Should I bother re-jetting?

My 98WR 400 has the stock jetting. It has the stock muffler with a Vortec tube installed, airbox lid off, I have not de-octopussed but did put a P38 and a Power Now Valve on. Currently the bike seems to run good and I'm having a great time in the tight twisting trails of Chadwick Missouri. So whats the problem you ask? The problem is I was rummaging through my tool box and found a OBELN - EP Standard Needle jet still in the little Yamaha Factory parts bag, which I purchased long ago and forgot about. The problem is I have an Engineering degree and like most Engineers I believe all mechanical things fall into one of two catagories:

1. Things that need to be fixed. and 2. Things that will need to be fixed after I've had a few minutes to play with them.

Should I step away from the carburator or put the OBELN Needle in? Does anyone else run this setup? If so what clip would I want to put it on and would a new main jet/ pilot jet be in order. Would the performance gains be worth the risk of getting tangled up in a potential tail chasing exercise? Any thoughts from you carb experts?


I'm by far the farthest thing from a carb expert but I can tell you this. My 2000 WR400 ran very good with stock jetting, but once I started to mess with it, it only got better. I'm currently using the EKN needle on clip 4, a 40PJ, 65PAJ(S), 158 Main Jet, 160MAJ. Not that these will work for you, but all the trial and error was worth it.

ELN may be a hair rich here and there, but clip 4 is as good a place to start as any. I can't be more specific than that because I have a 426, and ride at altitudes higher than your avg. MO riding area. Probably.

Swapping in a "E" taper needle for the stocker in a 400 is an old trick here at TTalk, and I don't think I've ever heard anyone say they were dissapointed in the results.

If your bike is jetted perfectly on the main right now, going with an "E" taper needle may require than you go down a size or two on the main.

Hope this helps.

Thanks guys, Based on your comments and Firemnwhyt's jetting profile it sounds like starting at clip #4 and changing the stock 165 main to a 160 main and the stock 45 pilot to a 42 might be a good start. I'm assuming this would also require some other corresponding adjustments. I'll research and see if I can figure out how all of these variables will affect existing carb adjustments before I start.

If it were me I'd just throw the needle in there on clip # 4 and see what happens. I do think an E taper needle will be an improvement, but I don't know that you need to go leaner on pilot and main, especially now that it is getting colder (not here really, but I'm imagining MO).

Of course there is only one way to find out, but it is easy to get confused by changing a bunch of stuff at once. If you spend an afternoon changing one thing at a time and then riding if for a bit you will understand what is going on by the end of the day and should also have it pretty well dialed.

Don't pay attention to anything the bike does until it is fully warmed up.

If you have any problems getting the carb and jets in and out do a search or make another post, TTalk members that have jetted their bikes have a lot of tips and tricks to make it easier.

Hope this helps.

Thanks Hick,

I'll eliminate as many variables as possible, and do only one change at a time. Hopefully the E series needle will improve the performance without creating any other issues.

KTM four strokes use basically the same carb as the WR and YZ four strokes. The richer ELN and EMN needles work very well in KTM's, however it is essential to use a smaller main jet (i.e. change from #175 to #165 or #162 for 520 EXC).

you don't have much to fear by going straight in with all the jetting figures that riders have come up with.

i argued that the MJ had nothing to do with a 'D' becoming an 'E' needle but alas there appears to be an element of truth in it.

something like


160 MAJ

ELN on c4



60 SJ

if you have an open pipe i think that 162 MJ will do and if a quiet pipe go for 158MJ.

that should work first time out.


I ordered a set of main jets and pilots about a week ago. I'll be ready to try out the various combinations when they arrive. I'll post again if I get into trouble. Thanks guys.

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