1980 CB750 - Page 3
Close

1980 CB750

This is a discussion on 1980 CB750 within the Project Builds forums, part of the Caferacer.net Forums category; which one?...

Page 3 of 4 FirstFirst 1234 LastLast
Results 21 to 30 of 32

Thread: 1980 CB750

  1. #21
    Senior Member catboy's Avatar
    Join Date
    Aug 2006
    Location
    Detroit Metroplex, MI, USA.
    Posts
    2,135
    which one?
    iphone generated fag

  2. #22
    Senior Member oneworden's Avatar
    Join Date
    Jul 2008
    Location
    franklin, WI, USA.
    Posts
    854
    For diagnosing this issue I would resort to shining a flashlight on the wall and making silhouette puppets while drinking so much that I collapse and stay on my garage floor till morning. I only say that because that would be just as productive as trying to get a 750 DOHC running correctly. I should know I finally got rid of mine.

    Honestly though, either keep the bike as stock as possible or get ready to make some real changes.

    http://www.cyclexchange.net/One%20Of...ind%20Page.htm

    Cycle Exchange has some great parts for DOHC's, they are mainly building choppers out of them, but the CR carbs are worth there weight in gold over the CV's on there now.

    Also if that Exhaust is clean (meaning no rust holes) polish it up and sell it on ebay as quick as you can. It's heavy, and you can get a good amount of money for it (atleast enough to cover the cost of a new 4 into 1 system).

    Other than that it could be a pretty cool bike if you take your time and use the right parts.

    Good luck,
    Did you hear about the pirate with two wooden legs?

    They caught fire and burned his ass to the ground!

  3. #23
    Senior Member Teazer's Avatar
    Join Date
    Mar 2010
    Location
    , IL, .
    Posts
    2,905
    Which diagnostic technique? Probably the it's sat around for years and based on years of experience, leads one to hypothesize that the carbs have a 99% probability of being full of stale gas, rust and fuel residue.

    Ignition and compression related issues could also exist, but the carbs are unlikely to be clean and usable as they are, so they should come off for a clean anyway.

    After the carbs are rebuilt and the rubbers checked - then is the time to move along the diagnostic path. For all we know at this point, the motor could have zero compression on any or all of the cylinders. That would suck, but teh carbs still need to be cleaned and the fuel flow checked.

    I'm not speculating as to what may or may not be the problem, but suggesting the carbs are a good place to start. And then compression and electrics.




  4. Remove Advertisements
    CafeRacer.net
    Advertisements
     

  5. #24
    Senior Member KeninIowa's Avatar
    Join Date
    Feb 2006
    Posts
    2,670
    quote:Originally posted by spacmanspif300

    shitting rainbows full of bad mojo.
    You can't shit rainbows full of bad mojo

    They're fucking RAINBOWS!

    Other unicorn caused damage, very possible

  6. #25
    Senior Member kenessex's Avatar
    Join Date
    Oct 2005
    Location
    Galena, AK, USA.
    Posts
    5,780
    quote:Originally posted by Teazer

    Which diagnostic technique? Probably the it's sat around for years and based on years of experience, leads one to hypothesize that the carbs have a 99% probability of being full of stale gas, rust and fuel residue.

    Ignition and compression related issues could also exist, but the carbs are unlikely to be clean and usable as they are, so they should come off for a clean anyway.

    After the carbs are rebuilt and the rubbers checked - then is the time to move along the diagnostic path. For all we know at this point, the motor could have zero compression on any or all of the cylinders. That would suck, but teh carbs still need to be cleaned and the fuel flow checked.

    I'm not speculating as to what may or may not be the problem, but suggesting the carbs are a good place to start. And then compression and electrics.



    I guess I wouldn't want to pay you by the hour to work on my bike. Cleaning the carbs because they might need it is not the diagnostic route I would choose. I might go on a more methodical journey towards enlightenment.

    I would ask myself, " Self, i says, what is the easiest thing to check that would prevent my new wonder bike from starting?"

    Having said that I would answer, " Spark!!! Does it have spark?"

    " I don't know, but if I pull all of the spark plugs and ground them against the cylinder head and then turn the engine over I can check all 4 for spark and get a look at said plugs,while I'm at it, to see if they give me any other hints as to what might be going on here."

    "Ooo, look, big fat blue sparks on 2 and 3 and no sparks at all on 1 and 4. What does that mean?"

    "I guess it means I better clean those carbs because they might need it. Then I will replace the faulty ignitor that thsse DOHC 750s are known for."

    o for me, I might try to figure out the problem before I start "fixing" stuff. By the way, since the plugs are already out I would probably do a compression test at this point.


    Of course that is just my opinion and I might be wrong.

    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!!

  7. #26
    Senior Member Teazer's Avatar
    Join Date
    Mar 2010
    Location
    , IL, .
    Posts
    2,905
    "I guess I wouldn't want to pay you by the hour to work on my bike"

    I'm not offering. And after you determine that it was or was not the sparks, you still need to clean those stinkin' old carbs. To be honest, there's two ways to approach the stated problem.

    One is your approach and if the bike wad been running recently, would be a perfectly valid approach that I'd probably use too.

    And then there's the approach that comes from the other end of the spectrum. This bike sounds like it has not run properly for a while and we know for experience with that particular bike, that carbs are a frequent source of heartache that maybe it would be appropriate to give it a good service, starting with the carbs.

    Start wherever. I already mentioned that the owner will have to diagnose the problem in a systematic way.

    Our only difference is where we think it's appropriate to start.

    My experience with that model is that the carbs are a PIA more often than the ignition system. Your experience may be different. Doesn't make either one more or less relevant.




  8. #27
    Junior Member PaulZ01's Avatar
    Join Date
    Nov 2009
    Location
    Brooklyn, NJ, USA.
    Posts
    28
    OK, i agree with you guys that the carbs are completely gunked up and they need to be rebuilt so I am just waiting for the $ to get it done since this is a project build.

    And I got one quick question for ya...I pulled the air box off so that the carbs are only attached to the rubber boots connected to the motor and I am having trouble keeping them on. The carbs keep falling / popping off the rubber boots. I tried moving them around and hitting them with some WD40 but no luck. Is it that I just need to get new ones or do you guys use something else when you run individual air filters?

  9. #28
    Senior Member oneworden's Avatar
    Join Date
    Jul 2008
    Location
    franklin, WI, USA.
    Posts
    854
    Running pods on the CV carbs will give you nothing, but headaches. They really need the pressure that the aribox provides.

    If I started over on my old DOHC I would pick up the carbs from cycle exchange. they are not cheap, but they will make you 100 times happier in the long run. plus you could ebay the old ones to get some cash back.
    Did you hear about the pirate with two wooden legs?

    They caught fire and burned his ass to the ground!

  10. #29
    Senior Member Victor52b's Avatar
    Join Date
    Feb 2009
    Location
    Eastford, CT, Sri Lanka.
    Posts
    172
    I gotta say, that while I'd have no problem recommending cleaning the carbs, I'm mostly with Ken on this one. Paul says it ran good when he looked at it. Even if the pilot jets are clogged or dirty, you should be able to get it to start.

    On many occasions I myself have initially blamed carbs for most any fault. Maybe that is because they are so mystical in function. I will have to say though that more often than not the fault was NOT carbs, and I wasted time fiddling with them when I should have been doing better diagnostic work. The last two times I initially blamed carbs it turned out to be 1) coil, 2) plug wire.

    My initial diagnostic method is to try to figure out what has changed since the last time the bike ran well. I note that it was apparently raining pretty good in the photos, and that the bike is obviously quite wet. I would first check for issues related to water in one system or the other.

    BTW... Paul, have you made any progress on this?

  11. #30
    Senior Member Teazer's Avatar
    Join Date
    Mar 2010
    Location
    , IL, .
    Posts
    2,905
    That sounds like a reasonable approach Victor. Spray WD40 over coils and connectors and blow it all out with a leaf blower or hair dryer.

    OR - it was running when I got there!! Started on starter fluid because the pilot circuits are all blocked perhaps.

    And yes. More to teh point, how far has it got?

Page 3 of 4 FirstFirst 1234 LastLast

Sponsored Links

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Similar Threads

  1. general problems on 1980 cb750
    By btuck in forum General
    Replies: 13
    Last Post: 05-04-2012, 11:19 AM
  2. 1980 Honda CB750 Problems
    By pistonhead in forum General
    Replies: 4
    Last Post: 05-01-2012, 07:08 PM
  3. What backrests will fit my 1980 CB750 Custom?
    By Corytrade in forum Technical
    Replies: 1
    Last Post: 05-16-2011, 10:13 AM
  4. "Bomber" My 1980 CB750 project
    By Psyco Diver 69 in forum Project Builds
    Replies: 22
    Last Post: 11-14-2007, 09:35 AM
  5. 1980 CB750 F Supersport F/S - REDUCED!!!
    By texmawby in forum Bikes For Sale
    Replies: 5
    Last Post: 09-04-2007, 09:37 AM

Search tags for this page

special cb 750

,
what is a burnt valve symptoms cb750
Click on a term to search for related topics.