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0bigsilver6

02 yz426 carb tuning issues

15 posts in this topic

Alright, I'm having trouble getting my carb set just right between the pilot screw and the idle knob. I followed the guide for setting the pilot screw, and then adjusted the idle down to where it wouldn't hang, problem is when it's down that low, when I try to blip the throttle from idle it will kill the bike. If I go really slow with the throttle most of the time it will be ok though. I've tried numerous times messing with the pilot screw and idle knob, but I can't seem to get it right. Anything else I might need to try? Or should I just leave the idle a little higher and deal with the hanging?

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Just got back from taking it out for a spin after trying to set the pilot screw again, and thinking I got it right. Now there's a hesitation when hitting the throttle sometimes. Not sure where to go.

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Can't remember on a model that age, but if there's an accelerator pump adjuster, I would try different settings to see what helps and what hurts. Just be sure to record your changes so that you can always go back to where you started.

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Would rebuilding the top end with a lighter wiseco piston mean the engine will run leaner? I'm thinking I need to go up a size in pilot jet as I can't seem to get rid of the popping on Decel. I don't remember this being an issue before hand though.

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Popping on decel is not a problem you have to get rid of.  For one thing, it's caused by leaking exhaust systems about as often as by lean pilot jets, and for the other, it's just kind of the nature of the YZF.  If it pops "some" some of the time, that's an indication it's set about right.

 

The Wiseco piston should not be significantly lighter than the stock piston (which is a very high quality forged piston, BTW).  If the weight were to change a great deal, the engine would be out of balance and vibrate more. 

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Popping on decel is not a problem you have to get rid of. For one thing, it's caused by leaking exhaust systems about as often as by lean pilot jets, and for the other, it's just kind of the nature of the YZF. If it pops "some" some of the time, that's an indication it's set about right.

The Wiseco piston should not be significantly lighter than the stock piston (which is a very high quality forged piston, BTW). If the weight were to change a great deal, the engine would be out of balance and vibrate more.

Thank you for the info. It's not only the popping I'm trying to get rid of, but the hesitation when hitting the throttle. I order a size up on pilot jet and new jets in my current size. So I will play with it and see if I can get it where I like it.

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Check the manual for the accelerator pump timing procedure.  A lot of times the bog is caused by a mistimed squirt.  Also, the '05 should have a leak jet, so you can increase the strength of the pump shot by going smaller on the leak, or reduce it by going bigger.

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Check the manual for the accelerator pump timing procedure. A lot of times the bog is caused by a mistimed squirt. Also, the '05 should have a leak jet, so you can increase the strength of the pump shot by going smaller on the leak, or reduce it by going bigger.

It's not really a bog. More of a, when rolling and you hit the throttle it will hesitate and then hit power, almost like when a two stroke hits the power band, but it happens in less than a second probably. I just did the accelerator pump upgrade with the shorter pin and the new spring. Will double check the timing of it as well, but I'm fairly certain it was fine when I checked it last. There is not a leak jet on the '02.

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There is not a leak jet on the '02.

 

Nope, sure isn't.  Too many '05's running through here right now, not enough coffee :smirk:

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Nope, sure isn't. Too many '05's running through here right now, not enough coffee :smirk:

Lol no problem! I will put in my upsized pilot and mess with the pilot screw again and see if I can get it any better, will report back with results! Thanks again gray!

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When you go up on the pilot, watch out for stalling when the throttle is closed quickly while under a moderate load at lower RPM. 

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When you go up on the pilot, watch out for stalling when the throttle is closed quickly while under a moderate load at lower RPM.

Why would it stall because of a bigger pilot??

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Excessively rich idle mixtures cause the opposite effect that lean idling does.  Whereas lean mixtures cause a "hanging idle", wherein the engine returns to idle more slowly than it should, a rich idle will causes the engine to drop back too quickly.  When you couple that with the fact that on a suddenly closed throttle, the engine gets a bunch of gas dumped in it that it can't burn because the throttle cut off the air supply, and the engine will often drop to a speed well below idle speed and simply quit, particularly if the clutch is pulled. 

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Excessively rich idle mixtures cause the opposite effect that lean idling does. Whereas lean mixtures cause a "hanging idle", wherein the engine returns to idle more slowly than it should, a rich idle will causes the engine to drop back too quickly. When you couple that with the fact that on a suddenly closed throttle, the engine gets a bunch of gas dumped in it that it can't burn because the throttle cut off the air supply, and the engine will often drop to a speed well below idle speed and simply quit, particularly if the clutch is pulled.

Huh. Never thought of that. I'm going to be trying a 45 pilot jet from a 42. Do you think that's going to cause issues like that? If I remember correctly, according to motomans jetting chart I should be at a 42. BUT, it just seems to be running too lean. Maybe just putting in a new jet of the same size will cure that?

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Got the 45 pilot in today. Bike seems to be running better than ever. Haven't had a chance to ride it to see if the hesitation is gone. But so far so good!

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