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  1. #1
    CB750 Member Daddyoo's Avatar
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    Mar 2019
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    1977 CB750F Carb Rebuild or Quick Fix Where do I start?

    Greetings! New to the sport with purchase of a 1977 CB750F. I purchased bike that had been sitting in sun and rain for 2 years. With new battery, new plugs and fresh tank of gas bike started with little effort. Would not run idling and died out randomly at 4K RPM when trying to warm up ( no choke, choke cable was removed to accommodate 4 individual air filters). If I sprayed Starting fluid over the air filters it started right up but ran sporadic with sometimes slight pinging engine noise. With not a great deal of time. I removed the carburetors from bike. Open just one of the float bowls and looked LIKE NEW inside with no varnish residue or debris. I blew out and just the sprayed carb cleaner in just the one carb. Reassembled with pretty much same result. Supposedly bike ran perfect till the day this was left to rot in the sun and rain for 2 years.

    I have a good deal working experience with old car and boat engines and pretty much fixing anything automotive. But, bikes seem to be bit more temperamental... So my question is: Where do I start? Before spending monies and effort in rebuilding the carbs. Is there anything I should check off list first be sure it is the carbs like timing, points cleaning or all the above. I was hopping to get this thing running initially to blow out any cobwebs since it was just sitting. Hate to tear anything apart if I do not need to...

    Lastly, If anyone has any recommendations of best place to purchase break calipers, lines, rotors and general CB750 parts. Seems like there are allot of Web choices just wanted some advice where can get the best parts for best price. Also, since these carbs have been torn apart by hackers before... I might want to get them gone through the first time by a pro service to know they are right. Any recommendations for a good Web based carb repair shop?

    Thanks for your help and patience in advance.

  2. #2
    Moderator dirtdigger's Avatar
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    Feb 2012
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    The pinging noise is probably too much starting fluid, not really the best thing for an engine, but anyway. Open up all the carbs and blow threw the jets and emulsion tubes. Make sure floats are free and adjusted to correct float height. The accel pump diaphram is probably hard and bad and the check balls are probably stuck. Doesnt matter how good it ran when they parked it with the gas the way it is it EASY could have plugged or at least restricted jets or passages. Need to get the choke working as these carbs are lean from the factory especially if the accel pump isnt working. The choke setup should work if they are decent pod filters, sometimes it just takes a little tweak of the linkage to get them to clear. The carbs are pretty basic so if you have worked on carbs before they really shouldnt be an issue. Dont really know of anyone that does just carbs, if you cant find anyone I could probably take a look at them or if you just want to open them up and take some pictures I could probably tell if it looks like it should.

    I wouldnt run anything till you do all the tuneup items....valve adjust, cam chain adjust, clean or replace the points, adjust the timing and dwell, sync the carbs once they are clean, fresh oil and filter. First things first I would do a compression test to make sure you arent trying to get a dead engine running.

    Some stuff is still available from Honda but not much. CB750 supply has a lot of stock type parts and they do a giveaway monthly on here so they help the site out. I do mostly performance related stuff on the 750's so my main suppliers are Cyclex, Dynoman and APE, but if wouldnt hurt for you to look at some of the stuff they offer for these bikes so you know whats available.

  3. #3
    CB750 Addict pidjones's Avatar
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    Sep 2018
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    East TN
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    There really are no shortcuts on bikes, but the 2 year abandonment has good and bad things. Good that it isn't nearly what has been seen (mine was in East Tennessee weather since 1999!) Bad in that it was probably ethanol gas, and that is much worse than 100% gasoline. I have a DOHC without points. If that model has points, they should be replaced, gapped, and the condensers replaced. Adjust the valves and test compression. The carbs really should be pulled, cleaned (SeanG procedure is great) and balanced. If you have pods, good luck getting good performance. If you have the stock airbox/filter, pull the filter and blow the rat/mouse/chipmonk nest out of it, and replace the filter after making sure it is good. Install an inline fuel filter after draining the tank, inspecting for and treating rust, removing petcovk and inspecting for operation and cleaning. Make sure any in-tank filter is in good shape. The carbs have very small passages and smaller jets. And fuel contamination will foul operation. Source and use only 100% gasoline if at all possible (high-test is not desirable and can cause carbon build-up unless you are racing).
    "Love 'em all.... Let GOD sort 'em out!"

  4. #4
    Moderator dirtdigger's Avatar
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    The sohc bikes run fine with pods, not an issue just appropriate jetting...easy to do.

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