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Makotosun

1968 DT1 rebuild/rejuvenation

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Replied by DaveinSC on topic 1968 DT1 rebuild/rejuvenation

As bad as that carb looked I might have it beat with the one from an AT2 I just started working on. I took the carb off last night and it was full of white powder. Probably the worst I’ve ever seen. It was a bear to get apart and cleaned up but I think it will be okay. I put it back on the bike and was able to get it to run for a few seconds but need to set the timing and a bunch of other things before actually trying to let it run.
29 Dec 2020 14:57 #21

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Replied by nhsteve on topic 1968 DT1 rebuild/rejuvenation

:EEEK

Yeah, amazing how the stuff corrodes things. Mine was a real fight to separate the bowl from the body. Bowl did not want to be released from the O-ring. Lots of lube and patience, I was considering leaving it in some real hot water for a bit to see if that would aid things. Just kept wiggling it back and forth until it eventually released (and all the lube I'd put in there dumped in my lap :ROFLOL )
02 Jan 2021 08:03 #22

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Replied by nhsteve on topic 1968 DT1 rebuild/rejuvenation

Progress so far this month has been slow, for multi0ple reasons. One of which has been the USPS, taking a lot of time to get parts in. Hopefully that will get better now that the holidays are done.

First pic is the left main case with the new bearings installed. My method is to freeze the bearings for a couple of days, and when ready, I heat up the case, and the bearing usually slides in easily. That is how it went with the main crank bearing and the output shaft bearings. The little steel-caged needle bearing for the main shaft not so much this time. I'm thinking that the steel cage and small size of the bearing allowed it to expand quickly. Also, it was the last to get installed, perhaps I should have started with that one. I do use bearing/bushing drivers sized to fit.

15 Jan 2021 10:19 #23

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Replied by nhsteve on topic 1968 DT1 rebuild/rejuvenation

As I am still waiting for one more bearing to show up (tomorrow, Mr. Postman?? Please-please-pleeeze??) for the right side case, I looked at the piston situation in the meantime. Crankshaft with new rod kit has been sent out for rebuild.

So apparently the current piston is 1st oversize, and it looks like a keystone-type top ring. Using a good set of feeler gauges I can easily get a .006" feeler between piston and cylinder, .007" not so much. Ring gaps measure .022"-.025" .

So I am thinking a 2nd O/S piston is next. My quandry here is not so much finding the parts, but in getting the piston, then finding that I really needed to go 3rd O/S during the boring process. I could wind up with parts that I will never use. When Enduronut was in operation he had sufficient supply to be able to do the job, and if the cylinder needed to go out one more step he could just do that. But that is no longer the case, so it looks like I may be headed to my local machine shop to see what they say.

15 Jan 2021 10:31 #24

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Replied by DaveR on topic 1968 DT1 rebuild/rejuvenation

Here’s a good read for some motivation !!
motocrossactionmag.com/yamaha-dt-1-the-b...ppened-to-motocross/

Some nice pictures of the early DT1, Kick starter bolt inboard, aluminum dip stick, hollow front axle on RH side..…
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15 Jan 2021 15:51 #25

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Replied by nhsteve on topic 1968 DT1 rebuild/rejuvenation

Very nice article, thanks for posting. The one I am working on has the alloy dipstick. The original kickstart lever was long gone and had been replaced by one from an RD350 (it looks similar to the one on mine, anyway). I suspect that style lever proved "unequal to the task" and was replaced by the sturdier one seen more frequently (which I have procured). #1717 has the smaller 8mm top rear motor mount bolt, that was upgraded to 10mm at some point. I will have to go look at the front axle.

My current thinking is to upgrade primarily in places where it would not be seen, as in the magneto/crank issue I mentioned earlier.
16 Jan 2021 08:02 #26

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Replied by nhsteve on topic 1968 DT1 rebuild/rejuvenation

Yesterday, I wrote this. I was getting pretty frustrated and finally retired to sit and think about things.

I am trying to reinstall the transmission into the cases, and am totally defeated and frustrated. I know several of the tricks (grease the little collars and shift fork pins and rollers to keep things in place, etc.). Rubber band the shafts together to try and keep things in alignment, etc. Found a youtube video about a 1970 DT1 transmission (in Japanese) where the man very carefully reassembles the two shafts and checks each gear, shim, washer, clip, and fork. And then (magically!!!) the next shot is of the entire assembly back in the case (poof! it's in!). I got the idea to try to loosely band things together from another for a YZ250F (not a bad idea).

So going to take a break from it now.
Last edit: 17 Jan 2021 11:44 by nhsteve. Reason: Information added
16 Jan 2021 12:26 #27

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Replied by nhsteve on topic 1968 DT1 rebuild/rejuvenation

Today is another day, and I have the transmission installed. It was my fault:Ugh . I was well aware of the split collar setup holding that first gear in place. I pulled that off to grease them up well so they would not disappear when I started installing, and then promptly put them back in place incorrectly. :Ugh

So..... don't do that.

Once I realized what I had done, I corrected that. I still liked the idea of using the rubber bands or a tywrap to assist me in keeping things together as I put the unit in place. You do have to tap on the ends of the shafts, and I was hoping doing this would help keep things roughly together during insertion. I slipped one around second gear (second gear is at the end of the shafts, right next to the case half) on each shaft (helped keep the gears engaged), and another one around the whole assembly once I had engaged the shift forks on their gears and set the roller pins (greased) into their worms on the shift drum. One of those pin/rollers engages the drum through a window in shift fork #1 (I think I've shown that).

Before I started lowering the rubber-banded assembly into the case, I made sure that I could see that the neutral stop on the end of the shift drum was aligned with the drilling in the case for the stop. This was hopefully to make sure the transmission was actually in neutral, not in any gear. (pic of that too)

The drive/output shaft is the longest at this stage, and as I lowered the assembly in place I tapped on the end of that shaft to encourage it to go into the bearing. As It went in I was eventually able to start tapping on the end of the main shaft as it approached it's bearing, and then the shift fork shaft (into the case). A little further in and I was able to just guide the shift drum into it's hole, and then continue to tap on the end of the shafts until I felt they were all home. Gears spun ok at this point. I clipped the rubber bands off and removed them.

So I am thinking I can go on at this point. I need to set the bearings into the right case, and then I will probably try a dry assembly of left and right cases, maybe with the neutral stop in place, and try spinning the transmission, maybe see if I can actually shift a gear, etc. Crankshaft has not come back, so I will move on, probably to the piston issue.

Pics:

The first pic is of the transmission right after I removed it from the case.
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Last edit: 17 Jan 2021 12:17 by nhsteve.
17 Jan 2021 12:14 #28

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Replied by MarkT on topic 1968 DT1 rebuild/rejuvenation

Congratulations!!! :likey
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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17 Jan 2021 12:33 #29

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Replied by nhsteve on topic 1968 DT1 rebuild/rejuvenation

Thanks Mark.

My mistakes are the only things I truly own!:ROFLOL
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17 Jan 2021 12:38 #30

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