Jetting gurus - help getting started

Need some jetting help from the crew here at TT...

1999 WR 400, White Bros Carbon E-Series with plate spark arrestor and 7 discs, stock head pipe, ride at sea level (Central Texas 0-600'), everything was running fine until a few weeks ago when I started fouling plugs.

Popped a new one in, bike ran like a top, started first or second kick, a little bog in the mid range but fine by me. Bike sat for a while, then dang hard to kick over, pulled the plug and found thick black coat.

I bought the bike used, previous owner appeared to take good care of it but I have no idea what was done to the jetting nor am I sure quite where to start to develop a benchmark.

Can someone offer some advice on how to start and/or share some secret targets for jetting? Ride mainly low gear (1-3) trails with some technical sections for low speed work.

I have searched the archives and found a ton of settings but not quite sure where to start or what would be best for my riding style and which way to go from here....

All help is appreciated!



If it was running fine then started fouling plugs chances are your problem is not isolated to jetting?

1) Have you changed anything else? (air filter oil, jets etc)

2) Have you changed the fuel you were using?

3) Is the carby blocked?

Swing the hole carby off and check it from top to bottom then test it all, fuel squirt etc. Make sure all the jets are clear.

Has the previous owner done the BK Mod? If so has the screws setting moved creating a longer fuel squirt and fouling the plugs.

If everything else proves ok then list your jetting spec's, needle etc and see if any of the guru's can spot any problems.

Put on a clean air filter and changed oil in September, rode one long weekend and then put the bike up until last weekend. No other changes in fuel, pipe, etc...

Always had a little bog in the mid range, carby does not appear to blocked but I will check this weekend. Was told the current jetting is stock, no BK mod, throttle stop has been cut, grey wire is intact, airbox lid is removed.

Haven't rejetted a motorbike in a long time, so curious to learn the best ways or methods to verify stock and test new settings.


When you clean out the carby record all the jet spec's

Needle - & clip #

Main Jet

Main Air Jet

Pilot Jet

Pilot Air Jet

Normally with an aftermarket pipe there are some jetting changes - though it is a little but trial and error.

If it sat for a couple of months possibly its just a bit of stale fuel, drain it and start again, try cleaning your airfilter again and make sure no oil has ended up in the carby.

I find that draining my float bowl helps even if only leaving for a few days - especially when transporting.

You may find it clears up after soiling a couple of plugs.

Good luck.


I am waiting for my OBELN needle from the shop and noticed that you run it. At what elevation do you typically ride? From a previous post of mine you stated that you were not completely happy with it yet, have you dialed it in yet? What's the difference in performance that you felt over the stock needle. When I put mine in, I will start with 3 clip, and everything else stock, then if anything needs to be done, I am figuring that since this needle is richer, I will start to lean pilot or main as needed. Any info will be greatly appreciated.

I am now almost happy with the setup.

It does not load up when riding at all.

The only problem I get is on extended idle - ie waiting at a gate or in a line to practise a jump and the plug loads up, you can generally clear it by giving it an open run but this is not always possible.

A couple of things I have been doing:

- shut motor off every time I stop

- drain float bowl before transporting

- clean plug between rides

I haven't had much time for testing lately but the last couple of things I want to test are:

- reduce the PAJ by a couple of sizes (jetting q's ratios)

- reduce the MJ by of sizes (have 160 & 155 in box)

Each of these changes are likely to help the performance.

I also want to do the BK mod but am holding off unitl I have enough time to do it and test it thouroughly. In the shed I have quite a bit of bog when snapping on from idle, though don't really notice it riding.

I think the BK mod would also help my idle problem - I have an old 2 stroke habit & tend to twist throttle while waiting = fuel squirt - if done a few times = lots of fuel = fouled plug on fancy 4 stroke.

I ride at various altitudes (guessing 0 to 1500m), but all pretty low. I have fitted a home made (Kouba style) adjuster on my fuel screw which allows half a turns adjustment here and there.

The performance after changing the needle is pretty noticable, much more direct mid range punch, where as with the old needle (DRR) it was less punchy - hard to loft wheel.

Leaner pilot, leaner main & higher clip (stock #4) where the changes I made - plus plenty of fowled plugs along the way.

Good luck.

With the 165 main jet and fuel screw at .5 out the plug was black around the edges and coughed a little on the second ride.

Ran over the weekend with the 160 main jet, open enduro loop, motocross and even a bit of messing around on the supercross track (no real tight stuff) not a fart or a cough so I was pretty happy.

Ran like a ripper, snap mono's, power slides etc - all good.

Only thing left is fuel screw which is currently only at .5 turn out.

I will check the plug and try 1 turn out later this week. still have the 155 main in the box.

Pretty close now.

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