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Stumbles opening throttle fast XR400

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Stock 01 xr400 with a White Bros muffler, stock jets.

 

Just had the carb apart to clean, starts easy and runs great but...If I crack the throttle hard like if I was going to wheelie or take off, it takes a second to react and kind of pops and stumbles, response is anything but immediate and won't wheelie unless I just roll on the throttle.

 

Is this a jetting issue? Need new ones (which ones?) or some sort of adjustment I can make? Something else? Runs great otherwise.

 

Thanks!

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Did a search and seems a lot of people have an "off idle stumble" as well.

 

A lot of people say buy a pumper carb, but there's got to be a cheaper fix right? Where should i start to fix this instead of throwing $400 for a carb.

 

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A lot of people say buy a pumper carb, but there's got to be a cheaper fix right? 

 

Wrong.  Only a pumper carb will give instant throttle response on a four stroke.  

 

The stumble isn't a problem in real world trail riding because you don't normally jack the throttle.  

 

Also, if you're good at jetting, you can minimize the stumble but not eliminate it with a non-pumper carb.

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Start with jets. Stock jets and a slip on muffler have you running quite lean. 100% sure of that. 

 

Start with 60/160, re-tune the fuel screw and see what that gets you. It should just about clear it up and it WILL let your bike run better and cooler than what it is now. Test it by trail riding like you normally do. Going from idle to WFO throttle will almost always cause a split second delay. And like Baja Rambler said, that really doesn't come into play while actually riding because your rpm's will be up above idle already so the difference in vac signal through the carb won't be as much.

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Agree with what both these guys have said. Stock jetting is 142/52, so pretty sure its running lean.

What's your elevation?

Learn to tune the fuel screw. You can't just swap in another pilot jet or ride the current one with the fuel screw as is and expect to improve throttle response.

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I can't twist my throttle fast enough to get a stumble while riding. Am running stock keihin with 60/160 jets and the needle in the 4th from the top. Running an exhaust with a pretty open flared custom, 1 3/16", tip.

Edited by lump_tha_thump

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Wrong. Only a pumper carb will give instant throttle response on a four stroke.

Wrong. I've never had this stumble on mine. Like Lump (last post), I can't twist my throttle fast enough to get a stumble. Response is crisp and immediate. Everything is stock in my carb. Snorkel removed, exhaust was opened a tad. I ride 3000-7000'.

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Wrong. I've never had this stumble on mine. Like Lump (last post), I can't twist my throttle fast enough to get a stumble. Response is crisp and immediate. Everything is stock in my carb. Snorkel removed, exhaust was opened a tad. I ride 3000-7000'.

 

Sure, you can jet it super rich and with a high enough idle that it won't stumble at the expense of poor gas mileage.  Don't take my word for it.  Just rub your tongue around the inside of your tailpipe and see how much black soot is on it.

 

A pumper carb is a better (and cleaner) way to go because you can jet for optimum power and you only get the extra squirt of fuel when you need it.

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Sure, you can jet it super rich and with a high enough idle that it won't stumble at the expense of poor gas mileage. Don't take my word for it. Just rub your tongue around the inside of your tailpipe and see how much black soot is on it.

These bikes ran lean to begin with, and my mods only made it leaner. Mine runs clean as a whistle and gets incredible mileage. All I was saying was that I've never had the problem, I've never felt the need to invest in a different carb, and I've never even cracked a seal on my carb since day one back in '01. Don't include everyone with a blanket statement that no 4t can run without flaws without a pumper carb just because you have shortcomings in the jetting department.

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I'm going to stand by my statement that:

 

 

"A pumper carb is a better (and cleaner) way to go because you can jet for optimum power and you only get the extra squirt of fuel when you need it."

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