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Old 03-09-2013, 08:16 PM   #1
Williamldarby OP
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1973 RT3 yamaha

Hello,
I currently own a 1973 yamaha RT-3 and i am expiriencing alot of vibration in the seat past 50 mph. so bad that after 5 min over 50 mph and my butt has gone completly numb. Is this normal? Enginge sounds fine and the seat has all of the rubber dampers as far as i know.
Best Regards,
-William
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Old 03-09-2013, 08:55 PM   #2
MightyChosen1
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What kind of tire are you running on the back? I would look at that first. Maybe its out of balance or like a lot of bikes I have owned mounted in the garage and never balanced at all.

I just sold my 73 RT-3. I still have some parts left over including a rare Hooker exhaust and most of the lights.
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Old 03-10-2013, 06:43 AM   #3
ken_the_carp
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Check your engine mounts are tight.
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Old 03-10-2013, 09:14 AM   #4
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As mentioned above.. look for loose, missing engine mount hardware. They shook like a whore in church when they were new anyway...

Omly the seat? Bars? Footpegs?
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Old 03-10-2013, 09:19 AM   #5
Foot dragger
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Its a large-ish old 2 stroke,they vibrate and buzz lots at the best of times.
Possibly a fresh tube of Vagicil can help.
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Old 03-10-2013, 10:37 AM   #6
Williamldarby OP
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Hello
I am currently running a cheng shin but it dose not look like there is any weights on it to balance the rear tire. Bearings are all tight.
Best Regards
-William
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Old 03-10-2013, 04:08 PM   #7
lrutt
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what gearing are you running? stock? or geared down?
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Old 03-10-2013, 06:14 PM   #8
Williamldarby OP
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I am running stock gear I believe it is 16/39 if I remember correctly.
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Old 03-11-2013, 07:10 PM   #9
smokin2stroke
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If the vibration was engine related, you should experience the same vibration at the same RPM in a different gear. Could be a crank out of balance, worn crank bearing, or loose engine mounts.

If it's speed related, it could be the tire youre using, say a knobby on the street with big cleates or an out of balance wheel, providing all the bearings and swingarm busings are tight.

If you drive the bike in 3rd of 4th the same RPM, does it vibrate the same?
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Old 03-12-2013, 11:20 PM   #10
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The is currently not running. The condenser just went out and should be here on Thursday. I will take it for a ride then and test the rpm in different gears. But as far as I remember it did not vibrate untill over 50 mph. I also just noticed the sproket is loose and I hear this is a common thing for this bike. Would that cause any vibration?
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Old 03-13-2013, 07:03 PM   #11
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You'd probably notice the engine a bit more jerky and snatching because of the slop when you let off the gas and hit it again, or it snatches a little on takeoff. I wouldnt rule out any part I found loose inn the drivetrain making for a weird ride.

If it feels more of a shake in the seat than a hum or buzz, likely its the tire out of balance unless it has some wierd cleat pattern that constanly rippling the tire on the asphault. you can try adding 5lbs to the tire and riding it again and see if the vibration changes. That would rule out the tread.

I'd think if the engine had the vibration, you may notice your hands getting tingly after a long ride at speed or parts on the engine vibrating loose, say stator cover or oil cover screws.

Have you tried jacking up the back of the bike, blocking it up,and running the bike in gear to watch the wheel and chain? If you can get someone on the bike and control it well, they should be able to turn the wheel fast enough to see if it starts hopping, rims bent, or tire is worn funny.
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Old 03-13-2013, 10:52 PM   #12
Williamldarby OP
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Well i think i may have found my problem. not 100 percent but i pulled the front sprocket cover off tonight and there sat a 13 tooth sprocket! stock is a 15 tooth sprocket. i have checked the runout in the rear tire and it is only 25 thousands. i still need to balance the rear tire but i think the gearing is my main cause of the vibration because of the increased rpm. Let me know if that sounds correct.
Best Regards
-William
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Old 03-14-2013, 06:34 PM   #13
smokin2stroke
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The 15 would definitely bring the revs way down. I'd give it a shot. I'm sure the engine would thank you. I'm surprised you got it that fast with a 13. It would have to be wound tight.
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Old 03-16-2013, 07:03 PM   #14
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Actually I was ridding up to 65 but it was revving pretty high that's why I checked the sprocket. But I am having a small problem around 3500 to 4500 rpm the engine is really jumpy like the felling you get when you ride a modern 125 and you are right on the edge of the powerband.
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