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Thread: '78 Yamaha DT400-how much compression?

  1. #1
    Brad200X's Avatar
    Brad200X is offline At The Back Of The Pack Arm chair racerAt the back of the pack
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    '78 Yamaha DT400-how much compression?

    Alright guys, I have a DT400 and it feels a little gutless for a 400CC 2-stroke. Can the decompression valve go bad? Is it adjustable? What kind of comp. should this bike have?
    Last edited by Brad200X; 03-09-2009 at 06:48 PM. Reason: my typing sucks.
    Rides I own:
    '85 200X(old smokey)
    '81 ATC250R(Bassani pipe, Wiseco piston, Vesrah rod, and AMS offset rims)
    '84 KXT250 Red(the new money pit!)
    '96 KX125(FMF pipe, Boyesen reeds, One Industries graphic kit, etc.)
    '85 KX125(vintage racer. Technically it was a practice bike)
    '78 DT400(the lone Yamaha)

  2. #2
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    I had one of those bikes and it was a 2-stroke, so it wouldn't have valves. It sounds to me like it needs a topend rebuild.

  3. #3
    Brad200X's Avatar
    Brad200X is offline At The Back Of The Pack Arm chair racerAt the back of the pack
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    It is a 2-stroke. The decompression valve is supposed to make it easier to kick.
    Rides I own:
    '85 200X(old smokey)
    '81 ATC250R(Bassani pipe, Wiseco piston, Vesrah rod, and AMS offset rims)
    '84 KXT250 Red(the new money pit!)
    '96 KX125(FMF pipe, Boyesen reeds, One Industries graphic kit, etc.)
    '85 KX125(vintage racer. Technically it was a practice bike)
    '78 DT400(the lone Yamaha)

  4. #4
    bandito90's Avatar
    bandito90 is offline At The Back Of The Pack Arm chair racerAt the back of the pack
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    I new a guy that had one of these years ago.I never rode the it,But if it is like the smaller dt/mx bikes it would have a broad power band .Not peaky like a yz or other similar motorcrosser.

  5. #5
    Brad200X's Avatar
    Brad200X is offline At The Back Of The Pack Arm chair racerAt the back of the pack
    Join Date
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    Yeah its power definitely comes on more like a street bike, as it is street legal, but it feels like should be faster. That's why I'm kinda thinking the decomp. is stuck open or something.
    Rides I own:
    '85 200X(old smokey)
    '81 ATC250R(Bassani pipe, Wiseco piston, Vesrah rod, and AMS offset rims)
    '84 KXT250 Red(the new money pit!)
    '96 KX125(FMF pipe, Boyesen reeds, One Industries graphic kit, etc.)
    '85 KX125(vintage racer. Technically it was a practice bike)
    '78 DT400(the lone Yamaha)

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