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Old 12-03-2012, 04:52 AM   #1
TheRadBaron OP
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'66 Yamaha Twin Jet 100. 60mph=2240 RPM!?

I just brought home a barn fresh '66 Yamaha YL1, aka Twin Jet 100. It's a 100cc, 2-stroke twin cylinder. I can't find my camera at the moment so I can't post a picture. Anyway, the speedometer has a little legend that says "60mph=2240rpm".
How can this be? This seems to be impossibly tall gearing for a little, tiny, 2-stroker. For example, my '72 Yamaha R5 (350cc 2-stroke twin) is turning about 5,000rpm at 60. My CB350 is turning 6,000. Hell, my 125hp FJ1100 is turning about 4,000 at 60. I can't imagine how this little bike has enough juice to move the bike that fast at that low RPM. If you were to ask me how fast I would guess a 100cc 2-stroke would be spinning at 60mph, I'd say about 8,000. Not 2,240.
Not that it would do it, of course, but that would put the theoretical, geared, top speed of the YL1 somewhere in the neighborhood of 240mph.
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Old 12-03-2012, 11:38 AM   #2
Tosh Togo
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Post picture of speedo.






And the rest that little puppy too.
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Old 12-03-2012, 06:59 PM   #3
dpforth
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Quote:
Originally Posted by TheRadBaron View Post
Anyway, the speedometer has a little legend that says "60mph=2240rpm".
How can this be?
Rotational speed of the speedo cable?
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Old 12-05-2012, 09:18 AM   #4
YamaGeek
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yes, this is for calibration of the speedo and has nothing to do the the engine RPM in top gear at 60 mph..

I'm not sure a Twin Jet 100 can make much over 60 mph as it is.
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Old 12-05-2012, 09:38 AM   #5
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Alright. That makes sense. I've just never seen anything like that on a speedometer before, and I've looked at a lot of them. I was having a really hard time believing that it was the engine RPM. Thanks.
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Old 12-06-2012, 09:58 PM   #6
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My single cyl. version won't (wouldn't) hit 50, and it was SCREAMING to even do that. Hoping my pipe and a few other changes will fix that, if I ever get it done. Love to see a pic, the twinjets sure are good looking bikes!
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Old 12-08-2012, 02:00 AM   #7
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Speedo drive wheel rpm (usually front wheel on these 60s Jappas). Even Honda 90 speedos of the time had the same.

Any reasonable Twinjet back in its day would just hit 70mph.
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Old 12-09-2012, 02:29 PM   #8
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Cry

Just for the heck of it here's a YL1 that I would like to buy from a friend but it's been promised to someone else Maybe someday I can sweet-talk it away from her. Looks like it would e a fun project and I have a minor life goal of someday owning a 2-stroke street bike.

If you look closely you can see the text the OP is refering to.









And here's some ad pics I've found aournd the interwebs..





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Old 12-10-2012, 08:47 AM   #9
YamaGeek
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I'm sure with a GYT kit, and some love and a light rider the TwinJet could just make 70, but my experience with one was it was hard pressed to hold 60 in stock trim. Sure sounded like it was going fast though.
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Old 12-10-2012, 06:56 PM   #10
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Re: The Twin Jet ad...

First and fastest what exactly?
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Old 12-19-2012, 05:07 AM   #11
McJamie
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Quote:
Originally Posted by TheRadBaron View Post
I just brought home a barn fresh '66 Yamaha YL1, aka Twin Jet 100. It's a 100cc, 2-stroke twin cylinder. I can't find my camera at the moment so I can't post a picture. Anyway, the speedometer has a little legend that says "60mph=2240rpm".
How can this be? This seems to be impossibly tall gearing for a little, tiny, 2-stroker. For example, my '72 Yamaha R5 (350cc 2-stroke twin) is turning about 5,000rpm at 60. My CB350 is turning 6,000. Hell, my 125hp FJ1100 is turning about 4,000 at 60. I can't imagine how this little bike has enough juice to move the bike that fast at that low RPM. If you were to ask me how fast I would guess a 100cc 2-stroke would be spinning at 60mph, I'd say about 8,000. Not 2,240.
Not that it would do it, of course, but that would put the theoretical, geared, top speed of the YL1 somewhere in the neighborhood of 240mph.
I think it's missing the "1" in front of the 2240.
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Old 12-21-2012, 01:43 PM   #12
Neil E.
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Quote:
Originally Posted by dpforth View Post
First and fastest what exactly?
First and fastest 100cc twin.

One of the guys had this exact bike when I was in high school in the sixties. He was well over 6ft tall and basically sat at the rear of the seat. He flogged the daylights out of this bike. Always doing wheelies on the 1-2 shift with the engine screaming. Went pretty good for a 100.
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Old 12-21-2012, 03:16 PM   #13
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Nice find!

Awesome find. A couple years ago I too had the urge for a 2 smoke street bike. Looked long and hard and ended up with a sweet Suzuki GT550 triple. Listening to that engine idle has to be one of the top 5 sounds of all time!..
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Old 01-29-2013, 03:43 AM   #14
Ian Robinson
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In Brisbane, Australia. The 1967, 4 speed, YL1 100 twin sold for $386 + reg. ( $13.60 first yr) the 5 speed of '68 sold for $410 + reg.

Fantastic little bikes for their time. I tried to race this one ( motor) on the dirt in '68, without success.

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Old 03-09-2013, 06:22 PM   #15
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Awesome find. A couple years ago I too had the urge for a 2 smoke street bike. Looked long and hard and ended up with a sweet Suzuki GT550 triple. Listening to that engine idle has to be one of the top 5 sounds of all time!..
When the baffles plugged up on my 76 GT550 I wrapped the baffles in steel wool because it was cheap and easy to replace. I was getting ready to leave work one day and one of my least favourite customers came in; a real know-it-all jerk and he commented on my bikes 'tough idle'. I told him it had a hot cam and he didn't know the difference. LOL
I put 21000 miles on that bike and once I figured out to put unleaded in it instead of leaded regular and used GM points grease to make the points last the bike was completely trouble free. It is the one bike I wish I could have kept.
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