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Thread: GS850 #4 cylinder not firing

  1. #1
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    Default GS850 #4 cylinder not firing

    So my bike restore is not going according to plan right now. It is a 1979 GS850 which was a "barnfind" and hadn't been run for 20 years. I have had the carbs off a couple of times and they have been cleaned using an ultrasonic machine plus some elbow grease and fine wire. The carbs were not "terribly" grungy but had a fair bit of dried fuel residue etc. Long story short is that the #4 cylinder refuses to light. Symptoms are:
    -cool exhaust header and popping/backfire which seems to be due to fuel starvation however I have checked these things.
    -compression is very close to the same on all 4 cylinders, 105lbs with a push in tester
    -switched the spark plugs with one from a working cylinder (no change)
    -switched plug wire caps (no change)
    -tried spraying flammable spray around the carb mounts (no change)
    - flammable spray directly into the carb throat (no change)
    -spark seems to be present and visible with the plug pulled
    -#4header much cooler than the other 3.
    -mixture screw set at 1 & 3/4 turn out-no difference in response if it is adjusted in or out unless it is closed entirely
    I will be taking the carbs off tomorrow for the third time and wondering if anyone has any suggestions. This is driving me crazy. Going to clean the #4 carb one more time but this has me and my mechanic buddy baffled.
    Any advice?
    Jonesz

    1979 GS850 current project
    1978 GL1000-naked
    1983 GL1100-Aspencade stripped
    1999 Valkyrie Tourer-long ride bike

  2. #2
    Nessism's Avatar
    Nessism is offline Forum LongTimer GSResource Superstar
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    My suggestion is to clean the carbs properly per the VM rebuild guide on Basscliff's website. Shortcuts are long cuts so just get on with it and ungang, soak in carb dip, replace all the O-rings, etc.

  3. #3
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    What Ed said, plus the fact that you don't realize you have two idle mixture screws points to the carbs not being clean
    1978 GS 1000 (since new)
    1979 GS 1000 (The Fridge, superbike replica project)
    1978 GS 1000 (parts)
    1981 GS 850 (anyone want a project?)
    1981 GPZ 550 (backroad screamer)
    1970 450 Mk IIID (THUMP!)
    2007 DRz 400S
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  4. #4
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    Thanks guys. Found the excellent article on Rebuilding VM Carbs by Paul Musser on the BikeCliff website.That will be a huge help. Will get back to you after doing another thorough cleaning.
    http://members.dslextreme.com/users/...rb_rebuild.pdf
    Jonesz

    1979 GS850 current project
    1978 GL1000-naked
    1983 GL1100-Aspencade stripped
    1999 Valkyrie Tourer-long ride bike

  5. #5
    Nessism's Avatar
    Nessism is offline Forum LongTimer GSResource Superstar
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    Quote Originally Posted by Big T View Post
    What Ed said, plus the fact that you don't realize you have two idle mixture screws points to the carbs not being clean
    Agree. And make sure the pilot fuel screw tips are not broken off in the carb body.

  6. #6
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    Quote Originally Posted by Nessism View Post
    Agree. And make sure the pilot fuel screw tips are not broken off in the carb body.
    Well you are a Suzuki whisperer!! Took the carbs out and started with # 4 which was the problem carb and going by the DIY article on Bikecliff & your heads up found that the pilot fuel tip was indeed broken off and lodged in the carb body. Does anyone have a clue or tip to get that broken piece of the pilot fuel screw out of the carb body without destroying it size/shape wise? Or am I now looking for a new carb body? Tried a lot of things to dislodge it but no joy so far and like I say can't be too aggressive.
    Jonesz

    1979 GS850 current project
    1978 GL1000-naked
    1983 GL1100-Aspencade stripped
    1999 Valkyrie Tourer-long ride bike

  7. #7
    Nessism's Avatar
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    Go back to Basscliff's website and look for the tutorial on how to remove the tips. It's a very common problem.

  8. #8
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    Thanks Ed you may have saved my sanity today. Will let you know how it goes tomorrow. We had tried all kinds of shorter thin/sharp objects but they all bent rather than punch the tip out.
    Jonesz

    1979 GS850 current project
    1978 GL1000-naked
    1983 GL1100-Aspencade stripped
    1999 Valkyrie Tourer-long ride bike

  9. #9
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    Well the tip of the pilot screw is now out. Didn't use exactly the method on bikecliff but used a piece of sewing needle cut off, held in a pair of needle nose vise grip pliers, inserted in the shallow depression/hole in the carb throat and with a flat blde screwdriver pushed on the backend of the needle and out came that tiny bit of brass pilot screw.
    Now it appears I am looking for a pilot fuel screw. Anyone have any good results with an aftermarket screw. Seems they are no longer available OEM. or maybe I can't find a part #. On amazon there is this one but I am kind of leery about aftermarket parts after reading about rebuild kits:
    https://www.ebay.ca/itm/153757472771...a5ImZ#viTabs_0
    Thanks for the help so far.
    Jonesz

    1979 GS850 current project
    1978 GL1000-naked
    1983 GL1100-Aspencade stripped
    1999 Valkyrie Tourer-long ride bike

  10. #10
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    Pattern screws will do the job as long as the taper is machined on centre.
    Here's a full set that i know are good and come with new springs and 'O' rings.
    SUZUKI GS750 GS850 GS1000 MIKUNI FUEL MIXTURE SCREW KAWASAKI PILOT MIXTURE SCREW | eBay
    Mikuni Viton Choke Plunger Seat Renewal.
    VITON Choke plunger seals .KAWASAKI Z1,Z900,Z650,Z1000,Z1R,SUZUKI GS1000,GSXR,RF | eBay

    Air Corrector Jets for Mikuni VM 24, 26 and 28mm carbs .

    https://www.ebay.co.uk/itm/254380193...84.m1555.l2649




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