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lean jetting on YZF250
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Posted by: Boozer---------------------
hello to all. how dangerous is it to run my bike on the lean side? i have a 250F and compared to my friends bike, it is jetted quite lean. the throttle response is fantastic and plug tests suggest it is jetted quite good, but i'm getting a bit worried.
i know a bit about detonation and sometimes it cannot be heard (noisy bike), but wouldn't there be a substancial power loss if this was occuring? the bike runs standard pilot and standard main, but the needle is 4 (think so) clips leaner than standard (next needle up- middle clip), with 96 premium unleaded. the exhaust tip has little carbon on it, and the carbon occuring is more than likely caused by the acc. pump. with it disconnected, i'm thinking there would be virtually no carbon on the tip at all. am i being paranoid?
Posted by: Rich Rohrich---------------------
The bike will keep going faster till it's too lean, so you are going in the right direction. Running lean can lead to detonation but doesn't always. It's usually going to show up if you are lean in an area of the fuel curve where you make large transitions at high load and lower rpm (lugging the motor and opening the throttle wide). Detonation can usually be seen on the insulator nose when running unleaded fuels. It will show up as speckles (black or brown) that look sort of like dirt on the nose of the plug. It's just tiny chunks of carbon being broken off of the piston crown and chamber during the sharp pressure rise from detonation. This is usually fairly safe but it's a warning sign that the next step is a dangerous and potentially expensive one. If you are seeing little blobs of aluminum on the insulator nose then it's detonating hard and will eventually hurt the motor as it strips aluminum off the piston crown. It can be fairly difficult to tell the difference between lead and aluminum deposits when running leaded race fuels.
The upside is the 250F like it's big brother is VERY fuel tolerant and can be run really lean without hurting it. I've found with the bigger engines that the engine will bog, and the performance will drop off from lean jetting well before it ever detonates even when running 14:1 compression ratio.
Jet it down till it slows down 
Posted by: Boozer---------------------
that was excellent. no more questions here.
Posted by: woodsguy2000---------------------
Thanks for the lesson. That was very interesting and informative.
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