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Noticeably more power but now she coughs

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I decided to do a couple easy mod's on my 01 XR 400 - Drilled 4 1/4 inch holes in the baffle, removed the snorkle and replaced with a UNI - the result was quite a bit more power, but the bike coughs at about 1/8th to 1/4 throttle more so when its cold, but will happen even warm ... I got it nice and warmed up, checked the plug, and it looked good, I'm curious if the plug just isn't giving it away, do I need to do any carb adjustment with this mod, based on the looks of the plug, I'm good???

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did you rejett or adj the fuel screw after the mods? now its gettin more air its prolly running alil lean.

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did you rejett or adj the fuel screw after the mods? now its gettin more air its prolly running alil lean.

no I didn't, and my thoughts exactly - which is why I checked the plug, but the plug looked okay, no dead giveaways ... any recommendations?

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id go up a size on ur MJ see if that helps. i think alot of people run 160mj and 55pj after doing the mods. not sure if that will work 4 you. but just do a seach on xr400 jetting u'll find lots of info. do you no what jets you have in there now?

search over in the xr250/400 for a thread (Gordon mods help!!!!!) hes dealing with some same issues.

T

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no I didn't, and my thoughts exactly - which is why I checked the plug, but the plug looked okay, no dead giveaways ... any recommendations?

!!!Merry Christmas!!! :smirk:

Unless you are riding over 6,000' I would think a #58 pilot would be spot on. after the engine warms up, adjust the fuel screw to find the highest idle. (should be between 2 to 3.5 turns out) If it takes more than 3.5 turns you would need to go up another size on the pilot.

A main jet size of #160 should wake up some more power for you. :bonk:

Perc

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you let more air in.Now you have to let more fuel in.Try the air screw in 1/4 turns. Each way. Remember where you had it to start. Next move the needle up (clip goes down) . I bet that fixes it.May have to go 2 notches. If that doesn't do it richen the pilot and check main too.

Im sure the xr forum has ppl that are very knowledgeable in xr jetting.

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!!!Merry Christmas!!! :bonk:

Unless you are riding over 6,000' I would think a #58 pilot would be spot on. after the engine warms up, adjust the fuel screw to find the highest idle.

so that's the goal, after its warmed up adjust to the highest idle and leave it there?

THANKS A MILLION FOR ALL THE HELP GUYS!

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you let more air in.Now you have to let more fuel in.Try the air screw in 1/4 turns. Each way. Remember where you had it to start. Next move the needle up (clip goes down) . I bet that fixes it.May have to go 2 notches. If that doesn't do it richen the pilot and check main too.

Im sure the xr forum has ppl that are very knowledgeable in xr jetting.

So I may not have to re-jet at all? SAWEET ...I'll try that ... merry xmas and THANKS FOR ALL THE HELP GUYS!

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I decided to do a couple easy mod's on my 01 XR 400 - Drilled 4 1/4 inch holes in the baffle, removed the snorkle and replaced with a UNI - the result was quite a bit more power, but the bike coughs at about 1/8th to 1/4 throttle more so when its cold, but will happen even warm ... I got it nice and warmed up, checked the plug, and it looked good, I'm curious if the plug just isn't giving it away, do I need to do any carb adjustment with this mod, based on the looks of the plug, I'm good???

Never look at the plug. It doesn't tell you anything.

If your bike 'coughs' when you snap the throttle from closed to 1/8-1/4, then it's a lean pilot circuit. If it coughs and sputters when you hold that throttle position, it's rich in the slide and/or early needle. That seems unlikely though, so I'll assume the former.

Get your bike all warmed up, then richen up the pilot screw until the response to a quick snap from closed to 1/8-1/4 gives an instant burst of RPMs. You can do this while on a stand. Richen it up until you *just* get the response instant. Take it for a ride it and do the same thing and fine tune if necessary - chances are you won't need to tweak it.

Few people have this adjustment set up correctly. When jetted right and adjusted, an XR4 responds to a snapped throttle by standing up and flipping over backwards before the bike even moves forward - it'll rotate right around the rear axle. The off-idle grunt of the XR4 is legendary...although my wife's 7HP TTR125 can do that too :bonk:.

Incidentally, I have the stock exhaust in my '98 XR4, and it'll tear your arms off. It's too loud even with that insert. Opening up the exhaust doesn't make any more power, other then possibly at the highest RPMs at WFO, and who cares about WFO unless you're on a MX track? The last thing we woods guys need is to make our bikes louder just for the sake of being obnoxious. Please put your stock insert back in.

JayC

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will do ... I could care less about noise either ... but I'm still learnin, I'm def a woods guy too ... however, the UNI definently helped ... I will try this ... thanks.

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