Yamaha GT80

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RageBall

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Not sure if I got the right area or not, apologies if I haven't.

Wife finally convinced her parents to give us her old bike from when she was a kid.

It's an old 1974 Yamaha GT80. Not running. Know there's a piston issue and one of the bolts holding the carby (I think{it's the part over the piston}) has been snapped off inside the casing. Possible other engine issues, long story short am just going to rebuild the motor. Avoid any other issues.

Have stripped most other parts and am slowly replacing. Most are easily found from eBay. If anybody knows of a good place I can find some in or around Adelaide then that'd be appreciated.

To the point though, I'm not a mechanic or anything. Small things are fine, I have picked most things up but I've never played with a motor.

How hard are these things to rebuild? Am I better off sending it to a mechanic? Haven't priced it up but someone mentioned it would be well over 1K. Lot of money to splash on a little bike that's only going to be for the kids to ride around the paddock.
 
Also forgot to mention, as it was her childhood toy, wife very much prefers to keep it original so replacing the motor is a last ditch option.
 
Cool little bikes, my mate has one sitting in his back yard, prick wont give it to me though... Been trying for years. As for parts i really don't think you'll find that much in Aus, mainly due to the vintage of the bike. My first bike was an MX80, exactly the same bike without the electrics/lighting. MY67XR on here is the best knowledge of the places to get parts from, i'm sure he'll be along soon :) As for doing the work yourself it really depends on what work needs to be done. Post up a pic of the piston/carby issue, and that explination is a bit hard to guage anything by.
 
I know for a fact piston rings are shot. Sorry for the poor description. Bike has been sitting in a shed for many years. As such there is excessive buildup of dirt and general crap. Basically I think it needs a full rebuild. Top to bottom.

http://i.imgur.com/PXbwMt2.jpg

http://i.imgur.com/ARa0pK5.jpg

Sorry for the poor lighting but you should be able to see the missing bolt. That has been snapped off inside the case. With the right tools it could be pried out but I haven't the tools for that.

EDIT: Forgot to mention there is a chip out of the top of the piston. Can grab a pic if you need.
 

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Bugger looks like a snapped stud to me. If you remove the other 3 nuts you should find the head/barrel will slip off, then you'll have to find a new stud somewhere?? That could be fun? Also how can you tell the piston has a chip out of it as it doesn't look like the heads been removed?
 
I pulled the head off but put back on as I haven't the parts to fix yet. Got no idea where I can find another stud lol. As you said earlier, parts are hard to find. Most of the ones on eBay I found are shipped from USA.
 
Hmm, looks like they will both be useful sites in my search for parts. So what are bike motors like on the inside? Fairly tech minded but no experience with motors. I've had some people tell me that if I label every item and document the stripping of the motor then I should have no problems putting majority of it back together (along with workshop manual)
 
With the internet these days it really is a piece of piss, im a slight I.T nerd and up until a few years ago had nothing to do with motors, but with a bit of google searching and youtoobs and a heap of info from this site :) and now i know my way around one resonably well.
 
yeah, the top of the engine is the head.
as timeeh said it's just a snapped head stud
is there anything of the stud sticking up out of the case ?

a top end rebuild will probably cost you under $200 for everything.
the carby will most likely need a rebuild, and the exhaust will need a burn out to get rid of the oil/carbon.
did you notice any markings on the top of the piston ?


what area are you from ?


don't forget to say g'day in our Introduce Yourself thread too while you are here.

cheers, craig
 
Yes there is a small bit of stud still hanging out. Have tried prying it out with vise grips and a lot of WD-40 but no luck. Unsure of the best way to get it out now.

By $200 for everything you mean purely parts I assume? Know any good SA shops I can find the stuff from or is it best just going online?

Top end rebuild easy enough to do myself?

Carby, same thing. How hard is it?

Exhaust is fairly rusted, was most likely just going to replace.

Piston was fairly clean and had no obvious marks except for a chip on the top and a ring was split in two. Will pop out and take a photo in a minute after lunch.


I'm out at Craigmore. So yes, Craig from Craigmore :p
 
RageBall said:
Yes there is a small bit of stud still hanging out. Have tried prying it out with vise grips and a lot of WD-40 but no luck. Unsure of the best way to get it out now.

By $200 for everything you mean purely parts I assume? Know any good SA shops I can find the stuff from or is it best just going online?


Top end rebuild easy enough to do myself?

Carby, same thing. How hard is it?

Exhaust is fairly rusted, was most likely just going to replace.

Piston was fairly clean and had no obvious marks except for a chip on the top and a ring was split in two. Will pop out and take a photo in a minute after lunch.


I'm out at Craigmore. So yes, Craig from Craigmore :p
Click to expand...
top end is easy to do but its better to know what your looking for.
measure the bore and work out the clearance
use a stud remover and heat to get that stud out mate..
carbies easy to do.. replace the piston for sure but
 
Stud removal should be pretty straight foward, as long as youv'e got some thread somewhere.
[video=youtube;66ATKL4G04M]https://www.youtube.com/watch?v=66ATKL4G04M[/video]
 
Ok guys, popped out and took a few photos. Also another thing, all the case screws are stuck and will strip the head pretty quickly. Any suggestions?

No thread. Stud remover? View attachment DSC_5465.JPG

On the piston, the scratch marks are from me. You can clearly see the chip in it. All the side looks clean, the inside of the piston hole (haha not sure of the name) is all fairly clean, can't see or feel any burrs or obvious defects. Does that mean I won't have to get it machined?View attachment DSC_5458.JPG
 
Hmmm, i would be trying an easy-out with that stud, don't think a stud-remover will do you any good. As for the piston, i would deffinately be putting in a new one + seeing the scuff marks down the side of it, i'm gonna guess the barrell will be in need of a hone if not a bore. Sorry :(
 
Far out mate that motor has seen better days that's for sure.

Really hope there is no play in the conrod. It looks really rusted so I wouldn't be surprised if the bottom end will need rebuilding as well.
 
Conrod? Yeah we intend to rebuild the bottom end as well. Your quite correct in saying it's seen better days. The only thing keeping this bike alive is the wifes nostalgia.

View attachment DSC_5455.JPG

Bit of a side view so you can see some of the crap. This is after a lot of scrubbing away.
 
The original electrics were also disconnected as well and half the wires ripped out. Unsure whether I'm going to restore the electrics or not. Not going to be a road bike, just for paddocks.
 
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