OT: DT400 help
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Thread: OT: DT400 help

  1. #1
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    Default OT: DT400 help

    Ok, I know this is not M109 related, but I'm hoping to find someone thinking "dang, I used to have one of those bikes back in the day..."

    I just bought a used 1978 Yamaha DT400 cheap. It's in really good shape, but need a couple of questions answered.

    1. How do you open / remove the seat? Most of the old trail bikes I had years ago had some kind of release for the seat. I can't find anything to release this seat, and can't even figure out what bolts might be holding it down. I want to get the seat off as it appears the access to the air filter is under there and I really want to get that cleaned out.

    2. Is there a way to ensure the oil pump for the 2 stroke oil is working correctly without just putting regular gas in the tank and hoping it mixes? If it is not working, what would the mix ratio be to premix?

    Now I just need to find a source for some cheap tires for this machine. It has 3k original miles (speedo / odo still work) and I'm guessing these are the original tires and are severely weather cracked.

    And for those that get offended at non-109 discussions, sorry, but nobody appears to be very active on any message boards I found for these old enduros.

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  3. #2
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    Default Re: OT: DT400 help

    not much help but here is a manual
    http://cgi.ebay.com/ebaymotors/YAMAH...QQcmdZViewItem

  4. #3
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    Default Re: OT: DT400 help

    Thanks! I did see those on there yesterday. I may just have to get one, but I was hoping to at least get the air cleaner and oil question solved by this weekend.

    Thanks again!

  5. #4
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    Default Re: OT: DT400 help


  6. #5
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    Default Re: OT: DT400 help

    Thanks again!

  7. #6
    Very Active Member Woody109's Avatar
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    Default Re: OT: DT400 help

    As far as the seat, I belive there are bolts, one on either sied, behind the side panels. For the oil pump, first top off the oil jug on the bike. Next mix approx 2 oz of oil to a gal of gas ( if you want to mix 2 gal add 2 more oz of oil) and add that to the gas tank. Next ride the bike around for a while and when your done check to see if you used any oil from the onboard oil tank. Keep an ear to the bike as you ride to make sure it isn't getting real hot and ready to sieze. I believe ideal mix is like 40 to 1, or 3.2 oz per gal of gas. If the tank is empty top it right off with pre-mixed fuel. That way when you get done with your ride you can see how much fuel and oil was used an figure out the ratio. Those old oil pumps are known for not liking sitting for long periods too. Good Luck jeff


  8. #7
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    Default Re: OT: DT400 help

    Still haven't managed to get the seat off, though I didn't try much. 100+ degrees is a bit much to be working out in the sun on something like that. I did put some straight gas in the tank to dilute the pre-mix the previous owner put in there and it is running a lot better. A lot less smoke, less sluggish, and less hesitation when I hit the throttle. There is still some hesitation / sputtering there, though. I'm guessing the carb needs a good run-through. I also need to find the fuel filter and probably clean that out. There is a bit of prior rust in the tank so I'll probably put a small in-line filter in there to catch that garbage if it comes through.

    Thanks again!

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