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Hi there my YZF R125 2008 keeps spluttering at around 8K Revs whilst riding, it’s had a full top end rebuild changed to a 125cc from 150cc. I have no clue what the issue is, I’ve tweaked about with my air mixture screw on the throttle body and nothing has come of it.
 

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2008 YZF-R125 (180cc)
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How are you adjusting the fuellign beyond the air mixture screw? Afaik teh air mixture screww only applies at idle so wouldn't realyl make any difference to the fuelling while riding.

If you are not adjusting the fuelling (e.g. no power commander, tuneboss etc ecu) then that might be your issue. You have put a bigger cylinder on but the ecu doesn't know that.

1) Super cheap option, put a slightly bigger injector on (e.g. 120cc/min, +20%) or find the R15 injector, they are on ebay now and then. basically this jsut makes a bigger hole to spray petrol through. No adjustment to fuelling etc. Your injector pulse widths (how long the injector stays open for) doesn't change but the hole is bigger so you flow more fuel.. IT's acrude solution and it'll probably will run better in some ranges and worse in others as the map isn't actually right for the cylinder anymore. Whether that's an improvemenbt only youc an decide, costs about £30, injector of aliexpress (can find a link if you want to try that)

2) add somethign to adjust the fuelling - e.g. power commander FC, Power commander 3usb, or the expensive but very fancy PCV. Then get it mpped on a dyno.

3) add somethign to adjust the fuelling and also take AFR readings.. .e.g.: PCV + autotune module. Very expensive intiially but automaticvally adjusts the fuel maap according to the engine's need. No dyno tune needed, very expensive but very cool. This is what I have on mine with the 180 kit and it rocks.

I've just recently published an interview I did with one of the higher-ups at Dynojet (who make the power commanders) about exactly thias subject. You might find it relevant.:

Also some fiurther explanation along with showing how I build a fuel map from scratch which also explains how these piggyback controllers work in a bit more detail, with diagrams etc:

hope that helps :)
 
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