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Makotosun

1978 DT400 General Advice resurrecting solid bike

  • RamadiRat
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I've been dragging this 1978 DT400 around the country and am now in a position to focus on her.  When I bought her 4 years ago she ran... but I know not how well - having trouble getting her started.

Here's what's going on now:
  • I have her topped up with oil.
  • She's half full of non-ethanol gas.
  • Have a new fuel filter
  • Have a new battery

No matter how much I crank away, she won't start.

*When I put some starting fluid in the breather box, she'll start and run but she races right up to redline SO I SHUT HER DOWN.

How best to proceed? I'm fairly conversant with carburetors in regards to mixture/idle adjustments (1976 TR6), however I think this is something else (everyone always wants to immediately start fiddling with the carbs.
20 Apr 2023 13:42 #1

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Without thinking too deeply, it sounds like the slide has been out then misplaced on refitting. Two things need to happen--slide needle down the jet & just as importantly the guide pin needs to find the slide--well other way around slide on pin. Otherwise it'll be on about 3/4 throttle & rev its ---off. Also, choke won't suck if throttles not fully closed or closed to an idle at least. Back out the speed screw initially just so you know its well down. Set the mixture screw on the rich side say one turn out & go from there. Also get into the carb if its been parked with old fuel. We have the worst fuel down here in NZ & it just goes off fast & grows varnishy crap & blocks up jets. In saying that , the choke jet is a very small fixed jet in the bowl cavity & need to look down in the light to see it--might need a poke & blow from each side, fed sideways in the bowl base, you'll see it. Im not sure but the tube that sits down in there might have a small hole too in its base. That's enough confusion from me for now. Bike looks nice. Could be other air leaks like crank seals but doubt it as pretty robust. Go for carb.

Just to add, as i said, throttle needs to be shut for choke to suck, so don't go fanging the throttle while kicking, leave it shut. If it does show signs after kicking till ya leg drops off then screw idle up a bit & with it hopefully now wanting to run then slowly increase revs. Alternatively if you have luck with engine start then just try that when ya sure throttle is bottoming out & might start & settle down.
Last edit: 20 Apr 2023 15:04 by RT325.
20 Apr 2023 14:54 #2

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  • RamadiRat
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Sounds like I need to remove the carb to check the sleeve, etc
20 Apr 2023 16:25 #3

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  • MarkT
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Clean the carb, specifically the pilot jet. If you're just a little careful with the float bowl gasket, you won't need any parts at all.

Slide should close with a "thunk". If not stuck, you might have an air leak.
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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20 Apr 2023 16:41 #4

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Replied by Pedalcrazy on topic 1978 DT400 General Advice resurrecting solid bike

Good looking bike. Keep looking into fuel delivery. Good luck.
1978 DT400E
1976 DT400C
1973 RT3
1971 RT1B
1968 DT1 (3)
20 Apr 2023 16:48 #5

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Great looking bike you can take the air box hose loose from your carb and look or put your finger in and see if your slide is at the bottom 
20 Apr 2023 16:49 #6

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  • RamadiRat
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Thanks for the tips/suggestions - I'll put some time on this tomorrow. This model is how I got back and forth to college in San Diego after getting out of the Army (LONG time ago), I feel lucky to have found one in such good shape.

So I removed the carb, cleaned everything...it looked pretty good as you can see in the pics.  I set baseline settings and it started just fine.  In trying to adjust the idle/air fuel mixture some adjustments would send her racing near redline, and those were minor adjustments. I suspect something is happening such that the slide is sticking but honestly I can't imagine that a slight adjustment of the idle speed screw would cause that. RIght now it's idling pretty reliably at 1400 and throttle response is decent (I think.. I have little to no experience to actually know).
Last edit: 21 Apr 2023 13:18 by RamadiRat. Reason: update on recommended procedures offered by members
20 Apr 2023 19:49 #7

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beautiful bike
I got one and LOVE it!
I found that its nearly impossible to remove the slide without loosening the exhaust so  you can rock it out of the way.  So maybe you cocked it a bit taking it in /out?  Its amazing how a tiny bit of dirt in pilot jet can impact these bikes.  Taking the carb out and cleaning really good probably good idea.  Oh, this model is also a  bitch to connect the throttle wire to the slide...its got a unique bracket with a couple of screws vs. the way the wire attaches to slides on my other bikes
Love to hear it run when you get it going.
oh - i bought a cool disconnect for the balance hose which makes removing the tank much easier and less messy
21 Apr 2023 17:07 #8

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  • MarkT
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Just curious if you removed the pilot jet and cleaned it? (Your pictures don't show how clean the "important" parts are.)

Anyway, if it redlines and you're sure that the slide is closing (listen for a consistent "thunk" when opening and closing throttle with engine off), then the uncontrolled revving is a symptom of an air leak... if it happens and using kill switch and key switch won't shut it off, then it's almost certainly an air leak.

Anyway, hope the work you've done fixed it and that she runs great for many years!
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
21 Apr 2023 19:59 #9

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  • RamadiRat
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I did remove and clean the pilot jet.  The comment regarding it revving even with the kill switch and key off are PRECISELY what happens when things go wrong.  So, having recently discovered the important of air/vacuum leaks on my Triumph TR6.... I'm particularly interested in chasing this down.  Another symptom that may confirm the air leak is that while it will idle reliably, it doesn't idle as smoothly as it probably should.... the tach bounces from 1000 to 1400... back and forth.  

So... question is for the group - where do I look for 'air leaks?'  Is this a function of an o-ring somewhere? A rebuild kit is not expensive so if that's a prudent thing to do it's an easy fix.
22 Apr 2023 08:36 #10

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