Yamaha hs1 90cc twin clean up

Hurco550

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I debated on making a thread, but I figured I might as well share the experience as I haven't seen any of these bikes on here. Bought this little Yamaha hs1 off of the second owner, who bought it used in 1972. Him and his daughter (12 at the time) went in halves and bought it for $200. He welded up a frame for the front of his camper and it was their ride for around the camp sites. The little booger even made it from Ohio out to the west coast once. (On the front of the truck, not under its own power. That would be a heck of a ride)

Anyhow, it ended up getting stuck out in the barn for years, and when I rode out to the guys grandsons place on the old Suzuki smoker, he rolled in and said "I have an old two stroke bike that i think you'd like". We made a deal and I brought it home yesterday.

To me, it's a very solid specimen for its age, just in need of a deep clean. It will need crank seals, brake pads ect, but it's a worthy bike in my book. The plan (as I told my wife) is to clean it up and try to make a few nickels off of it at mid Ohio, but I am kinda growing fond of it already.

The paint is probably an 8.5 out of 10 on condition, so I'll buff it out and leave it. It's only original once. Only thing not original is the po had the seat recovered. Kinda a bummer to. He said the original seat cover was perfect and he just wanted new foam put in. When he went to pick it up, the guy had recovered the whole thing... grrrr.

Anyways here it is as I looked at it.





And home with the first layer of crud washed off.



Dash area cleaned up and polished the rust off of the bars.





Still waiting on the bowl gaskets to come in, but I removed the years of varnish out of the carbs.





It's nice when the heads are small enough to fit in the little harbor freight ultrasonic cleaner.





I've never seen pistons this small on a bike either. Looks like they would be more at home in a weed eater out chain saw lol.



More updates to come.
 

grcamna5

New Member
That's a nice vintage Yamaha hurco even though the seat isn't orig. anymore.. I still think someone over at Mid-Ohio would fall in love with the little bike;the paint is Great ! 8)
 

Hurco550

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deviant said:
Ever find out about that carb mod?
Nope. I'll probably run into him at the local father's day car show. I'll ask him then. In the mean time, I'll just grab the ones from feebay
 

Hurco550

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Deviant was talking about this.


The p.o. cut the carb slides down from top side, and we can't figure out why.
 

Hurco550

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It's amazing what a little (er alot) of elbow grease uncovers.

Barrels off for a good cleaning and to check piston clearance.


Should have taken a before picture, but this is after about 1/8" of dirt was uncovered. Cleaning up nicely


 

adventurco

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Interesting on the carbs 'mod' never seen that before. Let us know if you find out the reasoning behind that from PO. cleaning up nice. Those pistons are adorable.


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grcamna5

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adventurco said:
Interesting on the carbs 'mod' never seen that before. Let us know if you find out the reasoning behind that from PO. cleaning up nice. Those pistons are adorable.


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From what I could imagine it would allow more mixture(rich)into the engine w/ the same amount of air at full throttle..
 

irk miller

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grcamna5 said:
From what I could imagine it would allow more mixture(rich)into the engine w/ the same amount of air at full throttle..
You do that on the other side of the slide. You can file away the bottom of the slide to take some of the angle away or flatten the arc to richen them up. What's confusing is it's the top of the slide.
 

Hurco550

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So far Noone can explain the carb mod lol it will be interesting to see what the p.o. has to say.
 

irk miller

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hurco550 said:
So far Noone can explain the carb mod lol it will be interesting to see what the p.o. has to say.
I've been googling the hell out this since you brought it up. It's taken me to some sled forums. They talk a lot about cutting the springs and drilling the slide to quicken throttle response. I wonder if he was going for something like that?
 

Hurco550

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deviant said:
I've been googling the hell out this since you brought it up. It's taken me to some sled forums. They talk a lot about cutting the springs and drilling the slide to quicken throttle response. I wonder if he was going for something like that?
May have been what they were going for, but I still think it may be an uneducated attempt to get the slides to open farther thus "increasing power". Once the new ones come in, I'll see if at full open they clear the through hole in the carb. Also, I have a service manual on the way. Sometimes they sent some weird service bulletins to the dealers, and it could have been a service mod taken to far lol
 

Hurco550

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Shoeman said:
What size are those carbs? I think I have a set of 26's of that type but that may be too big.
Thanks, but these are tiny. Vm16. I appreciate the offer though!
 

Shoeman

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If you're ever contemplating hot-rodding it a bit, I have the official Yamaha GYT tuning spec pages for the LS2, which shared the same basic layout and stroke, just up to a 38mm bore for 97cc displacement. It gives factory porting specs, head info, and part numbers for the race bits, all long superseded and discontinued but still cool to see.
I have a photocopy of the entire 1972 Yamaha Racing Tips publication. Very cool stuff to nerds like me.

Geoff
 

Hurco550

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Shoeman said:
If you're ever contemplating hot-rodding it a bit, I have the official Yamaha GYT tuning spec pages for the LS2, which shared the same basic layout and stroke, just up to a 38mm bore for 97cc displacement. It gives factory porting specs, head info, and part numbers for the race bits, all long superseded and discontinued but still cool to see.
I have a photocopy of the entire 1972 Yamaha Racing Tips publication. Very cool stuff to nerds like me.

Geoff
Wow, that's awesome! I enjoy reading through that kinda stuff to, but while my hands are on it, the plan is bone stock on this one. I am probably gonna sell it to fund another (bigger) Yamaha twin smoker.
 

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