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Thread: diagnosing rich condition (idle at rest versus under load)

  1. #1
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    Default diagnosing rich condition (idle at rest versus under load)

    After some tuning and plug reads, I need help diagnosing potential causes of rich conditions under load. After a carb rebuild I adjusted pilot fuel and air screws a couple times to get decent plug color and highest idle. Synced carbs too, though #4 gauge was wonky so I'll do that again (adjusted #4 by ear during sync and visually checked it afterward, slide looks to be consistent with others).

    Main puzzler: new plugs put in after bike was warm, and then run at idle for a few minutes in the driveway, then pulled. Little color to them so suggesting on the lean side. Plugs back in and taken for a ride, plug chop readings for 1/8 and 1/4 throttle were both rich across the board. Haven't been able to do reads at 1/2 and full yet. Before I do more fiddling with carbs, I want to entertain other causes for the difference in pilot circuit on the stand versus under load, namely:

    1. Could a leaky petcock cause rich conditions? This petcock failed on me once before, after rebuilding still leaks a very slow drip when tank is off.
    2. Weak spark? Coils (5 ohms) / wires / caps (NGK 5 ohms) are fairly new. Can test coil resistances next time I have the tank off.

    Pilot screw settings now (seem a little all over the place, started from 1 turn out on fuel and 1 1/4 - 2 1/4 air initially)

    Cylinder Fuel Air
    1 7/8 1
    2 3/4 2 1/4
    3 1 1
    4 1 1/8 1 5/8

    Other notes;
    Dyna S ignition
    Needle clip in middle position (moved down one after installing K&N pods)
    110 main jet (up from 102 after installing K&N pods)

    Any thoughts much appreciated. Thanks everyone.
    1978 GS 750 E

  2. #2
    Join Date
    Jan 2011
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    NoMo, MN
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    Default

    Pilot jet 15 (stock)
    Main jet 117.5
    Needle e-clip position 4 (from the top)
    Fuel adjust 1 full turn out from lightly seated.
    Air adjust 1.5 turns out from lightly seated. Then adjust using Gunston colortune - G4074.

    HTH
    -Mal

    "The only reason for time is so that everything doesn't happen at once." - B. Banzai
    ___________

    78 GS750E

  3. #3
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    Default

    Thanks, I want to use Colortune but hard to get one at the moment since production was shut down by Covid--I'm keeping an eye on suppliers for when stock begins to ship.

    Not sure I understand why you're recommending a richer position on the needle and a bigger main jet, though? I thought those would be moves for addressing lean conditions from 1/4 throttle to full.
    1978 GS 750 E

  4. #4
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    Many years ago there were a few on here with '78 750's - among us the memorable MisterCinders. He did a lot of the donkey work finding the right combination of adjustments (with a fair bit of humor) to make pods a workable option. Others may have slight differences to this setup but this is what I run and it works splendidly well.
    -Mal

    "The only reason for time is so that everything doesn't happen at once." - B. Banzai
    ___________

    78 GS750E

  5. #5
    Join Date
    Mar 2005
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    West Slope, OR
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    Carb rebuild - what method did you use?
    Idle mix - a few minutes might not be accurate
    A leaky fuel tap makes carb 3 rich, not the others
    Check out the Pipes, Pods, panic sticky above
    1978 GS 1000 (since new)
    1979 GS 1000 (The Fridge, superbike replica project)
    1978 GS 1000 (parts)
    1981 GS 850 (anyone want a project?)
    1981 GPZ 550 (backroad screamer)
    1970 450 Mk IIID (THUMP!)
    2007 DRz 400S
    1999 ATK 490ES
    1994 DR 350SES

  6. #6
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    Aug 2021
    Location
    Calgary, AB
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    Default

    What size is your pilot jet and have you checked the fuel level in the bowls (float height)?

    Puzzling indeed that it seems lean at idle but right at 1/8 throttle...I wouldn't have thought the needle setting would have much of an effect at 1/8.

  7. #7
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    Quote Originally Posted by allojohn View Post
    Many years ago there were a few on here with '78 750's - among us the memorable MisterCinders. He did a lot of the donkey work finding the right combination of adjustments (with a fair bit of humor) to make pods a workable option. Others may have slight differences to this setup but this is what I run and it works splendidly well.
    I read his stickys and they were invaluable to my learning. My pipes are close to stock (4-2, Mac) so maybe I don't need the bigger jets or richer needle position?

    Quote Originally Posted by Big T View Post
    Carb rebuild - what method did you use?
    Idle mix - a few minutes might not be accurate
    A leaky fuel tap makes carb 3 rich, not the others
    Check out the Pipes, Pods, panic sticky above
    Thanks, I'll try a bit more time at resting idle. I read original service manual, used Clymer's, watched videos, and used Paul Musser's excellent VM tutorial for rebuild throughout, which I found here. All O-rings replaced and only OEM Mikuni parts for the jets. I did use one float needle / seat from a kit as the old one was leaking.

    Quote Originally Posted by billyhoyle View Post
    What size is your pilot jet and have you checked the fuel level in the bowls (float height)?

    Puzzling indeed that it seems lean at idle but right at 1/8 throttle...I wouldn't have thought the needle setting would have much of an effect at 1/8.
    Pilot is original at 15. I set the floats when I did the rebuild according to spec (24 or 25mm as I recall), but I haven't done the tube/visual check of float level with the carbs mounted.

    Thanks everyone, as always. Big fan of this forum and all the people willing to share their hard-earned wisdom.
    1978 GS 750 E

  8. #8
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    Once you get your hands on the Gunston you'll begin to see what's happening in there! I see fleabay has at least one but it's pricey...
    -Mal

    "The only reason for time is so that everything doesn't happen at once." - B. Banzai
    ___________

    78 GS750E

  9. #9
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    Jun 2021
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    Quote Originally Posted by allojohn View Post
    Once you get your hands on the Gunston you'll begin to see what's happening in there! I see fleabay has at least one but it's pricey...
    $125 total cost and three days left on the auction, Yikes!
    1978 GS 750 E

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