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'77 GS 550 performance issues

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    #1

    '77 GS 550 performance issues

    Hey all,
    Just got my '77 550 back from the shop after rebuilding the carbs and cleaning it up for the summer. Took it out for a ride today and 35 miles out from home, my bike suddenly began to bog down with anything more than half throttle and if I didn't choke it, it would stall. I ran it full choke and half throttle back home (that was a LONG, slow ride) and now need to fix it. Has full oil and lots of gas, and is in otherwise good shape. I've also got an in-tank gas filter (stock) as well as one in the gas line running to the carbs, and the in-line one is clean and full of fuel. Any ideas? I'm guessing my high-speed jets are plugged, and if so, how do I fix it without having to tear the bike apart? Thanks for any suggestions!
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    #2
    Just because you see gas in your inline filter doesn't mean it's flowing well enough. Make sure the petcock filter is clean and also the cap vent. Vaccum lock could do that, cause the level in the bowls to get low enough to need choke to keep idling. If you paid good money to have the carbs cleaned, take it back and demand a re-do.

    Another thing that could have happened is that dirt got sucked in to the carbs, clogging them up. Check the air filter and airbox (carefully!). I thought my airbox was squeaky clean but something got into my carbs and I went from having a smooth powerband from 4000 rpm up to redline to having a big flat spot at 6 to 7k and surging when closing the throttle. I tried some gas additive (carb/fuel injector cleaner) which helped a little buit the problem hasn't completely gone away. I don't want to have to tear the carbs down either until after riding season so I just resigned myself to not having the power I once had. It was still good enough to leave most cars behind at the lights but passing on the freeway was another story. I've been riding like that for 3 or 400 miles now and it IS getting better gradually. I can accelerate up to 80 mph now without surging or flat spots but the power doesn't seem to be as strong as it was...

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      #3
      Okay, I'm hoping maybe a new filter and some carb cleaner will do the job. Any idea where to get a new filter for this bike that doesn't cost an arm and a leg?

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        #4
        If the air filter is the oiled foam type, just clean, re-oil and re-use. There's special air filter oil available but 30wt will work. Just don't overdo it and soak up as much excess with towels as possible before reinstallation. If too much oil gets sucked into the carbs you'll have more problems. Also trim to fit, if the filter is compressed to much it will restrict air flow. I think a new oiled foam filter is about $10-15 from bikebandit.com.

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          #5
          Could it also be something else?

          Before you go all the way down one path or another maybe some more diagnosis is in order. Pull out a spark plug (or two!) and look at them. You could have some plug fouling that is causing you to lose power. If it is black and sooty they are fouling from over rich fuel flow. If they are bone dry then I would follow the other path of lack of fuel. The fact that you were driving around with good performance and then bogged down could go either way. Before you go buying any parts and doing wholesale bike disassembly, let's find out some more information.

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            #6
            Guys,
            Thanks for the suggestions. Turns out that after some fiddling and prodding I've got a broken spark plug wire, right up in the coil. I had done the 'half-inch test' on each exhaust pipe right after I started having trouble with it and everything was hot, and now that it's cooled I can start the bike and hold on to #3 exhaust for as long as I want. If I give a good tug on the wire, all four start hitting again. So now the question becomes what brand of coil do I go with, and how do I wire it up? I've seen a bunch of suggestions for Dyna coils, and I have a set of MSD plug wires wires already. Any help here would again be appreciated, I'm glad to see that I don't have to tear the carbs off (again!), though I'm sure that bottle of carb cleaner now in my tank won't hurt anything. Once again, thanks for giving this newbie some good advice.

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