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Thread: Black/blueish smoke when revving engine

  1. #1
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    Default Black/blueish smoke when revving engine

    Hi,

    Hopefully this is in the right forum section.

    Recently my son and I have been putting back together 1982 GS750TZ that has been transformed into a cafe racer by my son. Bike was purchased as a learning project and time to bond, so we are noobs.


    Bike has 8822 miles on it before it was disassembled and transformed.

    The main symptom i'm chasing is the following:

    We recently got it running again but noticed black / blueish smoke when revving the engine. It mainly comes out of the left pipe but will come out of both pipes when revs are above 5000 - bike will rev thru full range.

    It will idle and it does start right up with help from choke and it holds a steady idle.

    Did a compression test and all 4 cylinders and they all have 120 or better compression but I noticed that cylinder 2 was wet and plug smelled like gas when I pulled the plug out.

    The good news is I"m sure no mosquitoes will survive in the garage after the exhaust smoke that has been generated while the engine is running.

    Any suggestions on what to try? Before I self diagnose it as bad valve seal in cylinder 2? The bike was not used for a few years before it was purchased by my son in November.


    When I checked oil sight window to see if a large amount of oil was being burned I noticed that the sight window was completely filled with oil - we previously had drained the oil and refilled with the correct amount of 3.2 qts - bike is on the center stand when observing I thought this was weird since we had changed the oil and made sure it was between the full and low marks as well.

    IMG_4937.jpg
    Last edited by gkoeber; 02-25-2020 at 12:00 PM.

  2. #2
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    Gas in oil is sign of petcock not completely shutting off. You haven’t left it in PRIme position have you?

    Had 850G for 14 years. Now have GK since 2005.
    GK at IndyMotoGP Suzuki Display... ... GK on GSResources Page ... ... Euro Trash Ego Machine .. ..3 mo'cykls.



  3. #3
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    Before advice, Welcome Home, you're in the right place. And great choice of bike, but I'm biased.
    We like pictures. (Imgur is great, link in my sig.)

    1. If you haven't checked/adjusted valve clearance, do it now.
    2. You likely have gas filling your crankcase due to:
    ___a. Faulty petcock/bad vacuum
    ___b. bad float needle valves in carbs
    ___c. improperly set float height in the carbs.
    ___d. all of the above.
    3. Clean, I mean really clean, your carbs.
    4. Drain that oil and fix your fuel issues before refilling the oil.

    Lots of great manuals and tutorials on BikiCliff's Website linked in my sig.
    Here's a copy of Suzuki supplement for your bike you may find useful. "Z" Supplement
    Last edited by Rich82GS750TZ; 02-25-2020 at 12:34 PM.

  4. #4
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    You can easily check the petcock for a leaking issue by disconnecting the fuel hose and visually checking it for the tell tale drip, that is if you didn't accidentally leave it in the prime position. If the crankcase has gotten fuel in it it is possible that the thinned oil is getting pushed up past the compression rings.
    GSRick
    No God, no peace. Know God, know peace.

    Eric Bang RIP 9/5/2018
    Have some bikes ready for us when we meet up.

  5. #5
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    Thanks for the quick replies and suggestions.

    After using the advanced search feature i found this article and what everyone has said makes sense.

    I'm pretty certain it was not left in PRIME but I will check to see if it is indeed leaking.

    My gs1000 started smoking from the left exhaust pipe. It is white smoke. It has an incredibley strong fuel smell. Also, my #2 plug is coming out black. When I clean the plug and put it back in the bike runs fine. However, once it sits and idles for a while it will foul again. It only fouls at idle. It doesn't foul any other time. What could be causing the smoke? It is not constant. And when I clean the plug it seems to really calm the smoke down.

    What it looks like now.
    CEUL3501.jpg

  6. #6
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    Quote Originally Posted by gkoeber View Post
    Hi,

    Hopefully this is in the right forum section.

    Recently my son and I have been putting back together 1982 GS750TZ that has been transformed into a cafe racer by my son. Bike was purchased as a learning project and time to bond, so we are noobs.


    Bike has 8822 miles on it before it was disassembled and transformed.

    The main symptom i'm chasing is the following:

    We recently got it running again but noticed black / blueish smoke when revving the engine. It mainly comes out of the left pipe but will come out of both pipes when revs are above 5000 - bike will rev thru full range.

    It will idle and it does start right up with help from choke and it holds a steady idle.

    Did a compression test and all 4 cylinders and they all have 120 or better compression but I noticed that cylinder 2 was wet and plug smelled like gas when I pulled the plug out.

    The good news is I"m sure no mosquitoes will survive in the garage after the exhaust smoke that has been generated while the engine is running.

    Any suggestions on what to try? Before I self diagnose it as bad valve seal in cylinder 2? The bike was not used for a few years before it was purchased by my son in November.


    When I checked oil sight window to see if a large amount of oil was being burned I noticed that the sight window was completely filled with oil - we previously had drained the oil and refilled with the correct amount of 3.2 qts - bike is on the center stand when observing I thought this was weird since we had changed the oil and made sure it was between the full and low marks as well.

    IMG_4937.jpg

    Hmmm, I would cease any running of the engine and pop your oil fill cap and see if you have a gas smell coming out. It does look like something is in the oil. If you do, DRAIN the oil ASAP. Black smoke indicates a rich mixture, blue smoke is oil. Could be the floats aren't set right, sticking open, and or petcock problems.
    MrBill Been a GSR member on and off since April 2002
    1980 GS 750E Bought new in Feb of 1980
    2015 CAN AM RTS


    Stuff I've done to my bike 1100E front end with new Sonic springs, 1100E swing arm conversion with new Progressive shocks installed, 530 sprockets/chain conversion, new SS brake lines, new brake pads. New SS fasteners through out. Rebuilt carbs, new EBC clutch springs and horn installed. New paint. Motor runs strong.

  7. #7
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    Petcock failure - a common problem after 35 years
    Don't try to rebuild it, many here find that does not work
    Buy an OEM or from Georgefixs

    Change to oil and filter now!

    Test it by removing the vacuum line from the petcock and plugging it with a bolt, golf tee, etc. Turn the petcock to Prime and see if the symptoms clear up. If they do, new petcock. If they don't, time to completely go thru the carbs and get a new petcock

    Go here http://members.dslextreme.com/users/bikecliff/ for a factory service manual and carb cleaning tutorial, plus all GS knowledge.

    Post up an intro in the GS Owners subforum and ask technical questions in the appropriate subform (this is the correct one for this problem)
    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
    1999 ATK 490ES
    1994 DR 350SES

  8. #8
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    Feb 2020
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    NNJ
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    Thanks again for all the advice......

    New petcock is ordered from Z1 and I will drain the oil and change the filter tonight.

    Once the petcock comes in I will keep everyone posted on the results.

    thanks again

  9. #9
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    Update - Problem fixed after replacing the petcock tank valve

    Changed the Oil and verified the level has not changed or filled with gas like before.

    Now I have to put stock airbox back on before I ask about why it is not able to rev past a quarter throttle approx. I have read the following thread: https://www.thegsresources.com/_foru...ewbie+mistakes

    so I realize I need to follow these steps before I ask more specific questions.

    After running the bike for a bit to see if the "smoke cleared up" I was able to rev engine to full throttle, but when i tried to ride it down the road, I was only able to rev it to approx 1/4 throttle before it would "bog down" - I'm assuming this is due to the stock airbox NOT being fully installed the bike is running with stock jets and stock exhaust. I checked the plugs and they were not fouled but clean as if it was running lean. sorry no pics

    Thanks again - this forum is an amazing source of information
    Last edited by gkoeber; 03-02-2020 at 12:38 PM.

  10. #10
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    Quote Originally Posted by gkoeber View Post
    After running the bike for a bit to see if the "smoke cleared up" I was able to rev engine to full throttle, but when i tried to ride it down the road, I was only able to rev it to approx 1/4 throttle before it would "bog down" - I'm assuming this is due to the stock airbox NOT being fully installed the bike is running with stock jets and stock exhaust. I checked the plugs and they were not fouled but clean as if it was running lean.
    Very lean sounds right to me. Stock jetting isn't able to supply enough fuel under load to keep up with the massive amount of air due to no airbox installed. Sounds like you're on your way. Good work.

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