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Picture Attachments Fixed? (18 Feb 2023)

Hopefully, the pictures uploads are fixed for the time being. Let me know if you continue to have issues. I am looking into a more permanent solution. Thanks for your patience. - Makotosun

Tear down & surprises on my '78 GT80

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Tear down & surprises on my '78 GT80 was created by worldmchn

I've been the new owner of a '78 gt80 for about a week and a half now after I got it out of a local scrapyard. Was a bit rough looking, but I think it'll come out looking and running good after a fresh coat of paint on the frame and an engine rebuild. Started tear down last night and finally made it down to the last section of the frame tonight. Found more than a couple surprises along the way, the least of which was the multitude of stripped screws throughout.
  


I knew there was a few missing parts (kick start and oil tank being the 2 I knew about when I bought it), but I was surprised when I took off the left engine cover and the whole magneto was gone, backing plate and all. Actually looks like all the original wiring is also gone and what little is there is the remnants of this 'universal wiring kit' with a cdi off amazon (www.amazon.com/Harness-Complete-Electric...iring/dp/B08CDM8TGW/).

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Next was the engine, took the carb off and cleaned/rebuilt it along with the fuel petcock, then took the cylinder jug off to take a look at the piston and rings since it didn't feel like it had a whole lot of compression and the reed valve looked fine, so checked it out and the piston and rings are broken (got most of the ring pieces out, looked like they stayed in the ring grooves for the most part, but I'll probably still take the crankcase apart just to make sure)


So now I'm looking at 1. Seeing what parts I can use from the 'universal kit' and piecing together a wiring harness from it. and 2. Getting a rebuild kit for the top end, but I need to figure out a magneto to put in there before I can start putting the wiring back together. I've done some looking around and it looks like a complete gt80 mag with the backing plate, points and rotor are pretty hard to come by, so I was wondering if anyone had an extra set they'd be willing to sell or knew of either another models mag setup or a good aftermarket one that'd work in it's place. Also any recommendations for a good top end kit or at least a piston and rings would be much appreciated.

Cheers!
-Nick
Last edit: 22 Feb 2023 21:15 by worldmchn.
22 Feb 2023 21:13 #1

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Replied by msavitt on topic Tear down & surprises on my '78 GT80

Congratulations on the cool little bike, I would love to get a street legal one!

I have had success buying used virtually perfect wiring harnesses on eBay for reasonable prices vs trying to patch up a big mess…I think I paid $40 for one for my dt400.  Same for magnetos but a bit more cash.  Anyhow just some ideas. Good luck with bike!
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22 Feb 2023 21:43 #2

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Replied by Schu on topic Tear down & surprises on my '78 GT80


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Replied by Schu on topic Tear down & surprises on my '78 GT80

When you open that item on eBay, click on the tab to visit the sellers store. Then enter GT80 in his search.  Your lucky day. He has obviously parted out a few GT80s because he has a lot of parts and his prices look fair to me.
Schu

CT1B, CT1C, JT1, JT2, DT360A, GT80B, DT100B, DT125B,
DT175B, DT175C, DT250B, DT400B, Z50, SCR950

Someday, you'll own some Yamahas
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Last edit: 23 Feb 2023 09:31 by Schu.
23 Feb 2023 09:30 #4

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Replied by worldmchn on topic Tear down & surprises on my '78 GT80

Thanks! I'm thinking on making it street legal since all I'd need is to add turn signals, a headlight, tailight, and a horn in my state, and If I'm going to get a wiring harness, might as well get one for a street legal 80. I don't think I'd be able to add a speedometer to it though since the hole that the cable drops down in looks to be plugged off.
23 Feb 2023 10:00 #5

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Replied by Schu on topic Tear down & surprises on my '78 GT80

Good luck in your search for parts. I have been successful at finding all those street legal parts for some of my bikes. But I have yet to be successful at making them all work properly. 

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Also be aware that the little GT80 only runs comfortably at about 37 MPH. Its only a 4 speed and could really use a 5th gear. It's really too small to mingle in traffic. 
Schu

CT1B, CT1C, JT1, JT2, DT360A, GT80B, DT100B, DT125B,
DT175B, DT175C, DT250B, DT400B, Z50, SCR950

Someday, you'll own some Yamahas
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23 Feb 2023 15:17 #6

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Replied by worldmchn on topic Tear down & surprises on my '78 GT80

Thanks for the parts recommendation, they're on their way. It's going to be trial and error for sure on seeing what parts I can get to work for me. And yeah I've heard the little 72cc engine doesn't like to do above 40 on a good day without screaming, so the only street use I'm thinking on taking it out on is some of the more scenic backroads that head up to a couple state parks where folks average no more than 30-35mph. I've made the trip on my 4-wheeler before, but It'd be nice to not have to worry on getting pulled over.
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23 Feb 2023 15:47 #7

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Replied by worldmchn on topic Tear down & surprises on my '78 GT80

Guess it's time for an update after a while! From where I left off in my last post I've since finished stripping the paint off the frame and repainting it with some rust barrier + low gloss black paint. Also stripped and repainted the fenders and oil tank in the closest silver I could find to the Yamaha original, as well as the muffler in a high temp matte black.
  


After a few weeks of waiting, my points, rotor, and piston kit finally came in, so it was time to put the engine back together! Added a splash of the same hi-temp matte paint to the jug and head as well as clean + set the points to .012".
 
 

So shes mostly put back together, but now I still have a lingering problem. No spark. Or rather my ignition coil (I've tried 3 different ones) isn't sparking. When I've hooked my test-light up to the input wire coming from the mag, I get power, but nothing when on the output side.
Last edit: 31 Mar 2023 15:46 by worldmchn.
31 Mar 2023 15:45 #8

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Replied by worldmchn on topic Tear down & surprises on my '78 GT80

Few other pictures I wanted to add. Re-soldered the original kill switch, but don't have it hooked up yet. Still confuddled on what the issue is with my ignition coils. Is there a specific one needed to work with a points ignition? So far I've used one off of ebay and an emgo 24-71532 since I heard that one worked from a couple of old timers to no avail.
 
 
 
Last edit: 31 Mar 2023 18:24 by worldmchn.
31 Mar 2023 18:24 #9

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Replied by MarkT on topic Tear down & surprises on my '78 GT80

You've posted nearly meaningless test results... could be original and both coils are fine and it's something else?

If you have an ohmmeter, what reading do you get between "input wire from mag" and and a good ground with:

1. Points open

2. Points closed

(no ignition coil connected during test)

My guess is that might be where the problem lies.
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
31 Mar 2023 18:53 #10

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