09 yz450f Leak jet, Engine breathes hard?

I am reading my service manual for my 09 yz450f and I have noticed that my bike runs quite good ( the tiniest bit of decel pop, very strong mid range and top end) except when I open the throttle quickly. It boggs badly and sometimes puffs out flame. Anyways My manual is telling me that this is caused by the leak jet not being correct. I am running it stock #55 with it adjusted to the manuals specs. My point is the manual must have been translated from another language and I do not understand it. It says -If the engine breathes hard in quick throttle openings select a leak jet having a lower calibrating number, then it says -If rough engine operation is felt in quick throttle openings select a leak jet having a higher calibrating number. Can any one tell me what the hell the difference is between these two definitions, my bike just boggs down I dont know if that means breaths hard or if that means rough engine operation. Does anyone know what the hell Yamaha means?


Stock jetting on 08-09s was good from sea level to a couple thousand feet anyway. If you slam the throttle open too quick from too low an rpm they will do that.

the books are translated from Japanese and many times are somewhat colorful in english. the leak jet is a strange deal its a fuel jet that when you put a smaller one in you get more fuel. Its sort of a bypass circuit in the AP system, So if you want more response from your AP get a smaller leak Jet. Before 05 or so the carbs did not have leak jets so you could plug it to try if you want. you might try lowering the clip of your needle 1 step first that will have some affect on the issue.

the books are translated from Japanese and many times are somewhat colorful in english.

That's one way of putting it.

Stumbling on sudden opening of the throttle is normal. Sort of. That is to say that it shouldn't do it under normal riding conditions, but even the best tuned YZF can be forced to do that if you idle it down far enough and slap the throttle open both far and fast enough. What's happening is that the suddenly opened throttle causes an abrupt drop in the intake vacuum while at the same time creating a condition where the carb has to provide a good deal more gas to compensate for all the air suddenly rushing into the engine. The air demand is instant, but the flow has to exist for a long enough time to start raising fuel up through the main jet and into the air stream. A smoother, more gradual rolling action on the throttle usually corrects it entirely, and as I said, unless it does it while you're actually riding so that it's a real problem, you shouldn't worry too much about it.

However, if it is more prone to stumbling than normal, then the info in the previous post is pretty accurate. The primary purpose of the leak jet is to PREVENT the accelerator pump from injecting fuel into the intake during small changes in throttle opening such as would occur when controlling traction leaving a corner or picking through some tight stuff. The leak jet is a calibrated leak in the AP circuit. If the throttle is opened slowly enough or only a short ways, fuel simply leaks out of the jet and never makes it all the way to the intake stream. Opened faster, the discharge from the pump overwhelms the leak and shoots down the carb throat. This allows the builder to have an accelerator pump that's strong enough to actually cover the engine when necessary, but keep it from loading up the engine under less demanding conditions. More info: http://www.thumperta...330#post4415330

Geofitt is correct about what the book says. A smaller leak jet will result in a stronger pump shot, and CAN be used to tune the AP, although that's not the primary design method of doing so. Raising the needle won't help. CHANGING the needle to one with the same length profile but a smaller diameter at the top does, however.

A pilot jet that is way too small will contribute to the problem, but one that's too big will cause excessive stalling when closing the throttle quickly at low speeds, and generally make the bike run ratty. The pilot should be tuned to make the bike idle correctly before anything else is done to it, and then richened only a little if absolutely needed. Read: http://www.thumpertalk.com/forum/showthread.php?t=327405

Take a look at what this guy did with his:


Wow you guys gave amazing responses, thanks a lot for that. This clears things up quite well for me. I guess four strokes simply are not meant to be opened up hard at idle unnecessarily. I have a yz250 and I slam it open all the time and ive never heard it bog once, totally different bikes though. Anyways I think their is some room for fine tuning of my bike I guess I am just going to take what ive learned and tune it when riding weather comes along. Its too cold right now anyways. Thanks again

The cold is another reason it's bogging. Lower temperatures mean denser air, which requires richer jetting.

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