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Thread: '80 GS1100 Cylinder 2 not firing

  1. #1
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    Question '80 GS1100 Cylinder 2 not firing

    Hi everyone! New to the forum, though a couple threads have helped over the years. I couldn't find an answer for this lovely new issue I'm experiencing so it was time to make an account.

    I've got a 1980 GS1100. I think the motor has been replaced at some point in its life, as the motor number doesn't match the frame number. The guy who had it before me was a NASCAR mechanic and he set it up and used it as a drag bike. I had a local shop go over it with a fine tooth comb and it ran like a top after that, but the shop has since shut down.

    After sitting for winter, I fired it back up and it ran great for about an hour, but on the highway the engine changed pitch and it lost a bit of power. If I gave it throttle and hit 5k rpm, the sound would go back to normal and I'd have full power. I figured a cylinder wasn't firing properly and started from there.

    Yesterday when I went to go for a ride, the bike was running like garbage and stalling out at idle. Backfired a few times. Didn't even get out of the driveway because it sounded so bad. I let it sit for a few minutes, re-seated the plug wires for #3 and #4, fired it up again and it sounded like it was back at 3 cylinders working. Weird, but hey it's an old bike they do weird things.

    So today I picked up new batteries and grabbed my temperature probe and checked the exhaust headers after starting the motor cold. Cylinder 2 isn't warming up, at all. I can go out and ride, and it'll run without that cylinder with just a bit of power loss. Around 5k rpm I'll get full power and firing, once the bike is warm.

    I hooked up an in-line spark plug tester and it's showing power to the plug. Is this the coils? The plug wires? Something else to check for? I don't have a compression testing kit nor do I have a garage, all of this work happens in my driveway in good weather. Help me out here, I could use some ideas.

  2. #2
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    Sounds like a failed petcock......dribbling fuel into #2 cylinder causing it to not fire. As rpms go up, the extra airflow lets it recover....then back at idle ,plug wash out returns
    1981 gs650L

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  3. #3
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    Quote Originally Posted by tom203 View Post
    Sounds like a failed petcock......dribbling fuel into #2 cylinder causing it to not fire. As rpms go up, the extra airflow lets it recover....then back at idle ,plug wash out returns
    I would never have thought of that, even though I already have a replacement petcock because it's on my to-do list. I'll toss that on and report back. Thanks!

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    Alright, old petcock off and new one is... well.

    Looks like somebody hacked the petcock hole open on this gas tank at some point. Here's a pic: https://i.imgur.com/Y6Gbn07.jpg

    As you can see, they gracefully opened it up to right where the gasket should seat. And also gouged out random bits of the tank that are supposed to form a nice seal. Always the optimist, I hooked it all up anyway and added a bit of gas to the tank. It started seeping out pretty quickly, but not so fast I couldn't stuff a rag under it and test fire the bike.

    Good news! Cylinder 2 started firing after a minute or so! Then I turned it off because, you know, leaking gas.

    So! Pulled it apart again, liberally applied fuel resistant RTV, and it's curing now. I'll reinstall the tank tomorrow and follow up again.

    As a bonus, here's a pic of the old petcock and some additional fuel filters that were rattling around inside the tank when I drained it. https://i.imgur.com/IAN4L28.jpg

    I'm assuming those larger filters were from its glory days on the drag strip, same with the Frankenstein petcock hole. You can see the sealant they used on the inside of the old petcock, and where it had flaked away causing the leak that started this whole mess.

    Tom203, from one Tom to another, thank you. You saved me weeks of headaches since I never would have thought to look for this.

  5. #5
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    Also, the motor number will never match the frame number on any of the GS bikes like they would for a car. Don't let that influence you on what to look for when working on the bike.'

    Also, get rid of that RTV FAST. you don't want or need that near those carbs. Do a full cleaning using the guides on Basscliff's site (https://members.dslextreme.com/users/bikecliff), get your o-rings from www.cycleorings.com and a set of bowl gaskets.
    Last edited by cowboyup3371; 05-08-2021 at 09:23 PM.

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    Quote Originally Posted by cowboyup3371 View Post
    the motor number will never match the frame number on any of the GS bikes like they would for a car
    Thanks for confirming this. I'd read something like that in a few places, but the idea is still weird to me. I'll get over it now.


    Quote Originally Posted by cowboyup3371 View Post
    get rid of that RTV FAST
    I'd love to. Truly, I would, but if you look at the pic they opened the petcock hole outside of where the gasket would sit. They make RTV that's fuel resistant now so that's what I'm using. I had to apply a liberal bead of the stuff around the outside of the petcock, gas kept leaking out of the area where the tank was opened up. Short of welding new metal onto the tank, this is what I have to work with.

  7. #7
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    Hi, if after sorting out the petcock, you still have a issue with #2 not firing. I would look at cleaning the carb or carbs. This is a really common problem with bikes that have been stored. It does not take much with today's fuel to foul carburetors. Filling up with good fuel ( Chevron 94 in Canada) , adding fuel stabilizer and starting the bike every month will pretty much eliminate this headache. Just confirm that #2 is giving spark before you tear the carbs apart. It is not really difficult but it is labour intensive. Also I use a skinny candle on the header to see if it is firing. Hope this helps
    1983 gs1100ed restro-mod. 1998 gsxr 1100 almost mint, 2019 kawasaki klx250, 2011 Beta 250 evo trials bike, 2017 Montesa 300rr trials bike, 2021 honda crf250rx woods weapon

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