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Thread: 78 GS750 Carb Rebuild - VM 26 (a few questions)

  1. #1
    Join Date
    Jun 2021
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    Ipswich MA
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    Default 78 GS750 Carb Rebuild - VM 26 (a few questions)

    Happy new GS owner here, Iím about to embark on a full clean of my carbs for the first time, and looking for advice on a few points before I break them down completely. Iíve read the sticky threads (and others) here, been watching various videos and grateful for the help (the amount and variety of opinions can be daunting). Iíve ordered a set of O-rings from Barr and need to order at least one replacement pilot jet, for one that broke (Iíll share how I got that out separately in case it helps someone else).

    Why I got started with the carbs: bike was running consistently and well enough (as far as I could tell) before, but after I had it running one day, #2 started dumping fuel intermittently out of the breather tube after the engine was offóthe root cause of which I figured out (quickly!) was that the petcock valve was not closing properly, so it was dumping fuel into the carbs continuously, at probably about half the rate of the priming setting. BUT I assume #2 float valve was also not closing properly, hence the overflow. So I pulled the bank and after inspecting decided to give them a complete clean and rebuild. Start from a good baseline, right?

    I do also plan to check/adjust valve clearances as advised, though that will be a first time for me as well.

    Current Setup notes:

    --Ď78 GS 750, stock VM26 carbs
    --Alpha Moto pod filters (I gather a lot of people hate pods, but please donít judge I inherited them...and everything else on the bike)
    --Exhaust: appears stock in configuration but tailpipes are aftermarket (see pics)
    --Jets are stock: 102 mains, 15 pilots
    --general, pre-breakdown: despite the combo of pods and stock jets, the plugs (and smell) indicated that all four cylinders were running rich.
    Please feel free to answer any of these individuallyónumbered for ease.


    1. Could #2 have overflowed because the float sank, not because the valve stuck or didnít seal? I have been told the new gas can cause that to happen to old floats (in outboards etc.) I havenít seen this in in my other old machines, but worth asking.
    2. Pods: Should I ditch the Alpha Moto pods for another brand, or are they decent? Hope Iím not inviting trouble hereÖ
    3. If I can keep them without sacrificing too much, whatís a good starting point for rejetting (mains particularly, I gather I can probably keep pilots stock)? This link in the sticky recommends 4 sizes up for adding pods. Alternately, if I really should get another type of pod, any recommendations for a jet / pod combo for an otherwise stock engine. (I cannot afford $160 for K&Nís at the moment though, honestly)
    4. Is the fuel rail as hard to re-seal as I have read? Barr includes O-rings but I gather the sealing bulge may not be a separate o-ring but incorporated into the tubes themselves.


    Thanks everyone! Looking forward to the journey with this machine, Iím in love already after only a few weeks of riding her.
    Attached Images Attached Images
    1978 GS 750 E

  2. #2
    Join Date
    May 2009
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    Norman, Oklahoma
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    http://members.dslextreme.com/users/bikecliff/

    look on right margin. also use the early 8 valve service manual

    http://cycleorings.com/

    orings and intake orings here
    MY BIKES..1977 GS 750 B, 1978 GS 1000 C (X2)
    1978 GS 1000 E, 1979 GS 1000 S, 1973 Yamaha TX 750, 1977 Kawasaki KZ 650B1, 1975 Honda GL1000 Goldwing, 1983 CB 650SC Nighthawk, 1972 Honda CB 350K4, 74 Honda CB550

    NEVER SNEAK UP ON A SLEEPING DOG..NOT EVEN YOUR OWN.


    I would rather trust my bike to a "QUACK" that KNOWS how to fix it rather than a book worm that THINKS HE KNOWS how to fix it.

  3. #3
    Join Date
    May 2009
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    Norman, Oklahoma
    Posts
    24,959

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    composite floats dont sink. check that the floats are perfectly straight...on both float in each carb. also move them up and down to see they clear the bowl gaskets sufficently. ive had to trim gaskets before.
    MY BIKES..1977 GS 750 B, 1978 GS 1000 C (X2)
    1978 GS 1000 E, 1979 GS 1000 S, 1973 Yamaha TX 750, 1977 Kawasaki KZ 650B1, 1975 Honda GL1000 Goldwing, 1983 CB 650SC Nighthawk, 1972 Honda CB 350K4, 74 Honda CB550

    NEVER SNEAK UP ON A SLEEPING DOG..NOT EVEN YOUR OWN.


    I would rather trust my bike to a "QUACK" that KNOWS how to fix it rather than a book worm that THINKS HE KNOWS how to fix it.

  4. #4
    Join Date
    Mar 2005
    Location
    West Slope, OR
    Posts
    11,951

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    1. Probably the float valve
    2. Not familiar with those
    3. Keep the pilots stock, raise the needles one notch and 4 sizes may be too much. Depends on your pods
    4. This has been debated to no end. I fall into the soak and use camp. Get a small jar, fill with gas and drop all 3 in during the carb cleaning. They should fit very snugly when installed. I'm still using the originals on my 78
    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

  5. #5
    Join Date
    Oct 2020
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    North Lincolnshire - UK .
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    For pod use.
    Use only good quality K&N, S&B, APE, UNIFILTER etc.. Cheap Moxi, Emgo, HiLevel etc are know to be restrictive, cause rich running and power loss).
    Fit air corrector jets ( see Ebay link provided ).
    leave pilot jet size and needle position etc stock...
    Increase main jet a couple of sizes.
    Air corrector jet kit. Suzuki GS1000,GS850,GS750.MIKUNI.LEDAR.PODS,K&N,FILTERS. | eBay
    Check out the seller feedback .
    Last edited by zed1015; 07-13-2021 at 05:25 AM.
    Mikuni Viton Choke Plunger Seat Renewal.
    VITON Choke plunger seals .KAWASAKI Z1,Z900,Z650,Z1000,Z1R,SUZUKI GS1000,GSXR,RF | eBay

    Air Corrector Jets for Mikuni VM 24, 26 and 28mm carbs .

    https://www.ebay.co.uk/itm/254380193...84.m1555.l2649




  6. #6
    Join Date
    Jun 2021
    Location
    Ipswich MA
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    Thanks everyone. I do have the original service manual already, and Clymer's. Have read the early one front to back at this point. O-rings are on the way.

    @Zed1015, what you say may explain why the bike was running rich across all four cylinders, if these pods are restrictive. I just haven't found much in the way of reviews etc about them, so thought I'd ask here.
    cheers
    1978 GS 750 E

  7. #7
    Join Date
    May 2009
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    Norman, Oklahoma
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    You can turn the side air screws OUT to lean the carbs out too. Normal starting off setting is around 1 1/2 turns out from seated. A colortune is the ideal tool for setting up VM carbs. How heavily oiled the pods are can be an issue too.
    MY BIKES..1977 GS 750 B, 1978 GS 1000 C (X2)
    1978 GS 1000 E, 1979 GS 1000 S, 1973 Yamaha TX 750, 1977 Kawasaki KZ 650B1, 1975 Honda GL1000 Goldwing, 1983 CB 650SC Nighthawk, 1972 Honda CB 350K4, 74 Honda CB550

    NEVER SNEAK UP ON A SLEEPING DOG..NOT EVEN YOUR OWN.


    I would rather trust my bike to a "QUACK" that KNOWS how to fix it rather than a book worm that THINKS HE KNOWS how to fix it.

  8. #8
    Join Date
    Oct 2020
    Location
    North Lincolnshire - UK .
    Posts
    143

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    Quote Originally Posted by Forden View Post

    @Zed1015, what you say may explain why the bike was running rich across all four cylinders, if these pods are restrictive. I just haven't found much in the way of reviews etc about them, so thought I'd ask here.
    cheers
    Those Alpha Moto pods are just cheap generic pods sold by Alpha Moto and not made by them.
    Most likely Chinese or the like with their logo stamped on the end cap.
    Most of their stock is rebranded Chinese/Taiwanese budget stuff that you will also find under numerous names on Ebay and swap meet tool stalls.
    I would ditch them and buy good quality K&N's ETC..
    They do cost more but there's a reason why and they are money in the bank long term.
    You get what you pay for.
    While you have the carbs apart make sure the air bleed holes in the needle jet tube are clear.
    If these are blocked they will also cause rich running.
    Last edited by zed1015; 07-13-2021 at 08:16 AM.
    Mikuni Viton Choke Plunger Seat Renewal.
    VITON Choke plunger seals .KAWASAKI Z1,Z900,Z650,Z1000,Z1R,SUZUKI GS1000,GSXR,RF | eBay

    Air Corrector Jets for Mikuni VM 24, 26 and 28mm carbs .

    https://www.ebay.co.uk/itm/254380193...84.m1555.l2649




  9. #9
    Join Date
    Jun 2021
    Location
    Ipswich MA
    Posts
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    I ordered the K&N RC-2454 pods--found a way to reduce the cost--so welcome tips on sizing main jets for those pods particularly, since I'm sure many of you have direct experience with them. The installation sheet specifies about 15% increase, or six sizes up, which is a bigger increase than I've seen recommended anywhere else.

    Also, 14mm colortune kits seem in short supply stateside (except gauged prices on ebay)--anyone know if they are still in business?
    1978 GS 750 E

  10. #10
    Join Date
    Oct 2020
    Location
    North Lincolnshire - UK .
    Posts
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    Quote Originally Posted by Forden View Post
    I ordered the K&N RC-2454 pods--found a way to reduce the cost--so welcome tips on sizing main jets for those pods particularly, since I'm sure many of you have direct experience with them. The installation sheet specifies about 15% increase, or six sizes up, which is a bigger increase than I've seen recommended anywhere else.
    See my previous post regarding jetting and air corrector jets to avoid bottom end rough running and off idle hesitation .
    Six sizes up is way more than you will need with just the pods and no other mods.
    That recommendation is just K&N being over cautious so you don't sue them for engine damage from running too lean.
    You should end up around 110 on the mains.
    Mikuni Viton Choke Plunger Seat Renewal.
    VITON Choke plunger seals .KAWASAKI Z1,Z900,Z650,Z1000,Z1R,SUZUKI GS1000,GSXR,RF | eBay

    Air Corrector Jets for Mikuni VM 24, 26 and 28mm carbs .

    https://www.ebay.co.uk/itm/254380193...84.m1555.l2649




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