03' 250ex pops a couple of times when letting off gas

[RazBarlow]

When you let off the throttle to change gears or going down a hill the engine will pop a couple of times . Is this running a little lean ? Or how do I make this quit ?

[tedwilley]

Sounds like you are running lean.
First read the FAQs, then search jetting in this forum.
Lots of post about jetting, you might want to read "Explaining the mysterious fuel screw" too.

[RideRed84]

My new 03 is doing the same thing, I'm gonna back the mixture screw out about half a turn and see if that helps.

[250exkid]

if its poping its rich back the fuel screw out a turn or 2 and that should work or raise the fuel needle 1 or 2 notchs

[rev-man]

it is runnin lean my 07 does the same

[RideRed84]

Yeah, if it's popin it's running lean. They usually come from the factory a little lean. And on the 250 turning the screw out richens it.

[250exkid]

hmm... iv heard the exact oppisite. like when it pops its not burnin up gas as it comes through so it back fires cuz if its running lean its gettin to lean and will run with power but will run really hot and then soo burn up

[RideRed84]

I though the same thing but I asked a few people and looked it up on wikipedia
http://en.wikipedia.org/wiki/Back-fire

[tedwilley]

When they are running rich enough to backfire it is usually along with stuttering and overall bogging. Lean causes it to run hotter and the gas in the cylinder goes off when it is not supposed to, like on decel.
You can talk yourself out of getting the jetting right, but it will eventually getcha!

[stew2010]

i had the same broblem with backfiring as i let off.

i took a turn out of the air/fuel scew in the carb. problem solved

[tedwilley]

Yes, most of the time you can adjust occasional popping by using the fuel screw to richen the pilot jet mixture, which is in use from idle to 1/4 throttle. It only slightly affects the needle jet range 1/4 to 3/4 throttle (the main works from 3/4 to full throttle).
So, it depends on where in the throttle range you getting the popping...

[stew2010]

it only popped when i let off the gas completely for me

[250exkid]

oh i c! :bandit:

[prairiemonster]

The real problem is in the head casting, lots of rough casting slag left in the head that causes hot spot that makes the backfire.Porting or polishing will fix this problem for good.

[tedwilley]

I do not doubt the casting may contribute to the problem.
But, are you saying that the fuel mixture is not the main contributor in most cases?
My concern is that some readers may think porting and polishing is the only remedy and therefore beyond their ability or price range, and choose to not fix the fuel mixture first.

[ryanh250ex]

Originally posted by tedwilley
I do not doubt the casting may contribute to the problem.
But, are you saying that the fuel mixture is not the main contributor in most cases?
My concern is that some readers may think porting and polishing is the only remedy and therefore beyond their ability or price range, and choose to not fix the fuel mixture first.

I agree 100%. While I'm sure the casting is less than ideal, there is something to be said about people poorly tuning their bikes. My bike does NOT pop at all, with the exception of a gurgle with the use of hard compression braking. I will bet all day long that this is truly a tune problem, NOT a head issue.


Prairiemonster, have you owned or do you own a 250ex? have you torn it apart to re-work the head?

[prairiemonster]

No I do not own a 250ex and never did,I just get to work on one about 20hrs a week!
I did not mean for a port/polish to be cure all for the popping when you let off the gas.I agree that a good tune job will get rid
of most ,if not all the popping on about 90% of the quads.It's that other 10%, that no matter what they try with jets, needles or
mixture screws nothing works.I'm just saying what worked for us
that's all.
I have been into a few cylinder heads in the last 59 years how about yourself?Our 02 recon head has very little slag in the head from the casting molds it is not well tuned and does not backfire at all.The 06 ex head was so full of this crap you could see why thier would be hundreds of little hot spots in the head.When this slag falls off and it will where is it going to go,guys.
Getting off the topic ,sorry.

[Hammer trx450r]

Listen... turn the air screw out a half a turn if it doesnt help give it another half.


When your done and happy tell us how many turns the setting is from lightly seated. If your within 3/4 turn to 2 1/2 turns then your good to go

[tedwilley]

prairiemonster,
I'm only 55 so allowing for my first Briggs and Stratton tear down at 12, you've got me by at least 16 years.
I wasn't questioning your knowledge at all. In fact I agree, most production engines I have seen, from single lungers through V8's, are not well finished internally.
My concern was that some readers of this forum are, let us say more susceptible to getting the wrong ideas. You should read how many don't want (or need) to address having the right fuel mixture, and believe running lean is normal.

BTW readers, if you are offended, I wasn't talking about you....I meant the other guys...

[ryanh250ex]

Originally posted by prairiemonster
No I do not own a 250ex and never did,I just get to work on one about 20hrs a week!
I did not mean for a port/polish to be cure all for the popping when you let off the gas.I agree that a good tune job will get rid
of most ,if not all the popping on about 90% of the quads.It's that other 10%, that no matter what they try with jets, needles or
mixture screws nothing works.I'm just saying what worked for us
that's all.
I have been into a few cylinder heads in the last 59 years how about yourself?Our 02 recon head has very little slag in the head from the casting molds it is not well tuned and does not backfire at all.The 06 ex head was so full of this crap you could see why thier would be hundreds of little hot spots in the head.When this slag falls off and it will where is it going to go,guys.
Getting off the topic ,sorry.

Whoo-weee, I think I struck a nerve here. I wasnt doubting your experience. You have come into this forum and gave out all this info that very few, if any of us- myself included- have heard elsewhere, so it's only natural for us to question your experience. I didn't mean any harm.

[prairiemonster]

Hey, ryanh250ex no harm taken, without good questions you can not get good answers.I did not want you to think I was a young kid trying to talk some BS or something along those lines.I'm just an old man trying to give all of you some new and different ideas that might help you out.
I can understand your doubt in me being new and all but give me a chance .

[RideRed84]

Where exactly is the mixture screw on the carb.

[tedwilley]

Underneath, near the front center of the carb.

[RideRed84]

Sweet, Thanks

[tedwilley]

BTW, the screw head is shaped like the letter 'D'
You need:
1. The carb tool for this Kehin carb available online-I forget where right now.
2. Remove the carb (2 minutes max) turn it upside down and using a small flat screwdriver, turn the screw in or out (my method).

[tedwilley]

I knew that I had read a post with the source for the adjusting tool, thanks to ALEXANDERJASON on this..
http://www.exriders.com/vbb/showthread.php?s=&threadid=287951

[prairiemonster]

I had the same problem so what I did was after I removed the carb.I gatherd up every pump bottle I could find then I removed the tops to get at that long plastic stem.Find one that fits over the fuel screw or smaller and heat it up and force fit it.I cut mine about 3" long and just let it hang down that way you can go a few hundred feet stop tune and keep doing this till you get right.Oh your going to need a glove it's real hot in there.When your done pull it off and keep it for the next time.

[RazBarlow]

it looks like to me that a short small flat screwdriver would work. Am I not seeing things right ?
Thanks,
Raz

[Hammer trx450r]

I use a driver tip out of a screw gun that are like 1.5 inches long. works great

[F9mechanic]

Ok, guys I know that this thread has been posted a million times. I tried letting out the fuel screw like suggested, it made the popping worse. I reset the screw to the starting postion and it actually bottomed out. Now, my question is, Do i need to re-jet, since the screw has bottomed out?

[Hammer trx450r]

first thing is make sure no vacum leaks. Then put the screw at 1.5 turns out from seated position. Warm up bike. while running turn screw out 1/4 turn, did the idle go up? yes then do another 1/4 turn until highest idle. No, then tun it in 1/4 untill highest idle. When done if you are withing lets say 1.5 to 2.5 turns then your fine, anymore that that then change pilot