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TOPIC: 1978 dt400 Needle jet

1978 dt400 Needle jet 10 Jan 2019 16:36 #1

  • Fpalbrecht
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I finally got to kick my 78 dt400 over. Fresh rings/hone, cleaned carb and new jets. Uni filter. Bike started in 5-6 kicks, idles fine. I can get it to 5000 rpm but it's not happy. Bogs. When I pull the plug it's dripping wet. My new from yamaha dealer needle says 6f9, needle jet says 166 Q-2, slide 2.5. Pilot 80, main I used a 210 to be safe. Is the needle jet correct?
Thanks I have manual and supplement but I'm not sure about the needle jet/ emulsion tube.
Thanks
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1978 dt400 Needle jet 11 Jan 2019 01:41 #2

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If it's been sitting for an extended period--years-- it's quite possible its got a belly full of oil that's gravity fed through the pump into the crankcase. This shouldn't happen but it 'does' find its way past the lightly sprung ball at the start of the small line from pump to cylinder inlet--or carb as some models have. Just get outa town & give it a good thrashing--so to speak lol. Alternatively is it likely the exhaust is restricted , especially the spark arrestor deep in the tail pipe muffler, or even built up in the main chamber where the tail muffler plugs in & ends. Not sure about the needle jet--needle combo but 80 pilot sounds rich but that's what the parts list says so should respond to the mixture screw at 2 turns out, like further out be lean & further in start to notice richening up unless it's getting fuel from elsewhere like a bad needle & jet combo. Choke not 'on' or held off the seat by the plastic seal cap thingy that if not clipped over the ledge can--on some--stop choke touching down. I'm clutching at straws lol, & woffling, so i'll leave it at that.--but wait, there's more. Give it a good rev if you haven't already, although do it slowly but surely because if it has a lot of oil down under the piston then it doesn't want it to all arrive up top too quickly. Also it'll get heaps of oil right out into the exhaust & smoke out the village for a long time unless you burn it out. Actually i was using a leaf blower the other day that i was fixing for someone & took the step down cap off the end & was surprised at the volume of air it put out so first thought--not leaves--but man if i could heat my pipe to get it burning initially then set the blower up in the front end of the pipe & let her go--think it'd work great.
www.partzilla.com/catalog/yamaha/motorcy...0e/carburetor-dt400e

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1978 dt400 Needle jet 11 Jan 2019 07:11 #3

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I think I found the answer from a previous post. I think stock is 166 O-2, I have 166 Q-2. Which would be way too rich. I placed a order with jets are us. And I wait again!

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1978 dt400 Needle jet 15 Jan 2019 11:53 #4

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That was it. Revs great now. But I need to wait to fine tune. 29° in WI, think I'll go ice fishing. More to come this spring. At least I can sleep now.
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