More ?Jetting? questions

4 replies to this topic
  • Rich_in_Orlando

Posted July 15, 2000 - 07:25 AM


OK, here's the deal. I've got a 99 WR with an FMF Megamax II muffler, stock header and the air box lid has been removed. The main jet is a 180, and I have the standard US needle at the #4 clip position. I'm not sure what the slow jet is because I haven't looked at it. Other than that, no other "breathing" mods have been done. I ride mainly between 0-300 ft above sea level. The gearing is the stock 14/50.

The bike runs really strong except for one minor glitch. When I'm cruising along between 45-55 mph in top gear and not moving the throttle, it has a little hesitation, almost like it's missing or something. It doesn't sputter or smoke, it just isn't absolutely smooth at those speeds. When I'm accelerating or decelerating, it is just fine. BTW, at those speeds, it looks like the throttle is just below 1/2. I just raised the needle clip from the 3rd to the 4th position this morning, because I wanted to richen the mixture just a little. (I checked the spark plug before I made the need clip change and it was a little lighter in color than I thought it should be-not bad, just a little light.)

What do y’all think- am I on the right track with this? I can obviously get back into the carb to raise the needle higher, but do you think I’m sniffing in the right place? Should I get a richer needle altogether? I think the #180 jet is the biggest one for that carb, so I don't think a bigger main would work. Or, should I check the slow jet. It starts on the first or second kick almost every time, and I don’t want to mess that up. I’ve never adjusted or even checked valve clearances, and it’s probably due. Could that be causing this (minor) problem. I don’t really ride it at those sustained speeds very long anyway, usually only when I’m on the road (the bike’s street legal.)

Thanks in advance for your advice.

Rich Bumar
Orlando, FL

  • James_Dean

Posted July 16, 2000 - 06:12 PM


To move to clip #5 would be very unusual with a DTM needle. Most WR's are in clip #3 or #4. Is your WR with YZ cam timing? With YZ cam timing I would suggest clip #3. WR timing - either 3 or 4. Check the carb over, making sure its completely clean inside. The hesitation may be lean as you suspect, but at 1/2 throttle you need to change the taper to the YZ426 needle EKN or EKP to get richer at 1/2 throttle and NOT at 1/4. See posting on the "YZ426 jetting boosts YZ/WR400 midrange power".
James Dean

[This message has been edited by James Dean (edited 07-16-2000).]

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  • Clark_Mason

Posted July 16, 2000 - 06:49 PM



If you are just crusing along you will be at just off idel in top gear maybe 35 to 45 mph and you get these little misses or whatever. I would suggest at your altitude you are just off idle 1/8 thorttle ofr less and need to richen up the pilot circuit I would go to 48 or a 50 PJ.


  • Rich_in_Orlando

Posted September 19, 2000 - 07:48 AM


It's me again. I took Clark's advice and went from a 45 to a 48 PJ, but it made it stumble even worse. (Sorry, Clark, but it did point me in what I hope is the right direction.) So, I figure that the problem is that it was a little too rich all along in the pilot/needle part of the throttle. (Who would have thought this would be this complicated?) I replaced the 45 PJ, put the DTM needle on the #2 clip to lean it out a little and now it's a LOT better. Not perfect, though. Now, I have very, very, slight deceleration popping (evidence that it's now a little lean??) and only a little hesitation at around 50 mph. It seems like it is a transition problem--maybe with the taper of the needle.

So, what do y'all think? Should I go to a DVP or some other needle or start playing with the fuel screw? I'm not sure fuel screw adjustments would fix the transitional problem but it might fix the popping. Thanks for the help.

Rich Bumar
99 WR400

PS- I have adjusted the valve clearance since the first post (they were all a little tight) and I run with WR timing.

[This message has been edited by Rich in Orlando (edited 09-19-2000).]

  • James_Dean

Posted September 19, 2000 - 12:22 PM


The fuel screw is an easy adjustment to make so try backing it out to richen the off throttle and transition as the throttle is rolled open down low.
The DVP needle will lean the jetting at 0-1/4 throttle. Using DVP#4 equals DTM#3 in the 1/4-3/4 throttle position range. If you want to get a crossover in the transition, then DVP#4 could be used in place of your current DTM#2. (leaner 0-1/4.... richer 1/4-3/4)
For your reference, the #48 pilot jet needs the pilot screw turned out at 3/4-1 1/2 turns on my '00WR. The #45 works better at 2-2 1/2 turns out.
James Dean


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