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  1. #1

    Slowly dying at idle with rough running.

    I have a stock '79 that I have been slowly working on as time allows. After an extensive amount of work overhauling the electrical, suspension, wheels, and tires I am tackling the carbs/engine.

    Behavior:
    Immediately starts with choke. Begins revving high immediately. Turn choke to 25% and bike idles ~1100-1500 and smooth. After 5-10 mins move choke to off and bike just slowly dies over 30 seconds.

    Observations:
    Heavy leak and air "puffing" out of manifold boot on cylinder 4. Carbs don't appear to have slid all the way in to the manifold boots, but are fully seated in the airbox boots. The airbox top "slide bolt" is only halfway down the rail. It looks like the carbs should advance toward the engine with the airbox another 1-1.5inches, but they don't. I can't tell what it is hanging on, but shouldn't the top bolt for the airbox to frame be all the way forward? You can see the carb lips inside the manifold insulators and feel air escaping with the bike running. Spraying carb cleaner causes the idle to skyrocket. Less so when spray the airbox side.

    Next Steps:
    I have new manifold insulators on the way as they are brittle. There are tears in the airbox insulators, but I can't seem to find them anywhere online. Do they pull out of the airbox? They seem like they just pop out, but in the event they don't and I can't find new ones I am leaving them be for now. I put a new accelerator pump assembly in, ultrasonic cleaned all the parts, and then sync'd the carbs on the bike at idle (it worked immediately after reassembling, but everything went to shit after we had a 38 degree morning for my first ride on the thing and it has been a pain since). The bike has had many owners and I just pulled the [I]original tires[I] off of the bike. Yeah, so I'd bet a hell of a lot of money that the valves have never been checked. I'll do a compression test, replace the manifold insulators, look for airbox insulators, and probably tear down the carbs again as everything looked okay when I cleaned it, but all the reading I am doing about this finicky CV set up has me second guessing the condition of the air cut off valves.

    Questions:
    Is there anything I am missing?
    I have heard a lot about ditching the finicky CVs and getting a makuni RS setup as a recommendation, but can't find information about anyone actually using them. Is this wise? I thought I knew carbs before but, good gravy does this set up give me fits. Are the makuni RS carbs a good idea? The simpler my life is, the better.

  2. #2
    CB750 Guru amc49's Avatar
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    Use the carbs you got, the other ones cost more than bike is worth.

    Compression test first to make sure you like everyone else are not doing endless work for no reason. The bikes are 40 years old yet everybody seems to miss that then the usual male testosterone thing of 'I will fix the carbs and it'll run fine'. They don't. The carbs DO give fits but it doesn't matter if the engine leaks at compression, it still won't idle correctly. Shove the carb bank up more fully forward too, the airbox bolt position has no bearing on that.

    The air tubes to airbox are available.

    https://oldbikebarn.com/products/hon...ubber-boot-set
    Last edited by amc49; 12-09-2019 at 03:16 PM.

  3. #3
    Quote Originally Posted by amc49 View Post
    Use the carbs you got, the other ones cost more than bike is worth.

    Compression test first to make sure you like everyone else are not doing endless work for no reason. The bikes are 40 years old yet everybody seems to miss that then the usual male testosterone thing of 'I will fix the carbs and it'll run fine'. They don't. The carbs DO give fits but it doesn't matter if the engine leaks at compression, it still won't idle correctly. Shove the carb bank up more fully forward too, the airbox bolt position has no bearing on that.

    The air tubes to airbox are available.

    https://oldbikebarn.com/products/hon...ubber-boot-set
    Thanks AMC. I ordered those linked insulators and an air filter. Great site. Now I need to get a compression tool and get cracking.

    I also need to figure out how to get the whole carb assembly to slide forward. No amount of force is getting it to budge.
    -79 CB750K

  4. #4
    CB750 Guru amc49's Avatar
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    You need new front rubber, the old cracks if you get carbs in then you have vacuum leaks so you haven't cured anything. On those engines the manifolds cook to be hard as rock.

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