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Carb jets mismatched - any suggestions

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  • #1

    Carb jets mismatched - any suggestions

    Good morning, I am finally getting my hybrid bike back together after fighting an electrical nightmare. Here's a quick catch-up. 79 XS1100SF with an 81 XS1100H engine. Got everything back in and tried starting (just wanted to find a starting point for carbs). Barely started then died, so I pulled the carbs back out for inspection. Oh, the carbs should be from the 79 as well.

    First, I pulled out the pilot screws (hearing how easy they are to break). The screws are ok, but it looks like someone broke one in the past and either flared the hole really bad by a pushin or they drilled it. Either way thats bad for me.

    Other than that, it didnt take much to clean them up, BUT the jet sizes they used are baffleing me. Here's the scoop. Should be a stock 81H motor, 79SF carbs, 4-1 exhaust, stock air box (not drilled) with K&N filter. Main Jet (130), Pilot Jet (42.5), Jet needle (5GZ6) clip on second step closest to taper, and air jet (180). Couldnt see the numbers for the needle jet emulsion tubes. I'm also at sea level.

    Everthing I've read including the Jetting Recommedations tells me this is way off. Especially the Main Jet.

    Thanks for any help and suggestions...Will
    Tags: None
  • #2
    .. in stock form, your 79 carbs should come with 137.5 mains and the needles set in the middle position. i have found that these things work best [here in fl] with your exhaust/air filter set up, with 142.5 mains and 45 pilots and the needles in the middle position.
    ..there is also a float height ritual that should be performed to make these things run smooth at idle.

    welcome to the forum

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    • #3
      Good to be involved on this site. Lots of great information and good people willing to help.

      I'm gonna try the setup you recommended. I put the carbs back in today after cleaning some more and I think I'm really gonna have trouble with the #4 carb that looks like it was drilled out. I cant even turn the pilot screw an 1/8 inch in or out and the rpms just go crazy.

      I think I'll start looking for a replacement carb or maybe a full set.

      Thanks...Will

      Comment

      • #4
        Hey Will,

        I think it was PGGG that recommended using some JB Weld to reform the hole. Use a good pilot screw, and mix up a little, put it over the hole and screw the pilot down into it until it pokes thru making the needed small hole, and then unscrew the pilot screw before it gets glued to the JB weld. Let it set up over night. Then sand lightly any burrs, etc. sticking out, and you should regain your fine idle mix adjustment!

        Having the 81 Engine only affects whether you are using the 81 ignition TCI and timing plate, or if you swapped the 79 IG parts out and put them on it instead....perhaps the electrical nightmare you spoke earlier about?

        But the difference in the jets for the carbs is due to the actual carbs, not the engine. the 78-79 series use the larger MAINS due to a shared fuel supply thru the main jet. The 80-81 carbs get their pilot circuit and main circuit fuel separately, and so their sizes are accordingly smaller.

        Welcome, keep at it, and let us know if we can help any further!
        T.C.
        T. C. Gresham
        81SH "Godzilla" . . .1179cc super-rat.
        79SF "The Teacher" . . .basket case!
        History shows again and again,
        How nature points out the folly of men!

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        • #5
          That sounds like a great idea to try. Heck...it couldnt hurt.

          I'll pull the carbs back out ( I'm getting pretty good at that ) and give it a shot.

          Will

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          • #6
            I have been thinking about doing the same thing. I think you would have to use JUST the right amount because the fuel feed hole merges from the side of the pilot screw bore. I don't know if I have the nerve.

            Sid

            Originally posted by TopCatGr58
            Hey Will,

            I think it was PGGG that recommended using some JB Weld to reform the hole. Use a good pilot screw, and mix up a little, put it over the hole and screw the pilot down into it until it pokes thru making the needed small hole, and then unscrew the pilot screw before it gets glued to the JB weld. Let it set up over night. Then sand lightly any burrs, etc. sticking out, and you should regain your fine idle mix adjustment!
            Skids (Sid Hansen)

            Down to one 1978 E. Stock air box with K&N filter, 81H pipes and carbs, 8500 feet elevation.

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            • #7
              Since tomorrow is Halloween, It will probably be Wednesday before I get them out. I'll take them to work so I can do things more precisely, and get them back in on Saturday morning.

              As soon as I fire the bike back up I'll let everyone know how it worked. I'll even try to get some before and after pics.

              Will

              Comment

              • #8
                I did a couple of carbs the same, the broken tips had been drilled out, you're only really turning a small pinhole into an even smaller pinhole, the amount of epoxy to use is tiny, smear it in from underneath with fingertips, don't let it squeeze up as far as the side fuel entry hole, then screw the jet down thru it, then slowly unscrew the jet back out.

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