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WhiteEagle78

98' YZ400F idol issues

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I picked up 1998 YZ400F a while back. Ive only ridden it around the block a few times. It seems to feel pretty good with the exception of some backfiring on decel, but whenever I stop... it wouldnt ido... it would just diel. The idol screw was all the way in and the rpm seemed to be at least around + - 1300 (no tach tough). I heard all kinds of great things about the ZipTy carb mod, and so I sent the carb to them to do the mod and check to make sure there where no other issues with the carb.

Everything came back the other day, and the jetting is a 45 pilot and 178 main jet. The notes from ZipTy recommend going to a leaner pilot for starting. Other than that everything looked good according to them. I never really have any problem with the start though, just idoling.

I got the carb reinstalled and upon starting her up... its the same thing. As soon as I stop playing with the throttle and try to let the engine idol, it just dies. The hot start is not stuck open or anything. Im not sure what to check next. Could the jetting be that far off? Or should I look somewhere other than the carb???

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Inasmuch as you are new to the bike, I wonder if you are aware that the idle speed screw is the black knurled knob on the left side of the carb?

Also, you may need to baseline your pilot fuel screw. Turn it in until genly seated, then back it out 1 3/4 turns.

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Yeah...

Ive got the idle adjustment knob figured out :ride: But even when the idle adjustment is maxed out, it still dies. I suppose I need to invest in an eloctronic tachometer tool to be sure, but the rpms seem to be well over 1000.

I'll baseline the pilot jet tomorrow and poke around the rest of the intake to make sure there are no air leaks such as cracks in the carb to intake boot or maybe a cracked hotstart air house.

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I have a 98 YZ400F that did the exact same thing. I had the carb apart a lot of times and found nothing. Let a friend check it out and it ended up having what looked like a small piece of rubber in the pilot circuit. Once that was out I haven't touched it since. That's been almost 2 years ago. Hope that helps.

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Yeah...

Ive got the idle adjustment knob figured out :ride: But even when the idle adjustment is maxed out, it still dies. I suppose I need to invest in an eloctronic tachometer tool to be sure, but the rpms seem to be well over 1000.

I'll baseline the pilot jet tomorrow and poke around the rest of the intake to make sure there are no air leaks such as cracks in the carb to intake boot or maybe a cracked hotstart air house.

If I am correct, proper Idle speed is around 1800 RPM, so well over 1000 is good.

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It looks like the idol issue has been resolved. The hot start button is mounted to the frame and there is a hose that goes from the hot start directly to the cylinder head. Upon inspection, I found a funky split/crack in the hose (kind of underneath the extra protection sheath thats over the hose), not that big, but apparently enough to suck in some air and probably lean out the mixture to much for the bike to idol. The hot start button is a little messed up anyways, and the hose is bad, so I just removed the whole thing and put a vacume cap over the intake nipple on the cylinder head and she's idoling like a champ now From what I hear, the hot start isnt needed all that often anyways, so I guess I can wait to find a cheap one on ebay or something.

Thanks for the help guys!

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