Results 1 to 7 of 7
  1. #1
    CB750 New Member PFND18's Avatar
    Join Date
    Apr 2019
    Location
    St. Louis
    Posts
    2

    Rough idle, backfiring, a number of issues-- a plea for help.

    Hey all,

    I bought my first motorcycle, a 1979 cb750k, last July. When I bought it, I really had no idea what to look for and couldn't spot any potential issues. When I bought it, the previous owner rode it over to my place and it didn't seem to have any issues with starting (warm).

    In the coming days, I came to realize that it was very difficult to start while cold, idled rough, and occasionally had weird acceleration/surging issues. Also, before it was warmed up, it had a tendency to die while idling. I didn't ride it around much after that because the drivers in St. Louis are genuinely terrible and I hadn't yet taken a basic rider's course, so I wasn't comfortable.

    Fast forward to this year. I cleaned the carbs and discovered a cracked accelerator pump as well as a float valve that was stuck from being gunked up. I replaced the accelerator pump. The bike starts up much easier than it did last year, so I think I've mostly remedied that issue. However, it still idles pretty rough and dies sometimes before it's warmed up.

    I've got a host of questions because I don't know what's normal for these old bikes and what's not, so I'm just going to list a few of them here:

    1. When I was cleaning the carbs, I noticed that the head of the pilot screw for carburetor #1 is raised much higher than the other pilot screws. I attempted to "lightly seat" all of the pilot screws and back them out to the appropriate specification outlined in the manual (I think 1 1/2 turns or something). I didn't really have much of a feel for what lightly seating the screws feels like, though, and I didn't want to damage them. After doing this, screw #1 was again raised up much further than the other screws. My question is this: is this normal, and for some reason carb #1 is designed differently, or is there debris/corrosion that made me think it was lightly seated when it really wasn't?

    2. While trying to diagnose problems, I ended up killing my battery from starting/idling/starting it too much. I walked away, waited about 20 minutes, and was able to start it back up. When I did that, I revved it up to ~3k rpm and held it there for a minute or so. I noticed what I think was backfiring during this and ended up hitting the kill switch because the pipes were very, very hot and I think I smelled burning and saw some smoke coming off of something, I think it was an exhaust pipe. Can backfiring be caused by a weak battery, and would you expect it to get hot enough to flash-vaporize water after about a minute of 3k rpm at a standstill?

    3. How long should I be holding the choke while starting it up? For the first few minutes, I have to fiddle with it constantly to keep it within an acceptable rpm range. Too much choke and it revs high, then declines and idles rough/dies. Too little, and it idles rough/dies.

    4. The engine sounds mechanically... off, to me, but I don't really know what I'm listening for. I've got a video on my phone that wasn't meant to be showcasing the engine noise but after listening it might give you an idea of what it sounds like at idle. Please let me know if you think something sounds wrong (knocking?).

    I've uploaded a couple videos that aren't really great quality, but they may be of some use.

    The first, I was filming the exhaust. I suspected that maybe one cylinder wasn't firing because the consistent puffs of smoke out of the left exhaust, but only intermittent puffs out of the right exhaust.

    The second, refer to point #4.

    #1:

    #2:

    If better videos would help, let me know and I'll upload as soon as I can.

    Sorry for the long post, I know it's a lot to read.

    Thank you all in advance, advice would really help me out. I am essentially a complete noob and have no idea what to look for, where to start, or even what's wrong in the first place.

  2. #2
    CB750 Enthusiast NicksHonda's Avatar
    Join Date
    Apr 2018
    Location
    Sacramento, CA
    Posts
    36
    #1 I noticed that the head of the pilot screw for carburetor #1 is raised much higher than the other pilot screws
    Yes this is possible. I had the same thing when I replaced a pilot screw.

    #2 Can backfiring be caused by a weak battery, and would you expect it to get hot enough to flash-vaporize water after about a minute of 3k rpm at a standstill? I doubt a weak battery would affect backfiring. Unless maybe your charging system isn't helping. MAAAYYYyyybe, your weak battery causes intermittent ignition. Not sure why it would go to 3K rpms unless the choke was pulled to bring RPMs up.

    #3 How long should I be holding the choke while starting it up? This will depend on how cold it is outside and your state of tune. For me here in California when the weather is nice, I put mine on choke for a few seconds with the rpms running high. I bring it down and off choke after about a minute or less and I can ride away.

    #4 Please let me know if you think something sounds wrong (knocking?). I can't really tell, but in the second video it sounds like you have the clutch clatter like I have on mine. Does it go away at idle when you pull in the clutch?


    Overall, I would say, confirm you have ignition on all four coils and spark/sparkplugs. Check your compression, check valve lash, check vacuum leaks around the boots. If you haven't already, rebuild the carbs and plug the ACV.

    Good luck.
    Havin' Fun in Sacramento!

    2006 BMW R1200RT
    2002 Honda CBR954RR
    1979 Honda CB750L

  3. #3
    CB750 New Member PFND18's Avatar
    Join Date
    Apr 2019
    Location
    St. Louis
    Posts
    2
    Quote Originally Posted by NicksHonda View Post
    #1 I noticed that the head of the pilot screw for carburetor #1 is raised much higher than the other pilot screws
    Yes this is possible. I had the same thing when I replaced a pilot screw.

    #2 Can backfiring be caused by a weak battery, and would you expect it to get hot enough to flash-vaporize water after about a minute of 3k rpm at a standstill? I doubt a weak battery would affect backfiring. Unless maybe your charging system isn't helping. MAAAYYYyyybe, your weak battery causes intermittent ignition. Not sure why it would go to 3K rpms unless the choke was pulled to bring RPMs up.

    #3 How long should I be holding the choke while starting it up? This will depend on how cold it is outside and your state of tune. For me here in California when the weather is nice, I put mine on choke for a few seconds with the rpms running high. I bring it down and off choke after about a minute or less and I can ride away.

    #4 Please let me know if you think something sounds wrong (knocking?). I can't really tell, but in the second video it sounds like you have the clutch clatter like I have on mine. Does it go away at idle when you pull in the clutch?


    Overall, I would say, confirm you have ignition on all four coils and spark/sparkplugs. Check your compression, check valve lash, check vacuum leaks around the boots. If you haven't already, rebuild the carbs and plug the ACV.

    Good luck.
    Thanks so much for the advice, man.

    My brother rode in from out of town to lend me some help. We cleaned the carbs again, much more thoroughly this time, and plugged the air cutoff valves as well. I think weve got the carburetors pretty much figured out, though there is a small fuel leak at the t-shaped fuel line on the carbs that the gas tank connects to.

    At this point its evident that there are some pretty major vacuum leaks at the carburetor boots. Id like to replace them, but I called a vintage motorcycle restoration shop nearby and theyre $96/boot new. Has anyone here replaced the boots on one of these bikes, and if so, where did you source the boots?

    Thanks again.

  4. #4
    CB750 Enthusiast NicksHonda's Avatar
    Join Date
    Apr 2018
    Location
    Sacramento, CA
    Posts
    36
    I found these generic ones for $22 on amazon.
    https://www.amazon.com/Carburetor-In...gateway&sr=8-3
    There is atleast one more with prime shipping for $40.
    Not sure about the quality tho.

    Im sure there are more on eBay.

    4into1.com carries a set. These are usually more and a little better quality.

    I think another source is vintage750.com. Iirc tho I think they have them only for the pre-1979 750s.

    Good luck.

  5. #5
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    2,293
    Check compression, most perceived 'carb' issues on these bikes comes from lack of compression due to owners never setting the valves, then they burn because on these the valve clearances commonly close up instead of open up like on most other designs. The backfire can be either that or the plugs wetfouled and that comes from the same issue, no compression makes the plug temperatures drop off enough to foul easily. Run the valves; you have zero clearance with readings of .002" on the valves, they do not run the same as what you get with engine not running. They burn valves at .002".
    Last edited by amc49; 06-12-2019 at 03:57 AM.

  6. #6
    CB750 New Member dh1125cr's Avatar
    Join Date
    Feb 2018
    Location
    oley pa
    Posts
    3

    manifold

    I bought the cheap ones from e-bay and i cant get two of them to seal. They look the same as the originals but maybe they are made with different rubber? I cant compare them to the 40 year old ones cause they are hard as rock. Is there a trick to lining them up and getting them to seal? Or is it pretty straight forward?

  7. #7
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    2,293
    Straightforward as long as the part is a quality one.

    The mixture screws SHOULD be close to even unless you can pull them out to find one is physically longer than the other on the HEAD only. Some screws have different heads ONLY, the rest of it should be the same. They should be the same at the start of thread to the very small needle tip. If all the same length then the difference in height is due to incompetents screwing them in too hard to screw up the seats and then you lose all concept of so many turns to closed. the damage done to seats precludes all ideas of that. If you can have carbs off and see the tip of the mixture screw sticking up into the carb bore then likely there is damage there, they should come close to but NOT enter the carb bore.

Similar Threads

  1. 93 CB 750 Nighthawk idle issues
    By Timberfield in forum The DOHC Garage (1984+)
    Replies: 4
    Last Post: 04-25-2019, 11:26 AM
  2. 1980 CB 750 K rough idle
    By Smithandbacon in forum Builds
    Replies: 1
    Last Post: 08-12-2017, 02:34 AM
  3. 1980 CB750K idle issues
    By powellskier in forum The DOHC Garage (1979-1983)
    Replies: 2
    Last Post: 05-27-2017, 08:05 PM
  4. High idle run on backfiring.
    By nighthawking in forum The CB750 Garage
    Replies: 7
    Last Post: 06-20-2014, 11:36 PM
  5. Throttle hesitates around 6-7 k RPMs, Rough idle.
    By narclark in forum Carburetors / Fuel
    Replies: 2
    Last Post: 08-01-2013, 02:18 PM

Tags for this Thread

Posting Permissions

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