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I've got a 1987 YSR50. Stock bore, tcr exhaust, mikuni/sudco carb with k&n air filter. After cleaning the carb, it starts, idles and runs beautifully, but at high rpms, it cuts out and stutters almost like a rev limiter sometimes. Does it at a standstill and when riding under a load. Can accelerate pretty well but it really seems like when I give it over 85% throttle or so it just wants to break up. If you baby it and get to higher rpms, it cuts out until you reduce the throttle then it picks up again. Tried adjusting the air fuel screw with little to no difference, and the needle being in the middle clip seems to be best, one clip up(more fuel) and it breaks up at a much lower rpm/throttle, and one higher clip(less fuel) and it seems to be worse then original position in the middle. Any ideas? Running a b9hs. Also running premis and oil injection just to be safe(bike is new to me,didn't trust the injection) Thanks in advance!!
 

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You might have to play with the jetting..

One of my bikes does this too but it took me so long playing with Jets just to get it running right that I don't want to mess with it anymore..

I thought it would be the CDI box but I changed it and I stuck it the same issue ..
 

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I've got a 1987 YSR50. Stock bore, tcr exhaust, mikuni/sudco carb with k&n air filter. After cleaning the carb, it starts, idles and runs beautifully, but at high rpms, it cuts out and stutters almost like a rev limiter sometimes. Does it at a standstill and when riding under a load. Can accelerate pretty well but it really seems like when I give it over 85% throttle or so it just wants to break up. If you baby it and get to higher rpms, it cuts out until you reduce the throttle then it picks up again. Tried adjusting the air fuel screw with little to no difference, and the needle being in the middle clip seems to be best, one clip up(more fuel) and it breaks up at a much lower rpm/throttle, and one higher clip(less fuel) and it seems to be worse then original position in the middle. Any ideas? Running a b9hs. Also running premis and oil injection just to be safe(bike is new to me,didn't trust the injection) Thanks in advance!!
Hello, I'm having the same issue with my 1987 Yamaha YSR50. My bike is completely stock with the exception of a new DG exhaust that I purchased and the calamari air box modification. I tried all different size jets (dynojet) from small to large, adjusted the needle position and adjusted the air mixture screw with no such luck.

The bike used to run nice and smooth, but not so much now. When it runs, it does have more power but l'm having the identical issue that Mitchell is having.

If you corrected your issue, can you tell me what you did?

Any help would me much appreciated.

Thank you
 

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1980 Honda Z50r
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I’m also having a similar issue.....I have a DAYTONA tachometer on mine. My YSR seems to do this whenI do a hard run up to top-speed ( around 50 mph..not happy with 50mph at all ) . It seems like I’m losing ignition or starving for fuel. But once slow for a bit it goes back to running ok. I happened to notice that when mine starts doing the “cutting out” , my needle starts drifting towards zero RPM as if its not firing. Maybe this is a clue.

YSR, GT80 motor, Calamari pipe, 20mm Mikuni round-slide.
 

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1980 Honda Z50r
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OK....so, I had a spare YSR50 motor the guy gave me with the bike. I removed the flywheel and swapped out the whole stator. PROBLEM FIXED. Apparently, even though the source coil OHM checked good on the bench it fails when running.

REPEAT.....BAD SOURCE COIL CAUSES THIS PROBLEM
 
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