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Thread: GS 400 engine problems

  1. #1
    Join Date
    Apr 2017
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    Apple Valley, CA
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    Default GS 400 engine problems

    I have been working on a 1978 GS 400 that I have had for several years. It has not been ridden a lot in the last 10 to 20 years. The engine starts and runs well at higher RPM's but due not like getting from low RPM to higher RPM (sluggish). Pulling spark plug wires show that the left cylinder is a little dead as pulling the left plug wire makes little difference as pulling the right wire kills the motor. I have done a complete rebuild of the carbs and I have put in new plugs, points and condenser. Here is where I stand as of this date:

    1. The air boot between the carb and air box on the left side was cracked and a replacement could not be located. Using my dirt bike past, I used a piece of 2" radiator hose in its place. I have heard that messing with the intake system can require rejetting. Can this be causing problems?
    2. Can the diaphragm carb sliders get old and stiff enough to cause these type of problems?
    3. When I put the new points in and adjusted the gap to specs, I had to rotate the backing plate all the way to one side to try and set the timing. I still fell short and had to alter the point gap a touch to get the timing spot on. I am at wits end trying to figure out what is causing this. Has anyone else run into this before?
    4. Trying to adjust the valves showed me that there are differing opinions on what the clearance should be. Some say the same for intake and exhaust valve, some say two different numbers. My Chilton manual says .0012" to .003", but I have reason to believe that this is way low. When the gap is less than what you can gauge, what is the best way to get close to how much you have to reduce your shim size?

    Thanks for your help.

  2. #2
    Join Date
    May 2015
    Location
    Summerland BC Canada
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    Almost a day gone by and nobody has helped you, so here goes.

    The boots must be totally airtight, and the air box and filter upstream must be stock and in good condition or the carburetors won't work properly. Messing with the air box will get you into a rabbit hole of jetting 'fixes' that never seem to get to perfection.

    Yes, the diaphragms can crack with age and are pretty scarce and expensive. Up to a point, they can be repaired, and there is a thread or two here about using thin nitrile glove material for the purpose. If you reach into the carb mouth and lift the slides they should slowly drop. If they just plummet back down you'll need to repair or replace.

    Aftermarket points - which is probably all there is now - often have a different rubbing block which can result in this problem. You can take a small needle file and lengthen to points plate slots to gain more sweep of adjustment.

    I won't ask you the reason you believe the official clearances are 'way low'. The aluminum head expands faster than the steel valve stem so the clearance will increase on warm up. Personally, I use .002 and .004 as my limits which allows for the clearances to close up over time; they never get greater unless you're about to have catastrophic cam bearing failure. The shims come in .002 increments so as soon as a .002 won't go in I switch to a smaller shim. You could go as wide as .005 but there's not much point and it could get clattery.

    If it's so worn that a .001 or .0015 won't go in check to see if the bucket still rotates freely. If so, you have SOME clearance, so going one size shim smaller should put you in the range. If the bucket is bound, try two sizes and hope you're lucky.

    I've never heard of 'differing opinions' on the clearance on this site, other than that a little looser especially on the exhaust side is not a bad idea especially for racing or if you're planning a world tour.

    I just set mine last week at .004 across the board and the bike runs perfectly.

    There is a shim exchange program on the site plus massive amounts of information on the 'bike cliff' site and various data banks and factory manuals and stickies so get reading.

    Before you install shims look into the zip tie method, mentioned on the site, of holding the valves open while you do so. NEVER EVER rotate the motor without a shim in the bucket. Always rotate the crank with the big 19mm or 3/4" nut rather than the little 10mm. There are also some aftermarket shims made by I think K&L?? that can shatter, and also some motor that uses a 29mm rather than 29.5mm shim which looks like it would work but isn't a good idea. I just buy OEM Suzuki GS shims that fit a GS500.

    I also have a pair of '78 GS400 intake boots that are still nice and supple low milers; you could PM me about their availability.

  3. #3
    Join Date
    May 2015
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    Summerland BC Canada
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    According to ebay, Ronnies Powersports in Lanesboro , Mass. has two NEW boots for 32.98 each. They only have two so don't hesitate.

  4. #4
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    Apr 2017
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    Apple Valley, CA
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    Thanks for the carb slide drop test info, I will try that if the problem continues. The diaphragms do not show any light when held up to a light source. Tell me more about the zip tie method.

  5. #5
    Join Date
    Jun 2015
    Location
    Shelby, Ohio
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    Quote Originally Posted by jpettigrew View Post
    Thanks for the carb slide drop test info, I will try that if the problem continues. The diaphragms do not show any light when held up to a light source. Tell me more about the zip tie method.
    Look here...

    http://gsarchive.bwringer.com/bikecl...ol_zip_tie.pdf
    Alan


    Weaned on a '74 450 Honda
    Graduated to an '82 GS850GL
    Now riding an '83 GS1100GL
    Added an '82 GS1100GL

  6. #6
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    May 2015
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    I'll admit to never having used the zip tie method; I'm good enough at timing cams that I just think about getting some zip ties for next time while I undo the cam caps and take the cams out. Usually I have to do all four, but if you just want to change one shim it would be a real time saver.

    There's a write up on the site somewhere, but the drill goes like this. Get a zip tie about 3/16 wide or so. Double it over in the middle and bend the doubled bit about 90 degrees. Rotate the motor until the valve you want is open and insert the 'tool' down the plug hole over the valve seat. Rotate the motor until the valve is 'closed' and there should be enough room to pry the shim up and slide it out and change it. Having the right size zip tie will matter.

    There's a little pry bar tool sold that is supposed to depress the bucket while you take the shim out, but the metal is so cheesy that it's notorious. I made one back when that worked perfectly but lent it out and never saw it again. Hopefully it isn't sitting in some toolbox somewhere with nobody knowing what it's for…..

    Method C is to measure your clearances and then remove the cams. I get the motor to TDC as per cam installation and then retract the cam chain tensioner fully and lock it with the locking screw. Then proceed with removing the cams; you might be able to pluck out the shims without losing the cam chain timing, but be prepared to retime them.

    The zip tie method is a site favourite.

    Hope you got to the boots in time; that's a great price.

  7. #7
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    Apr 2017
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    Apple Valley, CA
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    Assuming the slide diaphragms are sound, can they become stiff enough over time to effect the carbs from responding properly? How far do the carbs have to come off to do your drop test?

  8. #8
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    Apr 2017
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    Apple Valley, CA
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    How critical is the carb synchronizing?

  9. #9
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    Quote Originally Posted by jpettigrew View Post
    Assuming the slide diaphragms are sound, can they become stiff enough over time to effect the carbs from responding properly? How far do the carbs have to come off to do your drop test?
    No; and you'll have to get your finger in there somehow. The carbs come out pretty easily on these bikes.

  10. #10
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    Quote Originally Posted by jpettigrew View Post
    How critical is the carb synchronizing?
    I've never seen then go out unless they've been separated and dismantled. Usually they're fine unless they've been 'adjusted'. Imbalanced running is usually something else if it ran smoothly before.

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