1981 Honda CB650C aka "The Titanic's Anchor"

Discussion in 'Old's Cool' started by F16Viper68, Nov 15, 2011.

  1. F16Viper68

    F16Viper68 Trapped in the Sandbox!

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    I have a 1981 Honda CB650C I recently purchased off of craigslist which hasn't run in about ten years. It's in rough shape cosmetically but only has 8k miles on it. Over the course of past few weeks, I've rigged up a temp gas tank, adjusted the valves, performed a compression check (150 psi across all four) replaced the spark plugs, changed the oil/filter, changed the air cleaner, replaced the battery, and rebuilt the carbs.

    It's running now on all four cylinders. It was only running on three cylinders due to the slow speed jet on #3 being plugged. To fix that issue I disassembled the carbs (not the rack), soaked in pine sol overnight, flushed with water, air, carb cleaner, and air again. Did your standard carb rebuild and re-installed.

    It's running pretty smooth but I noticed a pretty wide temperature difference when I check the exhaust tubes (as they exit the motor) with an infrared gun.

    #1 = 100 degrees F
    #2 = 175 degrees F
    #3 = 268 degrees F
    #4 = 150 degrees F

    Is this normal? If not, can it be caused by the idle mixture which has not been fine tuned or the carbs being out of sync? Waiting on manometer.

    Dave...

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    #1
  2. Brad Felmey

    Brad Felmey Long timer

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    Short answer: yes.
    #2
  3. concours

    concours WFO for 41 years

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    Have you synced them yet?
    #3
  4. Motomantra

    Motomantra Registered Lurker

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    ASSuming the carbs are clean now, float level must be correct. Too low will make a lean condition.

    Be sure there is no air leak. Typically the carb boots/holders that attach to the head will get old & leak. Offering a LITTLE propane to each cylinder will tell you if any are lean. If the idle falls off it's too rich, if it raises it's too lean.

    Consider what the infrared tool is looking at. I've found it can be sensitive to textures. Like if one pipe is dirty, you'll get a different reading.

    Finish the job by syncing them & getting the mix screws adjusted. (Did I mention float levels must be correct.)
    #4
  5. Purcell69

    Purcell69 Mors ex Tenebris

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    And again, yes. The carbs not being sync'd and in proper mixture/float level will cause variances in your readings. Cooler tends to indicate a richer mixture and hotter tends to indicate a lean condition.

    -Joe
    #5
  6. Tosh Togo

    Tosh Togo Long timer

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    And going leaner than peak also results in lower temps. :deal

    Synch the carbs, and see what you get.

    fwiw, the inner exhaust cylinders' temps should be noticeably higher, especially when the bike's parked to check temps and the breeze has gone away. :evil
    #6
  7. High Country Herb

    High Country Herb Adventure Connoiseur

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    Good tips here.

    As Motomantra said, cracks in the rubber between the carbs and cylinders can cause those lean conditions. A good way to check that is with a small squirt of WD-40 around the "intake manifold". If the idle changes, you've got a leak.
    #7
  8. CycleDoc59

    CycleDoc59 Wrench Rider

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    Those temps are very low; either the pipes are double wall where you are getting readings, or the bike was
    only running for a few moments. And considering the low temps, the difference is small. Usually temps at
    idle where the pipes exit the heads run 3-400 or more, and if one shows 100 or so, that cylinder is not running,
    with that temp being just the result of pumping friction....

    Recheck where you took the readings, and let the bike run for 15 minutes before you take temp readings. Since
    you did not remove the carbs from each other, they are likely still in sync. Since the bike runs smoothly at
    idle, it's probably fine. You can pull off each spark plug cap in turn with the bike at idle. Each time one is pulled
    off, the rpm should drop about the same amount....
    #8
  9. concours

    concours WFO for 41 years

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    Yes, teh Jap bikes (with chrome pipes) have double wall pipes. The float level at idle will only be an issue if it's grossly misadjusted, not likely. The "high temps from a lean condition" are ONLY AT WOT AND ENGINE LOAD, or at least a nearly so throttle opening and load. At idle, a lean condition will show a cooler exhaust temp.
    #9
  10. F16Viper68

    F16Viper68 Trapped in the Sandbox!

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    Thanks for all the advice. :clap To answer some of the questions:

    Float Levels - The float levels on these carbs can't be adjusted, it's either in spec or the float needs to be replaced. I discovered after the fact you can check them but can't adjust them.

    Sync - No, I haven't sync'ed them yet since I don't have a manometer. I ordered Motion Pro manometer and adjustment tool last night.

    Idle Mixture - I have the screws set 2.5 out. I'm getting conflicting info on the initial setting (2.25 and 2.5). According to the manuals I have, I need a more accurate tach since the bikes tach is not accurate enough. Any suggestions?

    Temp Readings - The bike had been running about 15 minutes when I took the readings. I do admit the variances seem wide and low. Had me baffled. I'll recheck when I get a chance.

    Cylinders - I did notice a drop in RPM's when I removed the wires from each cylinder but the drop was more noticable when I removed #3 and #4. #1 minor drop, #2 more, #3 a lot, and #4 near stalls.

    Intake Boot Cracks - I tried the WD-40 trick when I was troubleshooting the #3 cylinder not firing and didn't notice any change in RPM. I'll check again when I recheck the temperatures.

    Thanks again for all the advice.

    Dave...
    #10
  11. anonny

    anonny What could go wrong?

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    If you are shooting an IR gun at the exhaust.... IR does not like reflective materials and the chrome will give false readings.
    #11
  12. DirtViking

    DirtViking SKOL!

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    I'd probably go back to the jets on #3 carb to ensure they are in fact clean... It may have looked clean, but since that was your troubled cylinder, it would be worth another look. Are you sure that the #3 idle jet is the same as the others? Stock? Jets are easily changed by owners fiddling with different intake systems and exhaust.

    Reading the plugs would give you a pretty good idea of what's going on. At a minimum, they can be compared to one another.

    You could also compare the plugs after riding at 3000 rpm or so and then chopping the power. Coast to a stop and read the plugs again. Try a higher rpm last, maybe 6-7000. The goal is to get the plugs to look the same at all levels. Those readings should indicate the effectiveness of your other jets.

    I'd probably sync the carbs last, after you are sure everything else is within spec. Not sure about that bike, but you never mentioned timing. If it's mechanical, a timing light and a dwell meter for the points are required.

    Jon
    #12
  13. F16Viper68

    F16Viper68 Trapped in the Sandbox!

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    That may be the case but it gives consistent false readings. :lol3

    Dave...
    #13
  14. F16Viper68

    F16Viper68 Trapped in the Sandbox!

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    I know for a fact all of the jets are clear. I spent a lot of time ensuring this fact since I'd already been in there once. While surfing another forum (yes they exist :lol3 ) I found a very good article on rebuilding Honda carbs. http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revD.pdf

    I replaced the idle jets and main jets. The slow speed are pressed in and can't be swaped out without major modifications.

    I'll do a plug check but the road check is going to have to wait. This thing needs a lot of work before it's ready for the street.

    I haven't check the timing on the bike. Good point and something I need to do.

    I just spent the past few hours trying to figure out why the sucker wasn't charging. Think I got it figured out. It confused the hell out of me because during my initial t-shooting the stator was reading shorted to ground. After removing the covers the short disappeared. Started chasing wiring and discovered the mess below. It was pinched by the primary cover.

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    #14
  15. F16Viper68

    F16Viper68 Trapped in the Sandbox!

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    Ok, today was about getting the fork seals replaced. As you can see below they've been leaking. ;D

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    Everything came apart pretty easily.

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    When I pulled off the dust seals I discovered I was in for a battle. The c-clips were rusted in there pretty good. I scooped out as much of the gunk I could, blasted it with air, and soaked them in PB Blaster for about six hours. After wrapping the fork tubes in blue painters tape I started tapping the clips with a drift pin to see if I could break it loose. After much fighting was able to remove the clips and washers.

    Removing the seals was accomplished by blocking the air hole, filling it with ATF fluid, and using a wood clamp to compress the fork which brings the seal up to the top.

    Despite my efforts the seals still came out crooked (not as crooked as the pic. That's after prying for awhile) so I spent about 30 minutes prying them out.

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    The longest part of this job was the polishing and buffing. Holy crap getting those DoD sticker off was a MoFo. I have a lot to learn about polishing but I think they look better than they did when I took them off the bike. ;D

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    Dave...
    #15
  16. CycleDoc59

    CycleDoc59 Wrench Rider

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    After removing keeper rings, seals are easily removed by removing the bolt at the bottom of the fork leg that
    holds the innnards together (best done with an air impact wrench), then pop the tube and seal out by collapsing
    then extending the fork like using a slide hammer...., for next time.
    #16
  17. F16Viper68

    F16Viper68 Trapped in the Sandbox!

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    Funny you mention that. I was working out a few hours ago wondering why I couldn't have done that. Because there's a bushing under the back plate (under the seal) which should pull the seal out if you perform the slide hammer action.

    I was following the service manual so I wonder why they advise you to use a press?

    Dave...
    #17
  18. Gham

    Gham Long timer

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    I'll be following this thread to get your impressions on performance.I think I test road one back in the late 70's,early 80's and eventually wound up getting a used 79 CB750K.
    #18
  19. CycleDoc59

    CycleDoc59 Wrench Rider

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    Donno; I've done quite a few w/the "slide hammer" trick - regular and upside down types. There is a sorta
    fun alternative on bikes with air forks, like a KLR; after removing oil and the keeper ring, they blow out
    with air.....
    #19
  20. F16Viper68

    F16Viper68 Trapped in the Sandbox!

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    I'm hoping to have it on the road by the end of December. A few things left to do (e.g. rebuild calipers, rebuild master cylinder, check the timing, fix intake boot leak on #1, sync the carbs, get the tank painted, replace the tires, etc)

    Ok, maybe check back in January. :lol3

    Dave...
    #20