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TOPIC: DT400 Ignition Mystery

DT400 Ignition Mystery 13 Jun 2018 04:07 #1

  • davisjj
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I bought a ’78 DT400 several months ago and it has never run well since I got it. It was initially very hard to start and kicked back viciously (once ripping out the side of my boot). The few times I got it running it seemed very “blubbery” at mid-throttle positions, although it idled well, and ran OK at full throttle. I checked the static timing with a dial gauge and set it to spec (2.8mm BTDC). I also took the carb apart and cleaned it; everything looked OK in there. None of that seemed to make any difference, so I took it to a local independent shop, where the real mystery begins (or at least it's a mystery to me). After verifying that the static timing was correct, the mechanic put a timing light on it and found that the timing was actually way, way advanced, so much so that he couldn’t rotate the base plate far enough counterclockwise in the adjustment slots to set the timing to spec. So he ended up drilling new holes in the base plate to allow him to rotate it far enough counterclockwise. The attached photo shows what it looks like now. He said that with the base plate in its new position it fires at the right point according to his timing light. It now starts easily and doesn’t kick back, but still runs poorly at mid-throttle, and frequently fouls plugs.

My question is this: something seems really wrong to me that he had to rotate the base plate so far out of its normal position that he had to drill new holes. I asked him about that, and either I didn’t understand his explanation, or he doesn’t know why either. Either way, he didn’t seem very concerned about it.

Has anyone encountered a similar situation, or have any idea what’s going on?

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Last Edit: by Devin. Reason: inserted picture

DT400 Ignition Mystery 13 Jun 2018 04:18 #2

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It might pay to check if you have the correct rotor for your bike .
The keyway may be cut in a different position . Just a suggestion .
I have no information on the rotors , but a more knowledgeable person may have that info .

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DT400 Ignition Mystery 13 Jun 2018 12:51 #3

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The mag rotor on my 78 dt400 have F3T350.

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DT400 Ignition Mystery 13 Jun 2018 13:32 #4

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My rotor is also stamped "F3T350" on the outside, so at least we have the same rotor. Given that, I suspect that I have the correct rotor. I wonder about the backing plate. Any way to tell if it is the right one?

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DT400 Ignition Mystery 13 Jun 2018 13:50 #5

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From a visual perspective all looks exactly the same, except that little dot mark on your pulsar coil is on the opposite side of mine. There is a wagon wheel mark on the backside followed by a dash (-), then 2. I have no clue of the significance.

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DT400 Ignition Mystery 14 Jun 2018 13:32 #6

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Let me rephrase my initial question: Is this something that should concern me?
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DT400 Ignition Mystery 14 Jun 2018 14:03 #7

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I would be curious if the stroke was changed so then the timing. The cylinder should have 2 stamps at very bottom of cylinder 397cc and 1m200, and stock base cylinder gasket. I have never heard of anyone stroking these motors. If it works then go with it i would say.

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DT400 Ignition Mystery 14 Jun 2018 14:31 #8

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I had this problem with the early Rexspeed shop settup, , was the engine pinking before you had to retard it this far ?.
As for the blubber in midrange , drop the needle 2 grooves,[ clip up] this should clear that ,
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If you aint mixin gas " you aint haulin Ass

DT400 Ignition Mystery 14 Jun 2018 15:42 #9

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Blackhat 250, could you elaborate a little more about the problem you had?

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DT400 Ignition Mystery 14 Jun 2018 16:11 #10

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Blimey, i'd put it back in the middle of the original slots then make sure key is ok then feel tdc on the rock which you can be very close too, then go back to about 2-oclock [no more] then felt pen or twink [correction white fluid] the flywheel to the case--anywhere convenient so then strobe it while someones kicking or preferably turning the back wheel or even better i saw someone with a reverse drill spinning a smaller motor for that purpose. Plug out of course & plug sitting on the head or at least earthed for strobe to work. Then observe where the hell the timing is in relation to the twink mark & should be close!!, maybe about 1 oclock at kick start speed. If 1/4 turn out which sounds like it probably is then 'what the hell'. Thinking outa the square, can the source coil be flipped over end for end as strange things happen, had that on a tt250 rewound coil but just reversed the red for the brown & was ok--rewind person said do that!!. I know yours is just one wire which is why i say maybe flip it over. Or--something way out of kilter with the pulser resistance. Just doesn't add up. On second thoughts it was years ago but my 465 i got a complete stator [think i've said this before] got it for the 'good source coil' & just bolted the complete thing on but fired like yours, too far out to run & was the pulser although didn't believe it till i swapped the old one & still going 10 years later. When i say 1 oclock or 2 just by feel back from tdc then doesn't matter where you twink it because you'll know that's 1 or 2 whatever for the strobe. Re-thinking that bit, on the flywheel there should be three scribe marks close together to strobe to but not sure where the stator plate mark is to observe it against, but you know that at kick start speed it'll fire 'just' back from tdc no matter what. Just a thought--rivets on the flywheel not let go? & been re-riveted in the next hole around--if fired from a notch in the center bit in relation to trigger coil. Or--a magnet been 'reversed' as think they're screwed in. Have seen weird thing happen over the years, then the customer gives up & brings it in then becomes our problem lol, & luckily there were a few wise old heads at work that enjoyed those kind of problem solving--just hard to charge all the time out but gives great satisfaction & you probably never come across the same thing twice but there's always something new & unexpected will come along. Hope that reads legible--cat's sitting half on the keyboard lol.
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