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Mikuni Carbs 1979 Suzuki GS750L

Discussion in 'Medford Region' started by Ordrock, Jul 30, 2012.

  1. My son bought or bartered for this bike and for a year we've been trying to get it to run, at least run better. The bike sat for years and the carbs needed some real attention. So when the boy first got the bike, he found out that the original owner and his nephew tried to clean the carbs. We'll the stupid thing about these carbs as told to me by the guys down in California at Mikuni are the smog adjustable screws on the bottom of the carbs. These stupid screws can only be correctly adjusted when the carbs are off the bike. Can't adjust while hot without a $78.00 right angle screwdriver. The repair manual even says leave them alone. But to do a good cleaning of carbs that have been all gunked up, require a complete cleaning.

    We'll the cleaning was done again and again, etc, to only leave us scratching our heads. The Mikuni guys say one complete turn to 3 turns. What happens is the bike has a vicious lean problem off idle and when pulling away from a stop. The tank has the vacuum petcock and had to be blocked off as well as the carbs.

    So my question if I was clear enough with the story is, does anyone have any experience with this GS 750's.

    We put filter pods in place of the stock air filter, thinking this might clear things up, but heck no.

    Any help would be great. Hate to have to pay those high repair shop hourly costs if we don't have to.
  2. Those pods leaned it out even further.
    There are some easy things you can do to fatten them up if you want without buying or even modifying any parts.

  3. There are some little plastic washers on the needles you can swap from above the clip to under it to shim it up a bit. Works pretty well actually to correct the overly lean factory setting.
    Hopefully you know someone that can help you sync them as well.
  4. Your bike should have VM26 carbs..

    You have 2 sets of screws.. Pilot Air screws near the carb inlets and pilot fuel screws under the carbs..

    You can adjust your pilot fuel screws with the carbs mounted using a little thumb screwdriber that take 1/4 drive bits.
    Let the bike cool to adjust and make small 1/8 turn adjustments.

    Now a few questions..

    Do all your pilot fuel screws have their tips? they should be sharp line a pin. If the tips are blunt the other end is probably stuck in the carb body.

    Did you dip the carbs and replace all the O-rings? Dipping is about the only way to get them truly clean..

    As for O rings get a set from Robert Barr

    He supplies rings the carbs kits don't have.. All you need for a rebuild is the top and bottom gaskets plus a set of his O-rings.
    $18 for a VM set

    I almost forgot.. How are the carb boots? any leaks here will make the bike nearly impossible to tune..
    Those boot have o-rings under them where they meet the head.. Yup.. you can get them from Robert Barr too for a few extra bucks.

    Step 1

    Join http://www.thegsresources

    Step 2

    Set your pilot fuel screws to 3/4 turns from lightly seated. (the bottom ones)
    Set your pilot air screws to 1 1/2 turns
    Bench sync your carbs.. Get all your slides set to the same level.. I do it using a small piece of solid core copper wire like a feeler gauge..

    If you have the complete factory air box and filter use it..
    Does the air box still have the rubber snorkel on the back? Lots of them got ripped off and tossed but that messes up the mixture.

    If you blocked off the vac port on the petcock how is it working? Was the diaphragm removed?

    Have the valves been checked? valve clearances close up over time and they shims need to be replaced.. When they get tight the bike becomes hard to start.
    Last edited: Jul 30, 2012
  5. As for tuning..

    Your pilot jets, fuel screws and air screws set your mix from idle to part throttle
    if your lean with the pilot screws backed out more than 3 turns it's either clogged or you need to step up a pilot jet size.

    The needles and clip position set your mid range mix..
    main jet is full throttle mix.
  6. How are you cleaning them?

    They need to be completely torn down and each carb and parts tossed in carb dip for 24 hours
  7. RedKat600

    RedKat600 Vintage Screwball Staff Member

    Get rid of the pods, go back to stock. Sync the carbs after you're done. Completely tear them down and dip them....remove every part including the A/F screws. Check the o-rings where the carb boots meet the head. Reseal or replace them if needed.
  8. In 79' when I bought my, 79' GS750L the first thing the dealer told me to do was open up the screws on the bottom 1/4 turn, I did this with a screwdriver tip with a piece of rubber hose on it with the carbs on the bike, I also ended up replacing the #15 pilots with #17.5's...bone stock bike, this cut the choke time in half and eliminated the lean condition. 1979 was the last year the Suzuki GS750L had 26mm Smooth Bore carbs.

    Last edited: Jul 30, 2012
  9. My 79 GS850GN was the same way. I tore them completely down and soaked each carb in Berryman's for 24 hours, blew all the passages and jets out with compressed air, and reassembled with all new gaskets and seals. Ran like a raped ape. I went up to 110 jets with my pods and had a slight lean condition, but it pulled hard and plug chops were the right color, so I ran with it.

    EDIT: also
    Last edited: Jul 31, 2012
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