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400EX jetting help..

  #1  
Old 02-25-2019, 11:27 AM
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Default 400EX jetting help..

just got a 400ex on Saturday itís an 04. Seems like itís running very lean. When I bang through the gears it keeps backfiring when I let off. Iíve ordered a new carburetor for it because the current one is very very Dirty and the machine has been sitting for a few years according to the owner! So I got a replica Klein carb off eBay for 50 bucks with a warranty if I wanna return it and rebuild the stock carb. Anyways, my 400ex has no air box lid, has a CT racing header and CT racing slip on exhaust, Iím located on Long Island in new York and my elevation where I ride is around 30-50 feet above sea level. Thanks for any help I can get!
 
  #2  
Old 02-25-2019, 08:38 PM
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Anyone?
 
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Old 02-25-2019, 10:02 PM
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First thing is to look at your plug and see how it looks, it’ll give you an indication of how it’s running. You might even identify what size main jet is in your carb and while it’s apart look at giving it a good cleaning. Not too sure about the cheap replica carbs
 
  #4  
Old 02-26-2019, 07:22 PM
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Originally Posted by Lt Granddad View Post
Not too sure about the cheap replica carbs
I am. That old KeiHin OEM carb is worth a lot, even dirty. If not corroded or damaged physically, it is FAR better than a new Chinese replacement. Clean it!

Backfiring out the exhaust on off-throttle deceleration is a feature of low-restriction mufflers. Reducing inlet air-flow resistance with a pod filter can also contribute to a lean condition. Plug-reading and possible rejetting is called for here.

 
  #5  
Old 02-28-2019, 05:02 AM
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Originally Posted by sr.ruedas View Post
I am. That old KeiHin OEM carb is worth a lot, even dirty. If not corroded or damaged physically, it is FAR better than a new Chinese replacement. Clean it!

Backfiring out the exhaust on off-throttle deceleration is a feature of low-restriction mufflers. Reducing inlet air-flow resistance with a pod filter can also contribute to a lean condition. Plug-reading and possible rejetting is called for here.
thank you for both your advice. I pulled apart the OEM carb and saw that the jetting was 38 pilot and 148 main. Sounds like Iím running very lean with the no air box lid and full exhaust. And also to take into consideration the low elevation of about 40-100 I ride on. Iím thinking I need around 170-175 main and 40 to 42 pilot.. any suggestions as to
if this is a good place to start?? Advice is very much appreciated!
 
  #6  
Old 02-28-2019, 10:51 AM
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Originally Posted by Parks1181 View Post


thank you for both your advice. I pulled apart the OEM carb and saw that the jetting was 38 pilot and 148 main. Sounds like Iím running very lean with the no air box lid and full exhaust.
You can get a set of 10 diff KeiHin pilot jets on eBay for less than the cost of one from the dealer. Over 40 for the size sounds good, but also those really small jets get clogged easily. As for the main, you can up it somewhat, but you may want to first try lowering the needle clip one position.

 
  #7  
Old 02-28-2019, 11:41 AM
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Originally Posted by sr.ruedas View Post
You can get a set of 10 diff KeiHin pilot jets on eBay for less than the cost of one from the dealer. Over 40 for the size sounds good, but also those really small jets get clogged easily. As for the main, you can up it somewhat, but you may want to first try lowering the needle clip one position.
maybe I should try a 42 pilot and moving the needle down one clip with the stock main? Or maybe get a 160 or 162 with moving it down one.. and just to clarify, moving the needle jet down one clip is what helps with running lean?
 
  #8  
Old 02-28-2019, 12:02 PM
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Originally Posted by Parks1181 View Post


maybe I should try a 42 pilot and moving the needle down one clip with the stock main? Or maybe get a 160 or 162 with moving it down one.. and just to clarify, moving the needle jet down one clip is what helps with running lean?
and also you think at my altitude of around 40-100 those jet sizes would be ok moving the needle down? I been looking seems that a lot of people with adter market exhaust, uni filter and no air box lid are running at least 170ís
 
  #9  
Old 02-28-2019, 01:25 PM
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Lowering the needle clip RAISES the needle, which richens the mix. You want the clip in the second from bottom
position to start with. You will have 5 positions, probably. A bigger jet also richens, but don't overdo it.
 
  #10  
Old 02-28-2019, 05:06 PM
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There was a pipe shoot out Dirt Wheels March 2001that had the main at 160 for the ct pipe but also came with their needle and different pilot which they didn’t disclose. I’ve seen a lot of people suggesting a 42 pilot which was eliminating the need for a choke to start the bike but I’ve seen as low as 3/4 turn out to 2 3/4 turns out on the fuel mixture.screw using the 42 pilot. Being an 04 you should have adjustable needle would try lowering 1 notch like sr.ruedas suggested 2nd from bottom.
 

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