Results 1 to 10 of 10
  1. #1
    CB750 Member les49's Avatar
    Join Date
    Jul 2020
    Location
    destin florida
    Posts
    5

    New guy with a choke question.

    Hi guys and gals. Great little forum you have here. I'm sure I will learn a lot. I have a 1980 CB750F in great condition and have a question about the choke. I don't believe mine is working properly as it is hard to start. How many of the carbs. does the choke effect? Is it a blade type choke or more of the enrichner type with a needle and jet?? It almost will not start by the choke but if I blip the throttle 3 or 4 time it might fire. Once it starts it runs perfectly. Pulls strong to 8,500 rpms in all gears.

    If I'm cruising at say 5,000 RPMs and pull the choke full up it does little or nothing to the performance of the bike.

    Is there a quick way I can tell if the choke is working properly??

    Thanks in advance for your help.

    Larry

  2. #2
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,069
    Pull off one of the outer back rubbers that joins carb to airbox, the chokes are in the last 3/4 inch at back of carbs. They are plate type and close completely shut if right. There is an accelerator pump that can help starting by repeat blipping of the throttle.

    Cruise at 5K pulling choke on should kill the engine. The engine running correctly should pull easily to 10,000 rpm, easily enough to damage it if you keep doing it.

  3. #3
    CB750 Member les49's Avatar
    Join Date
    Jul 2020
    Location
    destin florida
    Posts
    5
    Thank you for the reply. I will pull one of the outer rubbers and see what is going on. Yes it will just fly past 9,000 rpm but I try not to go above 8,500.

    Do all four of the carbs. have chokes??

    Larry.

  4. #4
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,069
    Yes..............

  5. #5
    CB750 Member les49's Avatar
    Join Date
    Jul 2020
    Location
    destin florida
    Posts
    5
    well I pulled the rubber for carb #1, and low and behold the choke blade completely closes. I see the shaft that closes the choke and it is moving going to carb 4. Can't see the middle 2. How can I pull the choke full on and it hardly effects the performance at 5,000 rpms ?

    I'm sure you can put that rubber back in in less than 1 minute but it is beating me up. Any tips???

    I have tried silicone, soapy water and still no go.

    Larry

  6. #6
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,069
    Don't know what to say as I have never encountered either issue before. I have no trouble getting the rubber back on and no special trick used at all. Using nothing to coat it.

    The choke cable of course can come out of adjustment to not close completely even if you can close the plates at the carbs by hand.

  7. #7
    CB750 Member les49's Avatar
    Join Date
    Jul 2020
    Location
    destin florida
    Posts
    5
    I decided to move the airbox back to see what the chokes were doing on the other three carbs. As you know and I found out that the left and right choke rods turn a spring and are not connected directly to the choke. I'm thinking that maybe at 5,000 or 6,000 rpms the incoming
    air overpowers the spring and the choke does not completely close.

    Anyway I put it all back together, pilled the choke and it fired right up.

    Honda had a reason the chokes are driven by a spring. Any good guess on why the used the springs???

    Larry.

  8. #8
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,069
    1. The spring provides for a breaking point to break the rack into pairs. One long rod too awkward to get together at assembly time.

    2. The spring allows for slight individual mistimings so that the chokes do not have to stay perfectly 100% in time, one side closes to hit and the other side then catches up to hit also and then evens up, spring allows that. It also allows for overtravel where if a simply completely positive cable action then operator pulling too hard might damage the plates. Spring is a shock absorber then.

    http://www.cb750c.com/publicdocs/Sea...anual_revG.pdf

    Page 31-32.
    Last edited by amc49; 08-02-2020 at 09:26 PM.

  9. #9
    CB750 Member les49's Avatar
    Join Date
    Jul 2020
    Location
    destin florida
    Posts
    5
    amc, thank you for the update. I thought about the issues you mentioned in 1 and 2 above. I will read the entire link you sent . Looks very informative. I'm going to do some more testing and reading. Do you or a friend have a 750 around a 1980 model? I would be very interested it what it does with full choke pull at 5,000.The Mine runs perfect I should probably leave it alone. LOL

    Larry

  10. #10
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,069
    I have the exact same bike and year but in pieces right now. mine would die even if the choke went on at fast idle warming up if it was past dead stone cold, it will not and should not stay running. Choke was absolutely need to start it cold but at over 90 seconds of running it quickly began to hate it.

    That should happen on anything using a choke, there is something wrong if you are able to have it full on at engine warmed up.

Similar Threads

  1. New member from CA with a question about the choke.
    By nightwolf0215 in forum The DOHC Garage (1979-1983)
    Replies: 3
    Last Post: 05-31-2016, 10:41 AM
  2. New guy introducing and question
    By Mack in forum Electrical
    Replies: 3
    Last Post: 01-30-2015, 06:05 PM
  3. hello new guy here with a question about wheels
    By iaddrummer in forum The DOHC Garage (1979-1983)
    Replies: 11
    Last Post: 08-27-2012, 07:37 PM
  4. New Guy, Suspension Question
    By SenorThumpy in forum The CB750 Garage
    Replies: 18
    Last Post: 07-18-2012, 11:52 PM
  5. Another new guy, Tank question
    By donmillerx in forum The CB750 Garage
    Replies: 2
    Last Post: 06-23-2012, 06:23 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
  •