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02 Sep 2019
Your admin has not been on the site much as of late as my main computer apparently seized up :( I have had limited access, but will be back soon. Thanks. Mako
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TOPIC: 75 DT400 minor running issues

75 DT400 minor running issues 08 Sep 2019 09:23 #1

  • Zjohnson878
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Me and my dad have been working on getting this Dt400 road ready. 4500 to 5000rpms and under it pulls well but has a stutter about 5000 and bogs/loads up. Carb was rebuilt and idles perfect. Came with a B8ES plug but I've also seen where a B9ES plug was used on these models. I have a feeling it may be a timing issue. I think I'm gonna go with a 12 volt ignition swap. From what I've read on here the stock magneto was suspect even from new. It's been fun wrenching on this old enduro, very addictive! We just replaced tires, tubes, brake shoes, brake lever, intake manifold, and chain and sprockets. Getting closer!
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My current children...
1975 Yamaha DT400b
1982 Yamaha Seca 650 XJ650RJ
2019 Honda Crf250l Rally
2019 Honda Grom

75 DT400 minor running issues 09 Sep 2019 19:08 #2

  • Pedalcrazy
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I've restored several and have a 76 DT 400 that is my baby. I've tuned and tuned and tuned this summer to get it the best it can be. Is your problem worse when cold? Better when fully warmed up? What main jet are you running? Mine bogged at higher rpms until I reduced the main jet. I'm running a 150 and it doesn't blubber out at high rpms. You could also verify if it gets better or worse next time you ride by flipping on the choke when it bogs. If it gets worse consider a smaller main jet....better..... go bigger on the jet. There are many other possibilities too...dirty air fi!ter. Factory plug is the 9 not the 8. The needle in the carb could be raised or lowered a notch too. Higher rpms though probably main jet. Oh yeah...Factory magneto/coils will run thousands of carefree miles. Everyone I've restored still had factory 6v setup and still do. Good luck...post if you get it sorted out.
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75 DT400 minor running issues 10 Sep 2019 15:37 #3

  • run103
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Make sure not to over oil the air filter Take it for a short pavement run with no filter and you can tell it that is the problem
Doug
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75 DT400 minor running issues 10 Sep 2019 15:48 #4

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Make sure not to over oil the air filter Take it for a short pavement run with no filter and you can tell it that is the problem. I tried a twin air with no oil and it couldn't get enough air at high rpm. Also it helps high rpm to remove the tool tray under the seat because it has that upside down cover on the airbox intake boot
Doug
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75 DT400 minor running issues 10 Sep 2019 16:35 #5

  • RT325
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Very unlikely to be a timing issue although i think "75 still had the two stage pulser & mine on my "74 packed up in the retard/starting half of it so it plain wouldn't spark/go so organised it to start on advance which simply meant it kicked back like a mule but it ran perfect--just kick it right & kick it hard. Anyway--doubt any of that is your issue. Lack of top end is usualy due to a restricted/carboned up tail pipe muffler especialy deep in that tail where the swirly spark arrestor is. If you live out of town far enough just give it good rev down the road minus the complete tail miffler--but man she'll be LOUD. Don't think those would have the side mounted main jet type carb like the RT3 etc, but if so the cause will be the needle jet extension Oring where it seals into the bowl base so the main jet gets drawn on direct & not drawing past an ill fittin Oring. Ignore all that i'd say. If you hve the flywheel off then make very sure it's TIGHT TIGHT not just tight--seriously, they shear off crank keys otherwise. good luck!!.
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75 DT400 minor running issues 10 Sep 2019 18:01 #6

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Can't wait to try some of your suggestions. I'm out on a boat delivery now so it'll be about a week before I can get to tinkering again. I will keep you guys updated on my progress! I had it at a constant 60mph the other day so it must be close to running right and hopefully something simple. My bike came with a K&N filter that didn't look too dirty or over oiled but there is an overstuffed not stock took kit sitting under the seat that takes up a lot more space than the stock took kit. I'll be interested to see what happens when i take the tools and the tray out plus a correct spark plug B9ES and 93 octane fuel. My stepdad filled it up with 89 octane before we rode it and I didn't realize it until later.

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My current children...
1975 Yamaha DT400b
1982 Yamaha Seca 650 XJ650RJ
2019 Honda Crf250l Rally
2019 Honda Grom

75 DT400 minor running issues 10 Sep 2019 20:21 #7

  • MarkT
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89 octane should be fine.
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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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