Vintage Dirt Bike Q & A

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 Post subject: 78 DT400
PostPosted: Fri Oct 21, 2011 9:42 pm 
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Joined: Mon Sep 05, 2011 10:45 pm
Posts: 49
Location: Poulsbo, WA
I just picked up a 78 Yamaha DT400 along with a parts bike. It's in pretty good shape, has great compression but no spark. I'm thinking the problem is the kill switch, but can't see how to isolate that. Can I just pull a connector at the CDI, or is there any other recommended way of ruling this out?
As a more general question, is there anything to watch for on these bikes? I plan on using it mostly for commuting, but I'd like to run around in the woods every now and then too.


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 Post subject: Re: 78 DT400
PostPosted: Sat Oct 22, 2011 3:00 pm 
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Joined: Mon Sep 05, 2011 10:45 pm
Posts: 49
Location: Poulsbo, WA
Well, the previous owner replaced the coil with an internally grounded unit, and hooked the single wire from it to ground :roll:
I connected it properly to the harness, and it runs now.
Pictures:
Parts bike:
Image
Runner:
Image


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 Post subject: Re: 78 DT400
PostPosted: Sat Oct 29, 2011 8:53 am 
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Joined: Mon Sep 05, 2011 10:45 pm
Posts: 49
Location: Poulsbo, WA
And I killed it- sort of. I was doing a quick test ride, and was almost back home when it started making odd noises and then died in the driveway. It would kick over with almost no compression, so I tore it down and found one of the rings folded on top of the piston. The cylinder is quite scored and may or may not be fixable.
The parts bike has a good 86mm bore and piston, but no rings (?) so I'll get a ring set and a gasket set and be back to rideable just as it starts snowing.


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 Post subject: Re: 78 DT400
PostPosted: Wed Feb 08, 2012 7:57 pm 
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Joined: Tue Aug 31, 2004 1:44 am
Posts: 283
Location: Maysville, AR
did you ever get it running again?

_________________
(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!


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 Post subject: Re: 78 DT400
PostPosted: Sun Feb 19, 2012 12:05 am 
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Joined: Mon Sep 05, 2011 10:45 pm
Posts: 49
Location: Poulsbo, WA
Not yet. I had a bunch of sleds to get snow-ready, and I don't have any indoor workspace. I need to order a piston, rings and gasket set for it, then it'll be my commuter. I'll likely put modern d/s tires on it to help with mileage. It'll be running once the little snow we got is gone.


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 Post subject: Re: 78 DT400
PostPosted: Sat Apr 21, 2012 2:33 am 
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Joined: Wed Jun 22, 2011 10:50 pm
Posts: 16
Try Yamaha enduro .com.Those are some great guy's with a wealth of information about anything Yamaha.Plenty of parts too.


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 Post subject: Re: 78 DT400
PostPosted: Mon May 21, 2012 6:41 pm 
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Joined: Sat May 19, 2012 10:51 pm
Posts: 7
Location: Laguna Niguel, CA
Be sure to inspect your brakes... old drum brakes like to delaminate if they get wet and dry wet and dry. In your case, hot cold wet dry who knows. I had a rear (thankfully) delam on me and locked my wheel right up. If this happens to your front wheel while commuting... well let's not think about that. As for tires, most enduro tires are 60/40 road/dirt or dirt/road. Long story short, if you have two sets of wheels, get road and off road tires. I put medium compound MX tires on my DT 250 and it hauls in the dirt but is super squirly on the road so I have a nice set of Kenda Enduros for road riding on a spare set of wheels. I look like a NASCAR pit member changing wheels in under 10mins...

_________________
75 DT250, 77 IT250, 78 YZ250, 93 YZ125, 02 YZ426


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 Post subject: Re: 78 DT400
PostPosted: Tue May 22, 2012 9:11 pm 
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Joined: Mon Sep 05, 2011 10:45 pm
Posts: 49
Location: Poulsbo, WA
Good to know about the brakes. Currently the bike is still sitting in Idaho while I get housing all set up here in WA. I got my Honda running decent so once that's registered I'll start using it as a commuter and work on the Yamaha once it's here. I'll probably play around with it a bit, like put a different reed setup and carb on it, maybe make a pipe, and put more street-oriented tires on. I have a Kenda K270 on my Honda and it seems decent in the little bit of riding that I've done on it, but I'll likely go with a modern d/s tire for the Yamaha. Something like a Shinko 700 or maybe a K761, I'll need to do a bit more research though.


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 Post subject: Re: 78 DT400
PostPosted: Thu Sep 27, 2012 4:39 pm 
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Joined: Tue Aug 31, 2004 1:44 am
Posts: 283
Location: Maysville, AR
Swampdog wrote:
Try Yamaha enduro .com.Those are some great guy's with a wealth of information about anything Yamaha.Plenty of parts too.

yep! that place is the resource there for sure!

_________________
(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!


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