I dont think it's lean with a 125 main jet. The stock is a 92 and I'm running a 112 and thats running on the rich side. I'm sure you need to go down. Even with a 112 you need to add more air in by venting the air box and replace the stock restrictive air filter with a UNI-filter.
My 250ex runs and idles perfect. It blows away my friends stock 250ex and he cant understand it.[img]i/expressions/face-icon-small-happy.gif[/img]

 

First of all what does your application with the pipe call for with or without the airbox lid...If you are popping then that means it is a lean condition...Or it could just be that you are running out of gas...check the simple stuff first and try a few different jets to see what seems to work best...And how many turns did the hmf directions tell you to go and is it set at the correct setting...get back to us

 

OK guys, Thanks for all of the responses. The HMF info did not include any specifics... the kit came with a 125 main jet but no instructions. After installing the pipe, I tried running it with the stock main and the bike was starving for gas and popping terribly. I then changed to the main jet and the bike ran better but bogged really bad at 1/2 throttle. I also have the fuel screw set 2 turns out. I then drilled several holes in the airbox lid with little improvement. I finally removed the lid completely and that eliminated most of the bog. It runs much better now but still has some bog and some popping. The bike definitely starts and idles much easier than the stock configuration.

I wrote to HMF and they actually recommended a larger main jet which I have not tried as it defies logic. I have also been told by the jetting expert on ThumperTalk to lower the needle (raise the clip 1 position). I haven't tried that as the bike is running pretty good now and my son has been riding his dirt bike more than the 4-wheeler. I'll get around to it one of these cold rainy weekends.... Thanks for all the suggestions.

 

The 250ex is a tricky little quad to jet after an after market pipe, filter and airbox vents have been done.

I put an FMF Megamax II on my wife's with a Uni filter and 4 airbox vents. I went up two sizes on the stock main and out 1/4 turn on the air/fuel screw. It seems to like that set up.
I found that the 250ex doesn't like removal of the airbox lid no matter how you jet it.

 

A couple things I have learned regarding this stuff. All of it is just read from various forums and my own logic so take it with a grain of salt.
1. The HMF setup was designed for the 2005 not 2006. I understand it will work, but may be an inch or two short. Again, I don't even have an HMF pipe, but I read this in the forums on HMF's site.
2. The Jet that came with it may in fact be for a 2005. Dynojet make two seperate kits for the 2005 and 2006. This is just my logic that they must be different, otherwise they would only have one kit.
3. And I did eliminate a lot of the popping by adjusting the idle screw. Not all, but a lot.

Personally, I'm still trying to figure out how to adjust the fuel mixture crew. I guess I'll try the suggestion of taking the screw out with needle nose pliers and making my own slot for a flat blade.

Again I am new to all of this stuff and I'm just passing along the info I have found.

 

Setting the Fuel Screw / Pilot Jet "by Ear"

Adjust the idle with the black **** until it is too fast. Then adjust it back down until it is around 1900-2000 RPM or if you don't have a tachometer (see below) until it sounds just a little high.


Before you start adjusting, count the turns required to tighten it up lightly.

Then start the bike with the slightly elevated idle and turn it out 1/4 turn, 1/2 turn, 3/4 turn and so on until you get to 2 turns. Listen for best RPM and best response to a quick 1/4 turn tweak of the throttle at each position of the fuel screw.

Now turn back in 1/4 turn at a time doing the same thing. By now you should have been able to distinguish the speed of the idle and the responsiveness to tweaking the throttle.



If it gets better between 3/4 and 2 turns out, set it at the best location and leave the rest of the pilot circuit alone.

If it is getting better turning it in or is best less than 3/4 turns out, replace the pilot jet with a smaller one and go through this procedure again.

If it is getting better as you turn it out or best at more than 2 turns out, replace the pilot jet with a larger one and go through this procedure again.

Here's how I do it
Just turn up your idle to about 1800-2000 rpm using your ear. then turn your fuel screw in or out until you hear where it runs it's best then turn back down you idle screw.