Results 1 to 7 of 7
  1. #1
    CB750 Enthusiast Chuck Hahn's Avatar
    Join Date
    Dec 2017
    Location
    Norman Oklahoma
    Posts
    49

    Unpredictable idling

    I have read many threads and dont seem to find what addresses the symptom. Bike is a 750 Four Super Sport with mechanical slde carbs. SOHC. First let me give you the back story. Bike hadnt run since 2009 so of course the carbs need gone through and here is my procedure::
    Overnight soak in Berrymans after splitting the rack and 100% disassembly of all 4 carbs.
    Overnight soak of the jets in Berrymans can #2.
    Carb spray rinse and lots of air in all passages.
    New intakes between carbs and head with new OEM clamps.
    Adjusted the valves .Hope i set them correctly. .05MM on intakes and .08MM on exhaust.
    Synced them with the Morgan Carbtune Pro ( bike started easily with full choke ).
    Bike has what looks to be an old MAC but the baffles missing.

    Now the problem. I forgot to count how many turns in it took to gently seat each pilot screw. So, I set them at 1 3/4 out. Bike idles great but when I blip the throttle it wants to hang up. I tried carb spray around the intakes ( I was out of starter fluid ) and no change what so ever in RPMs. So how many turns out should the pilot screws be out? And could this be my trouble??

    I lubed all the linkages and even checked to see if the chokes were opening fully by looseneing the clamp and allowing more slack and that didnt change anything either. Checked the air filter and even removed the filter all together...no dice. So can some add to my quandry with some other suspects to cehck?? Thanks.
    Last edited by Chuck Hahn; 05-04-2019 at 07:32 PM.

  2. #2
    Moderator dirtdigger's Avatar
    Join Date
    Feb 2012
    Location
    Black Hawk SD
    Posts
    2,749
    valve clearance looks right. If no leaks on boots and carb sync is good I would look at the ignition advancer. Probably the most overlooked thing that has a huge affect on running. The advance springs get weak and loose tension over time but the biggest issue I have seen is the advancer sticks or doesnt move freely. When you rev it the timing advances and if it doesnt snap freely then timing will still be advanced which will cause a higher rpm. after a while the speed sometimes settles back down after the sticky advancer finally allows timing to return to base. This is a good time to also check the condition of the points and clean and adjust point gap, dwell, and timing and lube the rub blocks.

  3. #3
    CB750 Enthusiast Chuck Hahn's Avatar
    Join Date
    Dec 2017
    Location
    Norman Oklahoma
    Posts
    49
    MARTEK 440 on it, But yes i can see the advancer as a problem, Will investigate...thanks.

  4. #4
    CB750 Enthusiast Chuck Hahn's Avatar
    Join Date
    Dec 2017
    Location
    Norman Oklahoma
    Posts
    49
    Found the main issue. Was with the Martek unit control box. Two wires right next to each other were worn and sporadically arching between themselves. Put heat shrink on wires and now runs on all 4 without issue. Now that its running "CORRECTLY" its time to revisit the carb sync. This should help quite a bit.
    Last edited by Chuck Hahn; 05-05-2019 at 01:30 PM.

  5. #5
    CB750 Enthusiast Chuck Hahn's Avatar
    Join Date
    Dec 2017
    Location
    Norman Oklahoma
    Posts
    49
    Got it all sorted and the test ride was amazing. This old girls got more balls than I expected.

  6. #6
    Moderator dirtdigger's Avatar
    Join Date
    Feb 2012
    Location
    Black Hawk SD
    Posts
    2,749
    Glad you figured it out. Yes the old sohc do run well when tuned properly.

  7. #7
    CB750 Enthusiast Chuck Hahn's Avatar
    Join Date
    Dec 2017
    Location
    Norman Oklahoma
    Posts
    49
    This baby yanks the front end up real good. Slapped her around and ran up to 6G between gears and shifted fast as I could to keep her in the power band...YOWZA!!! Lots of fun.

Similar Threads

  1. Rough idling low rpm + cam chain rattle
    By g10rg10 in forum The DOHC Garage (1979-1983)
    Replies: 2
    Last Post: 08-05-2018, 05:12 PM
  2. won't stay idling
    By lukers in forum The CB750 Garage
    Replies: 6
    Last Post: 04-06-2016, 10:31 PM
  3. Idling too high!
    By CivilPenny in forum The CB750 Garage
    Replies: 2
    Last Post: 06-09-2015, 05:46 PM
  4. leaky carbs and high idling
    By AAchops in forum The SOHC Garage (1969-1978)
    Replies: 0
    Last Post: 05-26-2015, 09:10 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
  •