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  1. #1
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    Default About to burn this bike to the ground

    I built this 1976 cb750. Can't figure it out. Running keihin carbs, pod filters, straight pipes, dyna coils and electronic ignition. Fully rebuilt engine. The bike will start up and idle fine, but as soon as it's given throttle it cuts, misses and dies out. I've tried everything the Clymer says and can't figure it out. I need to get this thing on the road. Any advice is greatly appreciated.

    https://youtu.be/xFr9U9nX6tE

    There's a video of it being a shit bag. Tell me what you guys l think or what to try.

  2. #2
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    By the way I suspect it's a carb issue but I've tried everything I know. I'll pay someone in as much beer as you want if you can come make this run. Literally cases upon cases upon cases.

  3. #3
    JetBlackII
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    Sounds like it is idled really high, do you have the choke out or in?

  4. #4
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    I feel your pain! Maybe we can burn our bikes together?

  5. #5
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    Choke was out when it started then I pused it off.

  6. #6

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    It sounds nice, there's a lot of power there - can't be a lot wrong.

    Sounds like fuel starvation to me, i.e. kinked fuel line or maybe a blockage in the fuel cap breather.

    What sort of condition is the fuel tap in, is there good fuel flow from it? is the bowl at the bottom of the tap fuel of rust?

    Or have you tried all of the above already?

    Nick

  7. #7
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    What size jets are you using? From what I've been reading and experiencing, its really hard to get them to run right with pod filters. Sounds like yours is running out of gas but with the stock jetting it will not run right with the pods. Velocity stacks are supposed to be much better than pods but I haven't tried them yet.

  8. #8
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    its for sure a fuel/air thing. Its always a fucking cunt, especially when you do pods and pipes at the same time and have no baseline of how far up you have to go.

  9. #9

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    Quote Originally Posted by deathmetaldan View Post
    its for sure a fuel/air thing. Its always a fucking cunt, especially when you do pods and pipes at the same time and have no baseline of how far up you have to go.
    Indeed. What baseline did you work against or was it running ok with the same setup before you rebuilt the bike?

    It it was running ok before using the same setup then something has been misplaced during reassembly - as I say, kinked hose, sediment in the tank stirred up during reassembly, idle screw settings for instance.

    If its a brand new setup then its a tuning issue and you would need to get some guidance on what pipes, filters and jets will work well together.

    The other thought I had was that my old Honda 4 did used to take a while to warm up and would die if taken off the choke too quickly.

    Just a few thoughts that might not be in your manual.

    Nick

  10. #10
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    Steady fuel flow from the tank to the carbs. I pulled the plugs last night after this was posted and they were fouled completley black so I'm assuming too much fuel.

  11. #11
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    I can't remember exactly what size jets I'm running, but I'll check it out. I thought it was a jetting issue but don't know what size to run or a good place to get them from.

  12. #12
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    I had the motor running in the stock frame/setup for about a day and then pulled the motor so I don't really have a base to go off of. I know it's something with fuel and air delivery/mixture but not sure how to go about getting it set right. It'll fire right up but when I put it in gear and try to pull out it sputters out. Carbs are my worst nightmare on any bike.

  13. #13
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    There is lots of info on tuning for pods on the SOHC forum. Your bike has a different style of carbs than my bike but from what I found out 2 sizes bigger on the pilot jet and 3 sizes bigger on the main jet will get you close.

  14. #14
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    You Must der the needles in the sliders higher; and check the high of the float gauge

  15. #15

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    Quote Originally Posted by jthomasbtm View Post
    Steady fuel flow from the tank to the carbs. I pulled the plugs last night after this was posted and they were fouled completley black so I'm assuming too much fuel.
    It's starting to sound like that isn't it?

    That's a good point made about the float height as well - also worth checking the floats aren't ruptured or split and do actually float.

    Also, it would be really helpful to make notes on jet sizes, number of turns on idle screws and such and how it ran with those settings - I always found it so much easier to refer back to notes that relying upon my memory.

    Don't worry, we will get you there

    Nick

  16. #16
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    Let's figure this out. Tell me which Keihin carbs you are running and we'll get them dialed in. Float heights are important. If you have the carbs with the spring clips, that's good because you'll be able to drop the bowls pretty easily. You may have to pull them to change things depending on what you've done.

    Don't start messing with jetting or changing needle positions just yet, in order to get it right, you have to start with getting it to idle properly, everything else for tuning is useless unless you get that right.

    Now stock sizes and setting for the jetting on the "roundtop" carbs is 40 idle jets, needles in the middle position, and usually 105 main jets. These numbers will probably change, but it's better to change little things at a time. Make sure you have the baseline things working first. The jetting of the midrange and main jets doesn't matter at idle. You should start with your air mixture screws at 1 turn out on each carb. Float levels on the carbs are measured at 26mm from the notch just above the gasket surface. This is the most important. You have to make sure you get that part right.

    Here's a great visual guide.

    http://www.hondachopper.com/garage/c...at_levels.html
    Last edited by CarloFreeze; 05-31-2015 at 10:54 AM.

  17. #17

    Default

    Done this over n over again jetting cb750 chops. If you had stock jetting in there before then it's definently gotta change or will run totally wack with pods n straight pipes. It is possible to get it right tho so don't give up . The idle jets stock are #40 n that's usually good at 1 turn out on pilot screws or one size bigger if you can't get the idle good with all the wide open stuff ur runnin, the mains gotta go up almost 4 sizes most the time. That means you need to get something between 120-130 on the main jets( Kiehin main jets in that number range go up in weird increments usually by 5) lotsa of sites online sellin whole sets of mains for cb750's , honda dealers will have them to but usually expensive.

    Also double check that the coil wires were put on the right cylinders (long lead coil is for the outside cyl. 1 & 4 , short lead coils for the inside 2 & 3) when it was put back in the frame and make sure the centrifugal advance behind the ignition plate isn't sticking or broken n double check timing with a timing light should be idling on the "F" mark. You can feel the pipes to right after it starts up to make sure all cylinders are firing if u got one or two cold pipes u got a problem due to ignition or fuel, don't burn your hand, good luck

  18. #18
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    I'm gonna swing over to the garage after work and check all these things and I'll let everyone know about everything as soon as I get there. Thanks for the help guys.

  19. #19

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    Quote Originally Posted by jthomasbtm View Post
    I'm gonna swing over to the garage after work and check all these things and I'll let everyone know about everything as soon as I get there. Thanks for the help guys.
    I cant wait to hear!

    Nick

  20. #20
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    Quote Originally Posted by Freewheeler View Post
    I cant wait to hear!

    Nick
    Yea me too. Following your thread hoping to figure out my own 650 issues. Test run after work today. . . .gonna be a long 9 hrs lol

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