250EX air box & jetting question

[Ride4Fun]

My wife's Sportrax 250EX has been running a little rough lately. It sputters at high rpm. I checked the plug and it is running a little rich. The ATV is in stock condition except for a Twin-Air air filter. We typically ride at 5000' to 9000' elevation.

I don't want to do any expensive mods, but I would like to get it running a little better.

Is there a good way to open up the air box to get more air? Or would I be better off changing the jetting? If so, what should I change the main jet to?

Thanks!

[2fun]

1st off, I see that you are a newer member here (well, few posts anyway). Just wanted to say welcome (back) to the site!

As far as the jetting question, is the air filter clean? Sometimes a dirty air cleaner can make a bike run rich.

Also, from the altitude you are riding at, I can't help but think it is running a little rich from that also.

Do you have the ability to remove the airbox lid?

What kind of conditions and terrain do you ride in?

If it is too dusty or muddy, I'd leave the lid on and just go down one or two jet sizes. Might also have to drop the needle one position by raising the clip on the needle one position. This will effectively lean it out.

This is assuming everthing else is in good working order, and that the sparkplug is not too old or fouled.

[TRXman86]

YES, WELCOME TO THE SITE, HOPE YOU HAVE A PLEASANT STAY, BRING IN THE FAMILY FOR SURE.

i would check air filter also. you could buy some 1" air box vents from White Brothers. start with like one and work your way up. i would not go over say 3 or 4 since it is stock, this could cause more damage then good so be sure and jet accordingly. most often it is good to vent with a slip-on or full system exaust. the good thing with the 250ex's are there is no true way to get RAW power. this is why i dont like them, they remind me of an air compressor motor on wheels LOL. BUT EXCELLENT LEARNERS
to be honest, you may want to listen to other remarks besides mine. i am like a shade tree mechanic and learned as i went. 2FUN, both WOODY's, and the more top posters are good advisers. hope someone can help you out. but sometimes people try to help that arn't the best LOL like me. good luck

[2fun]

I think most of us have learned as we went.
That is sometimes the best knowledge. 1st hand experience..........

[TRXman86]

hey 2fun, that is really good to know. i am glad that someone else thinks that also. some have a problem with those who are not sure. i am by far the SMARTEST when it comes to tuning, but i am a little knowledgeable(SP) about somethings.

[Ride4Fun]

Thanks for the info. I just ordered a couple of air box vents. I think I will start with that. At my altitude it should help to get a little more air in (I do keep the filter clean). Once I get the vents in I will see how it runs and then change the jet or needle if necessary.

Where is the best place to install the vents? On the rear of the box? Or the sides? Or the top?

[hardlydangerous]

Usually the top is best.
if your not riding in the wet/muddy stuff the vents should work fine or even removing the lid. if you ride in the wet, drop the main jet down a size or three.
If the plug is brown drop 1 size
if the plug is black drop 2 sizes

Also the OEM foam filter may flow more air. Twin air filters are dual foam are they not (foam filter wraped in a pre-filter) I've seen a few cases where the dual filters restrict more then OEM.

[Ride4Fun]

Hmmm...

I bought the Twin Air filter because I thought it was supposed to give me better air flow. Maybe it doesn't, and that might explain why the ATV is running rich and it seemed okay before I changed the filter.

What is the best air filter to use? I have seen a lot of mention on K&N filters. Are they a good choice? Can I use my Twin Air oil on a K&N filter?

[hardlydangerous]

For the most air flow
A signle stage FOAM works best.
OEM is fine when oiled properly, for slightly more flow go with an UNI or Amsoil foam filter.

I've been testing K&N style VS foam for a few years along with a few other mechanics. IMO if your going to run dusty conditions use a foam filter with a GOOD foam air filter oil such as UNI or belray.

[Woody_YZF]

In most cases bikes/quads come from the factory jetted to sea level, which of course you are aware, we are much higher than that (I'm in Salt Lake too). I am no expert on the air filters themselves, but everything I've heard and read gives the thumbs up to the twin air. The air box vents should help quite a bit, but I wouldn't be surprised if you still need to go down a jet or two on your main. Keep us posted.

[Ride4Fun]

OK - thanks again for your suggestions.

I ordered the air box vents from Rocky Mountain ATV, and they will probably come today or tomorrow. I should be able to install them this week and then test it Saturday at 5 Mile Pass. Not the highest elevation, but it should still be a good test.

Is there a good way to clean the plug so that I can check the color after this next ride without prior contamination?

[2fun]

Originally posted by Ride4Fun
Is there a good way to clean the plug so that I can check the color after this next ride without prior contamination?

Seriously, I'd just grab a couple of new ones.
They are inexpensive, and IMO, are the best way to check for color.

I tried to use an old plug to check jetting on my 250R after I had it ported, yada yada.
The plug looked paper bag brown, but the bike was running WAY lean!

I took it home, and did a run at the house with a fresh plug, and it was white as white could be!

I wound up going like 7 jet sizes before I got the nice tan color I was shooting for!

So I say only use a new white plug for your plug check!
A new plug is cheap insurance!

I have several new spares at all times!

[Woody_YZF]

That would be my suggestion also. Go out and get you a new plug or two, it will be much simpler.

[TRXman86]

thats one area that i have been slacking off. i normally change mine after about every 3 or 4 races when i change the oil. well, you can say i have been slacking alot on basic maint. i just did clean my air filter, man it was bad.

[Ride4Fun]

OK, first I added four 1" air vents. It was still running too rich, as expected.

Today I changed from the stock 95 main jet to a 92.

It seems to run well (around the yard anyway), but it backfires when I decelerate.

What causes the backfire? How do I fix it?

[Ride4Fun]

Actually, I guess it is more of a popping sound rather than a backfire. Is that okay? Or is it a problem? If so, how do I fix it?

Thanks,

[2fun]

Sounds like it may be running too lean on the pilot circuit?
Sometimes a lean condition in the pilot circuit can cause popping upon decel.

However, I recently read somewhere that if the pilot circuit is too rich, backfiring can also occur.

So my FIRST recommendation would be to richen up the pilot circuit and see if it gets better (Go up on the pilot jet, or adjust fuel screw to allow more fuel in). Try this first as you have leaned out the system by allowing more air to flow in.

If richening it up makes it worse, then try going down on the pilot jet (to lean it out), or adjust the fuel screw so less fuel is metered.

[Ride4Fun]

I opened up the air box AND went down one size on the main jet, both of which will make it run leaner. Before any of these changes it would stutter at high throttle. Now it runs better, except for the popping on decel.

I haven't yet given it a good trail ride or checked the plug color. I will do that next time we go on a ride.

It sounds like my next step would be to adjust the fuel mixture screw. Should I turn it in or out? How far?

[2fun]

I am new to 4-stroke carbs (just recently got the YFZ 450), so bear with me, but I BELIEVE in order to richen up the pilot circuit, you need to BACK OUT the fuel screw.

If it were an AIR screw, you'd need to turn it IN to reduce the amt of air coming into the carb.

As far as the FUEL screw, I would start out with 1/2 turn at a time to see if it gets better.

[hardlydangerous]

Too rich causes a back fire on decel (usually one big bang)
Too lean caused decel pop. Quiet multiple poping sound

I agree with 2 fun. It sounds as though the pilot circuit is too lean and you are hearing decel pop.
He is also correct on most 4 strokes you turn the mixture screw OUT to richen witch is the case with your machine.
1/2 turn out at a time works well.