99WR400 popping

Sometimes when firing it up about an hour or so before a race, I'll let idle w/choke for about a minute or 2. Then I'll cut the choke off and let idle for a few minutes,then hop on to get it warmed up. Sometimes after cutting the choke off and giving a little gas it starts popping and running rough(like the plug is going). I usually have to ride it WFO to get it to clean out and run ok. Sometimes if it goes dead I'll have to change the plug or it'll never crank. It's happened at 2-3 races, once it took about 8 miles to clear out(1 lap). I typically put in a new plug before each race to be safe since some races are 3 + hrs away and don't want to take any chances.I bought it used about 2 years ago,and raced about 5 or 6 races,haven't ridden since(new house,kids,etc).I'm going through withdrawals and plan to get back serious soon and want to get this fixed. the add ons are Whit Bros E series Header and pipe w/ turn down tip,Hot start,and Twin-Air filter. I run 93 only. I tried cleaning the carb and it did seem to help, but the main problem persisted. I'm thinking about jetting it to see if that will help, sometimes it seems like its flooding out. I also heard that there are problems with the 99 carbs? Anyone experience this type problem?Any suggestions? Thanks

Hey bud, No jetting expert, but does sound like your running a bit lean. If you can let us know some details about your set up it would help.

Is the jetting stock, what elevations and temps are you riding at? Without knowing where your at, I'll throw this out as a start........If its down low (0 - 1/4 throttle, idle)your most likely lean on your pilot jet circuit, not positive on '99s, but stock on an '01 426 is a 42PJ. Where do you have your pilot screw set?? You should be somewhere between 1 1/2 - 2 3/4 out, if you need to go more than that, you'll need to go up one on your pilot jet. Guess what I'm getting at, is if WFO is good, your needle should be close, and your main should be right on.

Dodger :):D

Hey harescrambler,

I have a 99 WR also, had a similar popping noise.It was after riding a bit ,stalled, relit using the hot start. The bike then started to pop. It ended up being the hot start valve was not closing all the way so it was just like an air leak.

Only other time I had a fouling problem was getting to carried away with the filter oil, the oil bleeds then gets into carb circuit not good.

I've got the same problem as you, work prevents me from getting to ride as much as I'd like so I dual sported my bike. It's actually really fun,

good luck with the bike, TT threads will get you going.

When I mentioned WFO, I ment that its the only way to keep it running and from fouling a plug, eventually it'll clear up and run fine at all rpm's. I'm not sure exactly the elevation. I ride N. Alabama, TN, KY, & S. IL. Lower elevations I suppose. The last time I rode was summer 01(early Sept). I had cleaned the accelerator pump, Drained carb and tank. I haven't got into the carb yet(jets etc). I cleaned my garage last night and moved my bike out of my shop to a warmer place to work. I plan to remove the carb tonight and will see what type jetting has been done, if any. I started reading the manual last night because I plan on checking the valve clearance while I'm at it. This looks like a pain,is it? My last 4STK was an 85 XR600 which was simple. This is pretty much all I can give for now ,I'll have more specifics after tonight when I get it tore down. Thanks

I plan on checking the valve clearance while I'm at it. This looks like a pain,is it?

Checking........no, it's not really a bid deal, quite easy in fact. Now shimming on the other hand is supposed to be a whole nother story. I havn't had to shim yet, my bike was completely withing spec as of July...........dang it's been that long, bout time to check them again.

If the time inbetween your rides can be measured in "months", I would also start to suspect your gas. Gas doesn't stay good for too long, maybe a month or so for real freshness, or maybe even less.

Let us know what you find out, or at least post your jetting setup and elevation.


Dodger :D:)

One thing to look at is your pilot set up. Sounds like it may be too rich. What is your jetting specs? My bike was doing the same thing when the temps started getting cooler & I'm now in the process of leaning it all out. Check our Taffy's settings. Just search for "Taffy" and you'll see his jetting in his signature. I think he runs the same bike as you. It's really lean, but it's supposed to work realy well. You just have to do all the settings not just parts of it. Also look for Yamakaze, he's supposed to know a lot about jetting too.

Second, check your valves. I just checked mine for the first time since last march and everything is really tight. I need to decrease each pad/shim one side to get back into specs.

First thing, your 99 carb blows (sucks?).

Remove the octopus (air cut valve) COMPLETELY. The octopus interferes with setting up your pilot jetting system to work PROPERLY.

Install either the KL or Taffy mod to your accelerator pump. My pump stroke time was > 2 seconds (I have heard of 4 seconds!). The magic number is ~ < 0.5 seconds

Re-jet your carb. Bill (senior moderator) has decent jetting #'s.

The accelerator pump on the 98/99 is super wicked prone to seizing up with mud & water. This needs to completely disassembled and THOROUGHLY cleaned. When you disassemble this, work over a real good area where, when you drop the little o-rings (2) inside the AP, you will find them.

You should make it a ritual to VERY THOROUGHLY clean your carb every couple of months (dependent on your riding time on the bike). This does include the air jets. These reside under the air horn on the airbox end of your carb. Use carb cleaner and compressed air to blow ALL passages clear.

Setting Valve clearanc is very easy to do, along w/ shim replacement.

I have some questions to ask before I can give any suggestions for your jetting problem.

1st thing to do is check for air leaks. This includes intake and exhaust. This "octopus" another TT member was talking about is a contraption. You need to eliminate this. Just to eliminate air leaks 1st, Buy a vaccum cap from your local auto parts store 30 cents. Remove the octopus completly and cover the air inlet valve on the head with a vaccum cap. Start your bike and see if it stll pops. Next, if it is still popping put a light behind your bike and wrap it and see if its spiting out raw fuel immediately folowed by fire and a loud pop. I know it sounds strage but a WR with a worn Cam Chain will jump time. Have you done a compression test?

Tighten all air boots and Check that darn air jet in the bottom of the air horn and the filter side of the carb. This thing has a tendency to get crammed with crud and can be easily overlooked. You have to remove the air horn off of the filter side of the carb. Im not sold on cutting the duration of accelerator pump.

Thanks for the replies. Saturday I took the carb off and cleaned the accel. pump. I took the needle out and put on #4 which is the STD setting. I pulled the main jet and its a 172.Drained tank and carb completely.Put in a new plug. I had a race Sun. so I kept it simple so I could ride a little. I haven't messed with the pilot yet.I'll probably go to a 168 at first and try starting from stock. It ran fine Sat. but got to the race and cranked first kick, off with the choke, it sat there and purred. Get gear on, fire it up, take off and soon as I start to "get in it" the Popping started again! I think its rich, but if you've ever forgot to turn the gas back on after a spill, you notice how it starts missing and popping?Thats what mine does, So am I wrong and its actually too lean? If it is lean how is it fouling the plugs? I don't think its the cam chain, I bought it used from a good friend of a friend(I know,I know) and he told me that a new engine was warranted a few months before I bought it.Its very tight and strong compression(by feel). I'll try the removal of the octopus junk next. Thanks.

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