Help with GB/BK mod on 02' YZ 426

I need some help here with this mod.I did the mod and it really made a big difference on start up.It starts with 1-2 kicks and I don't have to use the hot start all the time now either but I have a bog issue now that I didn't have before.The bike is pretty much stock being it has an aftermarket air filter on it. Could it be running to lean do to the GB mod?I did turn the pump timing down from 2sec. to .65-.70 sec.I was going to do the powernow mods also but would like to see what each one does.Would in be help to get a jet kit like the JD kit for the bike? Would it even help performace seeing I have no other mods yet?

Well developed jetting kits such as JD's will always help. It sounds like you have another problem though. I would think you are probably a little lean in the pilot if you are never having to pull the hot start. Is the bog only on the stand? Especially if you grab a handful of throttle quickly? Have you adjusted your fuel screw at all? :thumbsup:

Well developed jetting kits such as JD's will always help. It sounds like you have another problem though. I would think you are probably a little lean in the pilot if you are never having to pull the hot start. Is the bog only on the stand? Especially if you grab a handful of throttle quickly? Have you adjusted your fuel screw at all? :thumbsup:

Fastest gives good advise, I will add make sure your carb is spotless clean. Also your air filter, do not over oil it, this will add to the issue.

As fastest points out if your idle on a stand or whatever and stab the throttle it will bog this is not fixable and is normal.

Here is a link I did to a write up of the GBmod. It is taken from Motoman393's website but a bit more detailed only in Tuning.

GBmod Detailed Tuning guide

Also

What elevation you at? Temp / weather conditions

No I have not messed with any of the jets or fuel screw yet.I was thinking it my be lean as well because it seems a little hotter then normal.The bike doesn't seem as fast as it did before either.I will play around with it later this week.

The fuel screw is a must! Always adjust it first, I do before each day of riding. I suggest buying an adjustable one that takes no tools. A lean bike will always run faster than a rich one. Have you ever run it out of gas? At that last minute right before they die they run hard! You say it "doesnt seem as fast", what do you mean? As fast as it was this winter? If it was running well during the cooler parts of the year and it now seems weaker. It is because you would be too rich. Winter air (cooler and less humidity) is more dense and therefore contains more oxygen, so if the jetting was working well then, that is what we need to know. When was the last time it ran well? Has anything changed? Temps, humidity, altitude, modifications? Let us know. As JL stated cleanliness is next to godliness! Never over oil the filter either.

It has been about 3 weeks and temps were up about 15 deg and humidity is way up.When you say getting an adjustable one do you mean one of those from Zip Ty?Yeah I know anything lean runs fast but how do you know on these when it's to lean or to rich? :thumbsup:

If it's lean, the exhaust will "pop" on deceleration.

Yeah, I was referring to the Zip Ty or a homemade one. Lean in the pilot circuit is what causes popping on decel, not necessarily a lean main. Four strokes because of their oil supply arent as sensitive to jetting. Let me reword that, they are just as sensitive but they dont seize unless you are way off. You can check the color of the porcelain on the plug if you are willing to do a plug chop. I would just try a smaller main, turn in the fuel screw 1/4-1/2 turn and maybe drop the needle 1 notch. I would do these one at a time starting with the fuel screw. Turn it in til it just starts to pop on decel. If you eliminate the popping altogether you will lose a little throttle response. If you dont get anywhere with the fuel screw I would try a smaller pilot. It is not unusual to have to change jetting between winter and summer especially if there are 40-50 degree differences between seasons. Always take notes, keep the carb spotless and do one change at a time until you understand each circuit. :thumbsup:

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