Weird engine running on 950

Discussion in 'Crazy-Awesome almost Dakar racers (950/990cc)' started by Gustavo.Ramos, Sep 3, 2013.

  1. Gustavo.Ramos

    Gustavo.Ramos Long timer

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    A friend had his 950 serviced at a "friends" shop, (first mistake), who had no experience with the bigtwin to replace WP seals, timing chains, etc... big service (second mistake), and when the bike was returned, it sounded different than before (for the worst), and then he paid for the *service* while complaining (third mistake) for the different behavior while the "friend" was saying it was all his mind and the bike was better than before.

    After riding it:

    It sounds different, like a single cylinder, both cylinders are firing. Ran a bit weak with lots of backfire.

    After stripping it, one of the carbs was not fully seated, in fact was loose... can't figure how it still worked.

    Fixed the carb, synched it, went for a spin, was better, but still sounded like a thumper. plenty of power, power wheelies, etc...

    Today...

    Still sounds like a thumper in idle+low revs, then gets better, not much popping on decel. Had a 2005.5 950 and it didn't sound like this.

    Removed carbs, checked diaphragms, IMS position, needle position, all ok... removed cam covers and checked TDC for both cylinders + camshaft position.


    Any suggestion? Engine runs, but sounds like a thumper, not the rumbling twin, yet doesnt feel like missfiring...
    #1
  2. Katoom72

    Katoom72 Been here awhile

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    Keep your hands on the exhaust, like feel how the gasses get blown out. Any difference on both cilinders? Sounds like a timing issue to me.
    #2
  3. Orangecicle

    Orangecicle On a "Quest"

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    Probably did what I did the first time I timed the V-2 KTM. You have to find a TDC for the rear jug and lock in the crank. You set the timing on the rear.

    Now, the mistake I made the first time was to rotate the engine in reverse 75 degrees and time the front. That is a TDC for the front, but it is the wrong TDC. The book says TDC the rear, then rotate the engine 360 degrees, and then add 75 degrees. You can do this pretty simply just by looking at the motor. If you find TDC on the rear, set your wrench so that it is in line with the rear jug. Now, rotate in reverse 360 degrees to get back to where you were. The engine is a 75 degree motor, so rotate in reverse another 75 degrees so that your wrench is in line with the front jug. Lock in the crank. Now you are ready to time the front cylinder with the dots on the cams.

    I'm pretty sure that this is what happened to your friend.

    Also, when you go to remove the improperly timed front cams, make sure you rotate the engine around until the dots on the cams line up properly with the surface of the front head. Check with a feeler gauge to make sure that there is no pressure on the buckets. As long as there is no pressure coming from either cam, then you can remove the cam bridge bolts. If you try to remove the cam bridges with pressure on the bridges, you will destroy the bridges, and that is a very expensive mistake.
    #3
  4. Zuber

    Zuber Zoob

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    +1 on cam timing. You now have a 'twingle', a twin that fires like a single.

    Normally, the front cylinder fires, then 285 degrees later the rear cylinder files. Now, the rear cylinder fires, then 75 degrees later the front fires, much closer and more like a single. The dufus didn't have the right locking tool, or he pulled both cams at the same time and lost his place.

    Or, there's a small chance that he got the cams a few teeth off and bent a valve.

    Send it back to the dufus or ask for about 4 hours of labor back to get the cams timed correct. A bent valve can cost upwards of $1000 to fix.
    #4
  5. Gustavo.Ramos

    Gustavo.Ramos Long timer

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    While i said that checked the timing, was not fully convinced about it. used a locking tool for the crankshaft when having first rear, than front cylinders at TDC.

    For the rear the inside "X" on both cams didn't appear to aligh fully with the head gasket surface, the exhaust one seemed a bit recessed deep into the head, somehow like if a tooth was off, but the lobes didn't looked so different... yet as mentioned was not convinced.

    Katoom72, pulse from exhaust does not seem to be much different, however i'm more used to the 990 where the rhs can pulses more than the left one, so its a bit unconclusive.

    Guess i'm giving some bad news, lets dig into that engine again. He doesnt want the guy who messed the bike to touch it again... after seeing a carb not seated and the engine running like that i'd feel the same...
    #5
  6. Gustavo.Ramos

    Gustavo.Ramos Long timer

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    Orangecicle,

    Good thorough explanation! :)
    #6
  7. Sumi

    Sumi Long timer

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    +1 on the timing is in a "big bang" / "twingle" config. If it was only one tooth off, you wouldn't really even notice.
    #7
  8. kamanya

    kamanya Andrew to most

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    #8
  9. Gustavo.Ramos

    Gustavo.Ramos Long timer

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    To close the thread. Problem was related to:

    a) incorrectly seated carbs (masking the real problem)
    b) rear intake camshaft one tooth off the correct position.

    So these beasts actually run with slight incorrect timing
    #9
  10. Black Hills

    Black Hills Long timer

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    most engines will, my wifes Audi A-6 had a cam one tooth off for 75,000mi. the mechanic called me when he replaced the timing belt to come look at it. once it was properly set the little v-6 no longer fell on its face 500rpm before redline:1drink but it got worse mileage.
    #10
  11. FlyingPenguin

    FlyingPenguin Been here awhile

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    I routinely time my British singles, BSA,Ariel,etcc, with the can advanced one tooth. Makes Abigail difference in mid range and since they only Rev to 6-6500 the top end loss isn't noticed. One tooth on a 18 t pinion is 20degrees.

    Sent from my SAMSUNG-SGH-I727 using Tapatalk 2
    #11