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Yamaha RS100 (1Y8) putting out 25V instead of 6V

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Hello Lads,

Im havinga a bit of an issue with my RS100 that i got last year.

I restored it to "nearly" perfectly working order (left the patina on it:eusa_shifty:) last year and drove 130 km without stopping and had no problems whatsoever. Last year i had to throw in a new Battery (old one was missing) and gave it a new ignition coil. It needed a new ignition coil because when the bike came up to operating temperature it was loosing power. Replacing the coil fixed the problem completely... until now.

About a week ago i fired the Old Girl up and drove for about 3 km which it did fine. After the 3 km it started to loose power and the revs wouldn't go above 4000RPM. In fact it even struggled to get up to that! 

So I drove back home and decided to look after the Problem. Measuring the Voltage Output to the battery was shocking, 25V for a 6V Battery! The Battery was cooked inside and nearly empty of fluid. I fought that the rectifier had to be broken. Its not converting to 6V DC anymore. But replacing it did nothing.

I read my RS100 manual about a million times last week and i havent been able to find a soultion for my problem.


Maybe one of you has experienced a similar problem with a 2 stroke Yamaha bike of that era?


Thoughts would be appreciated


Schöne Grüße!

Aaron from Germany


Sorry if my English is a bit worse for wear:eusa_doh:


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  • Moderator

You need to change the voltage regulator not the rectifier. 


Dont spend a bloody fortune on one either, its the same regulator on a vast list of yams, something like this:- UNIVERSAL 6 Volt Voltage Regulator Fits All 1968 - 1983 Yamaha Enduro Models | eBay Will do fine.


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Okay quick update:

Yesterday i refilled the battery and charged it up again. Still holds about 6,5V so seems quite allright.

Fired the bike up an measured the voltage while the battery was still connected to the bike (the 25V were coming out because the Volt-Meter had no ground to the bike because of the removed battery, i know rookie mistake:eusa_doh:) and the measured voltage was 8,5V as the manual suggested.


Tomorrow im going for a ride and will update if something has changed. Could the damaged battery be the problem for the rough running after a while?


Anyway thank you guys for your suggestions and stay tuned!!!

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Nope, most yam 2 strokes generate their own ignition power, you could chuck the battery in the bin and it would run fine.

May well blow every bulb on the way back mind.

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The test drive wont be happening this weekend because of the terrible weather in germany right now.


Spark plug color is a strong blue one. Cant be the problem i guess.

On thing that came to my mind: It could also be the ignition switch. There is a small spot where the bike is running terrible if you put the key in that position.


Im gonna report back if the weather gets better and i made a test drive.

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