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xr400 throttle hesitation

fremontguy

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#1
I have pesky problem with xr4 gasping when I open throttle suddenly. Some times I'm able to smooth it out by tweaking air screw. Over all I think the jetting might still be a little rich, plug is black colored. My thought is to lower pilot jet to #55 from #58. Running: M155, P58, N-2nd notch, snorkle out, xrs only pipe at 4000'. Has anyone else had sucess with this issue?

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'99 XR400R '95 300EXC
 

penguin

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#2
you are experiencing a lean bog condition. all big bore 4 strokes do this to some extent, you can make it better but will never completely eliminate it.adjusting the airscrew will help. A pumper carb helps but you still will have a slight bog or hesitation if you whack the throttle open suddenly. Most 4 stroke riders learn to roll the throttle on instead of opening it all at once.

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You weren't born knowing this stuff.
 

mtngoat

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#3
At that altitude IMHO, I don't think you're rich. With your mods, my experience is a Main 155/158 and Slow Jet 58 works pretty good up to 7500'.
For CA bikes, the A16 dual-taper needle also helps a lot.
I'd check your pilot screw and consider the A16 needle.
Here's a link to some info on "what works when" for the stock carb.
On several XR4s I've jetted, the hesitation is not noticeable with practical riding.

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Noscitur ex socio, qui non cognoscitur ex moto artem
 

fremontguy

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#4
I'm currently running the a16 needle, clip in 2nd notch. Since I have tried 60 pilot(bad), 58 is better, maybe improvement on #55? Pilot screw is at about 1 turn out now for optimum response. Thanks for any response, I'm just looking for a cheap quick fix I guess.
 

mtngoat

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#5
First, I didn't post the link I spoke of. Here it is: http://john.rushworth.com/Motorcycle_Co_UK_archive/xr4faq/

Looks like the only significant difference between our bikes (running at 3500' to 7500' with no noticeable hesitation) is the pilot screw, which we left in the stock position. Everything I've read calls for more turns out. See the link above, which also has Honda tech bulletins.

Only other thing I can think of is the number of disks you're running in the pipe. I run 8 on an old FMF, my buddy runs 12 on the E-Pipe. The other new XR4 uses the Vortip and we jetted a little richer on the main.

I can quickly crack the throttle on these bikes and force the hesitation, but while riding we don't really need to and therefor don't notice it.

Hope this helps.

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Noscitur ex socio, qui non cognoscitur ex moto artem