Jetting for high compression

3 replies to this topic
  • levy1

Posted May 16, 2003 - 02:28 PM


I want to install a power-bomb header, a power-now valve and a FMF Can. My bike has always been pretty strong. I could never get it to run wide open, as it would bog at ¾ throttle. I never needed much on the throttle side because with a 53-tooth rear sprocket I can climb anything at half throttle. I want to get the jetting “spot-on as Taffy used to say. I have never jetted before, don’t know where the clip is but I can read so I am sure I can find it. I figure to start by taking off the carb. Pretty smart huh! Could someone give me a baseline start? How do I check it along the way? I know everyone is sick of this but if you were me who would you ask? Of course, fellow TTers. I have a high-compression motor and the head has been ported and a little polish. Just a little because I didn’t think it needed to be to shiny inside. Anyhow I am ready to jump in. Thanks, Fred

  • Indy_WR450

Posted May 17, 2003 - 04:17 AM


With all those mods and a WR400 I cant help you that much. But since you are not getting many responses here goes:
Be prepared to take the carb in and out 5 - 10 times over a weekend of jetting. Forget taking it to Taffy's level. You need to get your bike in the ball park first. Bog above 3/4 throttle should be a sign of rich jetting. If it stops pulling hard at full throttle you are lean on the main jet. Start with a newly gapped spark plug, 160 main, stock needle in the center clip position. I like to jet by the seat of the pants. Make sure you have run the bike for 5 minutes hard so it is good and hot for your evaulation. You want it to pull hard & climb revs smoothly. I am sure someone with a moded WR400 may have a better start point. But I have seen some ridiculous rich main jets on this forum. Some run as high as 185 mains. I suspect they have YZ timing and can run richer. I can not go higher than 172 main at sea level in 30 to 40 degree weather. Hope this helps. Good luck :)

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  • Taffy

Posted May 17, 2003 - 07:41 AM



the jetting on your ported and HC piston bike should be lower than normal by 2 or at most 5 on the MJ. otherwise just treat it the same as all the other WR400's you've seen jetted. my jetting is good for sea level or you can be a little more conservative with something like a 38PJ and EMN needle and a 165 on that open pipe. the rest-copy mine if you want.

if you're above 2-3,000ft then go start following people like hick and whoever.


  • levy1

Posted May 18, 2003 - 04:31 PM


taffy. It runs real good on the 170 main. should I use your baseline or does the relationship change with the 170?


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