Skid Jackson

Member
Nov 1, 2000
191
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I'm posting this for my brother who has an 01 xr 4 with a stock front sprocker (14t ?) a 52 t on the back Stock jet of 142 and has lowered the needle one notch from the stock setting.

The throttle response off idle is too abrupt for his liking. When in tight twisty stuff he has to blip the throttle. Kind of like hitting a light switch off and on. He is trying to get it so that you can roll the throttle on smoother. If a motocrosser or hare scramble guy got on the bike they wouldn't know what the problem is. My bro prefers to "tractor" through the woods, trail or no trail. any suggestions on carb set up for this type of riding keeping the stock pipe.
 

biker

Member
Sep 22, 2000
170
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I've never heard this complaint about a stock XR4. They're about as tractor like as you can get. Maybe he would have been better off with the XR 250. He could try going one tooth smaller on the front sprocket, to lower the gearing.
 

TR

LIFETIME SPONSOR
Mar 17, 2000
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14/52 gearing ! Standard is 15/45 and you should be go anywhere with that.
I run 15/44 and if the hill is at all rideable than I can usually get to the top in 2nd.

I would try experimenting with the gearing. The differences are noticeable and you might suprise yourself.

Regards
TR :eek:
 

John K

Member
Feb 22, 2000
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I had 14:53 on my XR6 when I bought it ..... WAY TOO LOW for what the trails I ride!! A couple of people I know were quite happy with 14:46 for their XR4's
 

Skid Jackson

Member
Nov 1, 2000
191
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mmmm I knew this wasn't going to be easy!! It's not the gearing It's already low enough to pull stumps!! It's the way the gas comes on when you twist the throttle. It's like the initial spurt of gas is too much. Admittedly it's being a little knit picky but my old xr did the same thing and it bugged me a bit. Plus my drz doesn't do it. As i mentioned above it's like the gas hits all at once you can't really roll into the throttle. No it doesn't go into full throttle or anything like that but it is kind of irritating. I guess we are trying to fine tune the throttle response off of idle. What inside a carb would you screw with to do that???
 

VintageDirt

Baked Spud
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Jan 1, 2001
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Could it be that you are experiancing the XR400 off idle stumble and go?
 

Hondaxrguy

Member
May 17, 2001
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I think he's just trying to smooth out the initial surge of power off idle. Easiest thing I can think of would be to use the clutch a little bit.

Jeremy
 

Skid Jackson

Member
Nov 1, 2000
191
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Ahhh!! now we are getting somewhere!! vintageguy talk to me about this stumble and go.... describe it...... is it an xr thing you have to live with or can it be fixed

H-xrguy a couple of riders have told my bro just to slip the clutch. But that sounds like a real pain when your doing psuedo trials-trail riding. Although i know nothing about trials riding.... do they work the clutch like madmen or stay away from it or just the usual clutching you'd expect??
 

snaggleXR4

Member
Aug 5, 2001
309
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Hey,
The 400's have a flat spot right off of idle, then the fuel surges, and you get a little hit. Check out bajadesigns.com, they have an XR400 section on their menus. It tells about the dual tapered needle, slide, and jetting, as well as the airbox mods. Supposedly, it fixes the flat spot problem. The needle and slide are relatively cheap also, so it's worth a try either way in my opinion. On the other hand, you can spend ~$400 for a Mikuni Pumper carb which also, supposedly, works wonders for the 400's. In the near future I will be trying the Pumper carb on my 400. I've got an Xr's Only pipe and jetting and I'm not displeased, even at low speed, low RPM riding. However, I think I understand what your friend is complaining about. Search around these forums for XR400 mods, Mikuni Pumper carbs, and check out bajadesigs.com. Good luck.
 

Mudboy

Member
Dec 3, 1999
95
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I had an '00xr400. If you really crack the throttle open at low rpm they will bog. Correct me if I'm wrong but I think they have a CV carb? So it has to build a little pressure before it lets the gas in. The richer the jetting the more the bog, the leaner the more it goes away. The answer is to clutch a little or give it a little roll before you snap it open. W/O changing to a pumper carb you won't get the response of a yzf or a 2-stroke.
 

VintageDirt

Baked Spud
LIFETIME SPONSOR
Jan 1, 2001
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Skid

Everything I know about an XR4 I posted in this thread, click here . I know it's not much.

Also, I don't think the stumble is cause by a rich condition. At least in my experience it's been due to a lean condition. I went to a richer slo jet and improved the off idle throttle response. Also, you didn't say what year your XR is (at least I didn't see it), I know the newer ones came with the dual taper needle, just don't know when that started.

One more thing. If your engine idle speed is set to low, you'll notice the stumble is worse.
 

Reeko

Member
Aug 9, 1999
67
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I had a 98 XR400 with the same stumble problem. Although I never thought the hit was excessive.

It is ussually caused by a Lean condition when opening the throttle suddenly from idle. I think this is a 4 stroke thing that the YZs solve with an accelerator pump giving a shot of gas.

A quick check tha proved to me it was too lean was to engage the choke 1/2 way and try it. It ran much better without the stumble for me. Then, a richer idle jet helped but never completely eliminated the problem.
 

mtngoat

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Jun 12, 2000
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This is a tough one. Most XR4 jocks complain about the hesitation, not the hit off throttle.

Here's a link that describes what-works-when:

http://john.rushworth.com/Motorcycle_Co_UK_archive/xr4faq/

Trying some reverse logic, you may want to richen things up to dull the off-throttle response. To do so, return the needle clip to the middle position (assuming you didn't really "lower the needle", you lowered the clip) and go one size bigger on the pilot jet. BTW, beware of too lean conditions if any mods (exhaust changes, open airbox) are done; explained above.

If the "hit" occurs when rolling and coming off idle, consider encouraging the use of the clutch to finesse the power.
 

Hillman

Member
Apr 16, 2000
1
0
Skid,
I didn't check out mtngoat's link, but I know this much, the XR4 needs to breath more. I agree that your strong hit at the bottom is actually the low throttle stumble described by every XR4 owner, myself included. I don't have the pumper carb yet, but almost eliminated the stumble when I opened the bike up a bit to breath more. I first tried removing the snorkel, coarse mesh on the air filter support and filing out the excess weld on the exhaust header. That helped, but it wasn't enough yet. So I drill four #3 holes in the exhaust end cap between the tubes, and replaced the filter with a K&N. Then I needed to rejet. 3000' 158/60. That's what I did and it is totally a different machine. I certainly didn't invent this stuff, everyone that has looked around the web knows about it, but I thought I would mention it in case.
Your brother may not want more power, but I couldn't get it smooth off idle until it flowed more air. FYI, it will never act like a machine with a pumper carb. When you wick open the throttle there isn't enough vacuum to drawn in the fuel needed. The accelerator pump on a Mikuni 36 compensates by forcefully injecting a small additional amount of fuel to reduce this lean condition.

Make sense?

Kirk Hillman
'99 XR4
 

TexKDX

~SPONSOR~
Aug 8, 1999
747
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While he's working on the carb make sure and read MTNGoat's link and set the float TRAVEL, not height. Two different things. Makes all the difference in the world in whoops and roots.
 

Skid Jackson

Member
Nov 1, 2000
191
0
OK my brother goes out and tools around on his xr with the choke set 1/2 way and says that is much better. Which i guess means he should try a richer idle jet. any sugestions on which size?? It will be used with the stock pipe!!

One more thing ...... this may sound really dumb..... but hey gotta learn this stuff some how.... Would riding around with the bike on 1/2 choke screw up the motor!! Just curious!!
 

lipskid

Member
Feb 12, 2001
11
0
No it won't, but correct jetting will work much better.:confused: When the bike is being ridden at a good clip, say, wide open in 4rth, and he chops the throttle, does it backfire? This would be one indication of the pilot jet being to lean.
You said he lowered the needle on position. Why? If the bike runs better at half choke, I would say the first order of buisiness would be to return it to the stock position.(please keep in mind I'm no authority on any of this, just puttin' in my .02) Also, have you adjusted the pilot screw? Turn the pilot screw clockwise untill it seats LIGHTLY. Then turn it out(counter clockwise) 2 1/4 turns. Warm up the engine to operating temp. Then adjust the screw in or out to obtain the highest rpm. Reset rpm with the idle stop screw.
Also, the stock countershaft sprocket is 15 tooth. Going to a 14 tooth is a fairly significant change(equal to about 3 teeth larger on the rear). XR400's have a lot of low end torque, lower gearing (which is what the 14 has accomplished) will only put MORE power to the ground FASTER.
Remember to only make one change at a time. If you do several at once, you never know which one is working(or not working).
The stock carb on newer xr4's work really well. I ride mine from 4500' elevation in Moab, to 13,000' in Colorado without changing the jetting.
Gotta love this bike! Good luck!

Steve
 
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