.
    No change at all from the first start up. Well maybe a little. Will start, easily, will idle about 1200 rpm, then rpm s go up to 2500. I loosen the idle adjust screw to get the rpm s back down then will not idle. I can maintain a prettty good idle with the throttle but it still revs up at will. From the beginning not much change.
    If I set the idle screw to get it to idle at 1200 it will idle for a few seconds then rev up, if I set the idle screw low enough to keep it from revving it dies, I can keep it running with the accel pump though. When it is idling it sounds pretty good...for a few seconds. This is beginning to feel like a carb linkage problem???
    I did not take the tops off the carbs or mess with the slides, have not synchronized the carbs. Suggestions, I am sure I am not the first one to have this problem.

  8. #68
    CB750 Enthusiast Old Okie's Avatar
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    This bike belongs to a family member, he has an uncle who is experienced with 4 cylinder Hondas and the uncle may finish this bike. I have spent too much time and energy and forsaken my other duties while trying to get this bike to run. The other party will likely continue with the info we have developed on this forum. I appreciate all you guys have done for me and I will report progress at it happens.

    Wheredoes the little black hose dangling from the carbs go??
    Last edited by Old Okie; 10-03-2018 at 07:03 AM.

  9. #69
    CB750 Enthusiast Old Okie's Avatar
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    Well, you are not rid of me yet. While tinkering with this bike I discovered that cylinder 3 and 4 are not running as they should, after some investigation I think I have some bad spark plug caps, more on that later. My question today is, does the 1978 CB750F SuperSport have tube type tires?? I started putting on the new ires and discovered that the old tires have tubes in them. The rims are not marked tube type or tubless. So what did this bike have originally??

    So some of my carb problems may be electrical??

  10. #70
    CB750 Addict Dave's Avatar
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    Quote Originally Posted by Old Okie View Post
    Well, you are not rid of me yet. While tinkering with this bike I discovered that cylinder 3 and 4 are not running as they should, after some investigation I think I have some bad spark plug caps, more on that later. My question today is, does the 1978 CB750F SuperSport have tube type tires?? I started putting on the new ires and discovered that the old tires have tubes in them. The rims are not marked tube type or tubless. So what did this bike have originally??

    So some of my carb problems may be electrical??

    You keep saying the carbs haven't been synced, that's kinda the first step.

    Link below shows a tube.

    https://www.cmsnl.com/honda-cb750f-7...ist/F++09.html
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

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