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1980 CB750

This is a discussion on 1980 CB750 within the Project Builds forums, part of the Caferacer.net Forums category; My 'puter won't play your clip. I'm under the impression that you have to jump it every time. It's never going to run right w/a ...

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Thread: 1980 CB750

  1. #11
    Senior Member KeninIowa's Avatar
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    My 'puter won't play your clip.

    I'm under the impression that you have to jump it every time. It's never going to run right w/a junk battery. You may get it to start and run for awhile, but it's drawing power from the ignition system trying to charge a brick.

    Start w/a fresh battery.

  2. #12
    Administrator borzwazie's Avatar
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    Engines need air, fuel, and spark to go.

    Have you cleaned the airfilters? (you can pull them off for troubleshooting)

    Have you cleaned the carbs? That means taking them off the bike, pulling off the float bowls, getting cleaner down all the little channels, and blowing them out clean. If you haven't done this, stop, and do it. Set the float height according to the manual.

    A shitty battery won't give you consistent spark. A shitty battery even with a jump will make troubleshooting fucking miserable. Get a new battery, less than 50 bucks at any auto-parts store.

    Do you have spark? Get a new plug, ground it out against the case or head, and crank. Do you have a fat, blue spark? If you don't, it's gonna run like shit. Check/replace plugs and wires. Clean the points, doesn't take much.

    Have you put a new set of properly-gapped plugs in? They're like 1.50 apiece, it's the cheapest fix ever. The guy at the counter will probably even toss in one of those round plug-gap tools for free. USE THE STOCK RECOMMENDED PLUGS.

    How's the flow out of the tank? Have you cleaned the tank? Is the filter screen at the petcock clogged?
    "This site is bullshit."

  3. #13
    Senior Member joe1028's Avatar
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    looking at the bike its been sitting for a while ......so carbs are probably in need of a clean/rebuild...bad thing is you have one of the worst keihins honda used with 4 seperate aircut off diaphragms. they may be bad from age, someone using carb cleaners,even some fuel stabilizers degrade(soften-leak)the diaphragms.you have to take each carbs off the brackets/linkage to replace/clean passages for three of them...i'd ck compression 1st,spark,plug condition, plug cap resistance (10k ohm stock), choke is all the way closed and your bowls have fuel(loosen drains)..joe

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  5. #14
    Senior Member Teazer's Avatar
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    Joe is spot on. Clean the carbs first and check the petcock/fuel tap for debris and other accumulated rust deposits. Those carbs are a total PIA to set up for different exhaust or filters, and not a lot of fun to fit with that airbox, but it has to be done.

    Start by making usre the fuel is getting to all 4 carbs and work form there.

  6. #15
    Senior Member spacmanspif300's Avatar
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    youve probably got a little unicorn in there, jumping around, putting holes in shit, and shitting rainbows full of bad mojo. you should let him out. after all, he'd rather be running through space with santa claus and old vans and falcor...


  7. #16
    Senior Member raven's Avatar
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    Also check the flow out each drain tube. (open the drain on each carb with the petcock on). Does it still have the vacuum petcock on the right side of the carb rack? (should be a round thing with two fuel hoses and a vacuum line from #2 carb going to it)

    Charging systems can be problematic on these bikes. They CAN NOT overcome a dead battery at idle. It aint gonna happen. You usually dont start breaking even until like 2k rpm with the lights on and a great battery. The ignition is not a magneto type, it is a battery driven CDI and as such puts a big demand on the battery at idle. Buy a new battery, new plugs, new wires, and clean the carbs as stated above. Check the rubbers between the carbs and the head and the carbs and the airbox for hardness, cracking, or molestation. Bad intake rubbers will DRIVE YOU NUTS. (funny how the result of bad rubbers tend to do that...)

    And check the valve clearances. If one is too tight it WILL burn the valve, sooner rather than later and sometimes it runs just fine with one too tight. Burnt valve=pulling the engine. Not pulling the head, the whole engine and then the head.
    Bike:
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    My collar may be a little frayed, and maybe I need a shoeshine. But nobody\'s got a mortgage on my soul. I own it. Free and clear. -Phil Lovecraft

  8. #17
    Senior Member catboy's Avatar
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    why do so many people immediately jump to carbs as the source of every damn problem?

    what school of trouble shooting does this philosophy come from?
    iphone generated fag

  9. #18
    Senior Member kenessex's Avatar
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    quote:Originally posted by catboy

    why does so many people immediately jump to carbs as the source of every damn problem?
    Acually it should be why [u]do</u> so many ....

    But, grammar aside, it is easiest to pick carbs first since so many vague symptoms, could be carburetion. Plus electrical is so full of smoke, mirrors and mystery that nobody could possibly understand, much less diagnose it over the interwebs.

    Of course it could be a tight valve, too, which means you have to pull the motor. If it was me, I would just strip everything down and plan on a complete rebuild which I would start by powder coating the frame and pounding dents in the tank. I would follow up by asking a series of questions about internal wiring in clubman bars, swapping forks and rejetting for pods and an exhaust that I want to use wrap on. Last, but not least, I would ask everybody about rebuilding the brakes and checking the charging system, while still using a crappy battery that won't hold a charge.

    Oh Yeah, what kind of tires, shocks, fork oil and plugs should I use and is it OK to paint it flat black? Don't forget to be nice and polite to me even if I don't fill out my bio and get snippy if I don't like your answers.


    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
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  10. #19
    Senior Member catboy's Avatar
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    but i corrected it already, sir!!!

    all by myself!
    iphone generated fag

  11. #20
    Senior Member kenessex's Avatar
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    Too Bad Catboy,
    I caught it and you.

    How's your 750 doing? Ready to ride?

    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!!

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