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  1. #1
    CB750 Enthusiast NicksHonda's Avatar
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    Under 4K hesitation

    Under 4K whether blipping the throttle or under load it hesitates. After 4K it takes off and runs great.
    Stock 1979 CB750L with 15,442 miles.
    It starts right up from cold with the choke on. After a little while I can pretty soon push the choke in and it idles fine.

    I sprayed WD40 all around the intake boots and no noticeable change in RPM.
    I have the same cylinder pressure on all cylinders.
    I rebuilt the carbs three times now. I double checked the float and they are all at 15mm.

    I noticed on mine, my #2 cylinder is running much cooler than the other cylinders. After it runs for a while, if I spray a little fluid on the pipes, the other three burn the water right off and sizzle. #2 doesn't sizzle so much.

    I pulled the plugs and the #2 plug was wet with gas. I changed the plugs.

    When I cleaned my carbs I found the accelerator pump pumps (what seems like) a lot of fuel.

    I was thinking maybe mine needs a throttle sync. I have heard from other forums you shouldn't have to sync the carbs. I have a stock 1979 CB750L with stock air cleaner. I found my pilot jets seem to run well at 1 3/4 out on all the carbs. I have the yellow mark on the carb nuts for the butterflies. Makes me think it has never had a sync done.

    I swapped ignition wires between #2 and #3. No change in #2 cylinder temp.

    I'm thinking it MIGHT be a tight valve on #2, or it needs throttle sync.

    Any other input besides swapping carbs with another brand?

    Thanks in advance.
    Nick
    Havin' Fun in Sacramento!

    2006 BMW R1200RT
    2002 Honda CBR954RR
    1979 Honda CB750L

  2. #2
    CB750 Addict pidjones's Avatar
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    If you've never checked them, you probably have tight valves on several cylinders. Check your compression and valve clearance. If you have really rebuilt (which requires splitting the rack) the carbs and not balanced them then yes, you need to balance them - at least a bench balance which gets them very close. Get SeanG's carb rebuild document.
    "Love 'em all.... Let GOD sort 'em out!"

  3. #3
    CB750 Enthusiast NicksHonda's Avatar
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    They are all tight. I can get my .0015 in all of them. I can not get .002 in any of them. Getting the tool tomorrow (hopefully) to get the shims out. I am guessing they are all stock at .280. I will find out before I order shims. Joy. I was hoping to order a kit of shims, but that looks really expensive. Then again, at $7 each I will need eight of them, that's $56.
    Havin' Fun in Sacramento!

    2006 BMW R1200RT
    2002 Honda CBR954RR
    1979 Honda CB750L

  4. #4
    CB750 Addict pidjones's Avatar
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    You can usually find them for $6 on ebay. Sometimes a bunch for not much.
    "Love 'em all.... Let GOD sort 'em out!"

  5. #5
    Moderator dirtdigger's Avatar
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    You need to use .004 to .006 for valve clearance not honda spec. So you want to hit .005" You also will need shims for 16 valves not 8 as the dohc is a 4 valve per cylinder. There is no "stock" shim size they will be whatever was needed to get the clearance. You will have to remove all the shims so you can figure out what size each individual valve needs.

  6. #6
    CB750 Addict pidjones's Avatar
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    And, please don't rotate the cams without shims in all of the buckets!
    "Love 'em all.... Let GOD sort 'em out!"

  7. #7
    CB750 Enthusiast NicksHonda's Avatar
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    Ok. So I知 measuring in mm. You池e telling me to measure in inches by your quote marks. Correct? .04 inches is .10 mm.

    My clymer manual says .08 mm gap. So a wee bit bigger.

    Fun find, my shims show 290 mark and measure at 2.80mm. One has a mark of 300 and is measuring at 2.80 mm. Sounds like I need a few 2.70 mm and a few 2.60mm.

    I知 off to measure the rest.

  8. #8
    CB750 Enthusiast NicksHonda's Avatar
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    I see I used inches to measure the gap in my first thread. Sorry about that. I plan to use mm going forward. As such, I can get a .04 mm in all of them. I can not get a .05 mm in any of them.

    Thanks.

  9. #9
    CB750 Addict pidjones's Avatar
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    I've never seen a shim over .03 off.
    "Love 'em all.... Let GOD sort 'em out!"

  10. #10
    CB750 Enthusiast NicksHonda's Avatar
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    I better check my mic. Thanks.

  11. #11
    CB750 Enthusiast NicksHonda's Avatar
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    I check the mic. It was off. I now have .013 mm exhaust on all except #1. #1 is now .010 on both valves exhaust.

    The intake are all .010 except #4 intake is at 013mm.
    I still have a #2 cold cylinder exhaust pipe.

    This seems to be a float issue in the carb. I also read someone said the o-ring on the #2 carb accelerator pump could leak.

    i will swap the plugs on #1 and #2 real quick and check. Even though I changed #2 plug out with a new one.

    Then I will check carb float height on #2. I am pretty sure they were all at 15mm or 16mm, but I didn't write it down.
    Havin' Fun in Sacramento!

    2006 BMW R1200RT
    2002 Honda CBR954RR
    1979 Honda CB750L

  12. #12
    CB750 Enthusiast NicksHonda's Avatar
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    Sooo.

    I知 under the impression the accelerator pump is expected to squirt fuel into all four carburetors, correct?

    This is not happening.

    I知 guessing the fuel rail looking piece connecting what seems to be the accelerator squirt to all four carbuerators is blocked. Only #2 is squirting.

    Can anyone confirm this?

  13. #13
    CB750 Addict pidjones's Avatar
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    Quote Originally Posted by NicksHonda View Post
    Sooo.

    I知 under the impression the accelerator pump is expected to squirt fuel into all four carburetors, correct?

    This is not happening.

    I知 guessing the fuel rail looking piece connecting what seems to be the accelerator squirt to all four carbuerators is blocked. Only #2 is squirting.

    Can anyone confirm this?
    More likely the spray nozzles in the others are blocked. The cross-over tube has a good size passage but should be cleaned to prevent crud from getting into the accel jets. On my '79F all of these hard to access jets were blocked. I had to use a high G string from a 12 string guitar set chucked in a pin vise to reach from the outlet end of each carb and poke around until it hit the hole in the little brass tube and spun the string in to open it up. Then I flushed it well with cleaner. Make sure you have good o-rings on the accelerator cross-tubes, and the #2 bowl accel pump o-ring is the correct size (too fat will plug accel pump flow).
    "Love 'em all.... Let GOD sort 'em out!"

  14. #14
    CB750 Enthusiast NicksHonda's Avatar
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    The accelerator pump wasn稚 the problem.

    The #2 air cut off diaphragm was ripped. Apparently the thing to do is block the air cut off valve. I did this and it all works great.

    I知 going to plug the rest.

    Thabks all for the help.

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