250ex and top end problems

[Maxz]

I am thinking my problems is with the main jet. Here's the breakdown. 250ex with White Bros. E-Series Slip On, K&N Air Filter. The first main jet I ran during initial jetting was a 96, 1 bump up from the 95 stock. This was the directions in the White Bros Instructions. I had the mixture scre turned out 2.5 times.

It was probably 95% dialed in, minor sputtering, once it got hot it tended to idle semi-poorly and die when you began to give it gas, the plug was a little dry-black (Rich) also at full throttle it would seem to reach a tope speed and then stumble or reach an rev limiter? (is there such a thing?) This got more noticeable after a long day of riding a week or so ago.

so I was attempting some fixes tonight, I turned the mixture screw so it is only out 2 times and this I think fixes the idle problem.

I am now trying to get the top end dialed in. I originally thought I would have to go down a jet to 94 to fix it based off thinking it was the "rich stumble" I was experiencing. So I tested a 94 jet (I figured it wouldnt work seeing how stick was 95) same thing. So I tried a 98, same thing. Now I am thinking keep going higher? Rev Limiter? Whats up? Does anyone have any suggestions or help?

[White_Knuckles]

You're on the right track. The 250EX definitely has a rev-limiter easy to hit in the first 2 gears. The fuel screw should be out 2-2.5 turns from seated to richen the idle circuit and correct for lean out pipe backfire. Your main jet could land between 100 – 105 if you pulled your airbox lid or drilled it. 98 if your lid is stock.

Reading the plug is tricky unless a proper “plug chop” is performed. Try this method it works great.

The Roll-Off technique is the quickest and is almost as accurate as the Roll-On method. First, one gets the engine warm on the way to a safe roadway. If there is room, use fourth gear as this allows more time to assess the result.

Now, get the engine rpm high enough that it is on the cam and in its power band. This may need to be as high as 4000 rpm with some cam choices. Apply full throttle. Let the engine accelerate for a couple of seconds until it has settled in and is pulling hard. Quickly roll the throttle off to about the 7/8ths position. When you do this, the mixture richens slightly for a second or so.
If the engine gains power as you roll the throttle off, then the main jet is too small and you need to fit a larger one.
If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you need to fit a smaller one.

[redroost85]

White Knuckles, thanks for all the free info! You seem pretty adept at this and I am having somewhat similar problems please help if you can. Here is the very short part of my problem.

At the track the other day (first time I have EVER had any mechanical problems with my ride....yes I dropped the f-bomb a few times!) Under acceleration the motor would cut out, like I would floor it and it would hit and miss until I put it in neutral and it would idle fine, but put it in gear to go and it would happen again. I am going to change the spark plug boot and the plug just because I have them as spares. Is this a fuel porblem, jet issue, I don't know WTF! Please help any thing is appreciated! Thanks again dude! If you need any more info, I will supply! :devil:

[Maxz]

Well I am off to do some jetting again. I am going to try running a 100 main with the current setup I have, then if that doesnt work try a 102. I usually run with the air lid on but will try it without, I dont play in the mud or water unless I go to badlands so I wont have to worry about that. Hopefully my top end "lull" will dissapear.

Thanks White Knuckles, if anyone else can suggest anything also to help myself and redroost85 we would appreciate it. I will check back later this evening and give an update.

[White_Knuckles]

R-Roost: sounds like Mr. lean is the likely suspect. This may sound lame but I missed it as well. The fuel tank petcock controls a filter screen wrapped rod that extends up 4 inches into the flat bottom tank. "On" is the top half of the tank, "reserve" the bottom half. So if you pull the gas cap and look it seems you have lots-o-gas. If the tank is set to "on" and near or at half (which looks fine) it will run as you discribed. I now run on reserve always so the carb is supplied with fuel and no surprizes.

I opened the carb drain screw and switched back and forth between the two settings trying to figure out why "on" with what seemed to be 1/2-3/4 tank (visually) was "f-bombing" my motor. Too many years with dirt bikes having a very small reserve.

If your machine ran good and then started a new symptom something changed. My money is on a fuel issue before ignition. PM me for further diagnostic tips.

[redroost85]

White Knuckles.......aka EX Master! Ahh, thank you very much. I have a pretty good idea what you are talking about. I have replaced the plug boot, just because I guess. I will attempt to look at the fuel issue and carb some more. This is great stuff man. I will let you know how all this turns out in a day or so when I head back to the track. Thanks for all your EX wisdom!:D