[Reece616]

I know basically everyone has a jetting thread, and i've tried searching but i cant seem to figure it out.
I put a aftermarket exhaust on my 04 250ex, and it was popping on decel.
I went to the local powersports shop and picked up a few different main jets as well as a larger pilot. (for the main, they didn't have any kiehn jets, but they had mikuni that fit, a 105, 110, 115, and 120) to solve the original issue, i figured i needed to up the main, so i tried the 105 to start and it was no good, so i jumped to the 115, and while i had the carb open swapped the stock pilot to a 45. I put everything back together and went to start it up and it fired right up. I took it for a little ride, and it was going good, slight hesitation at low throttle, but it was running and idling fine. I couldn't do too much on the throttle cuz it was late and people were sleeping though.
Today i went to test the higher throttle positions. It started fairly easy, and after a few i took it for a spin. The hesitation at low throttle became more obvious but the rest of throttle positions seemed good, no popping on decel. either. Then, all of a sudden, when going slow, it dies. Try to start it and it just cranks over. Pull the choke and it starts but wants to die when the throttle is released. Since then i have not been able to get it to stay running. It starts with the choke pulled but usually dies shortly after.
I tried playing with the pilot screw. I put it all the way in (lightly seated of course) and tried starting after each half turn out. I got it almost all the way out and the best i could get it starting with the choke pulled but it would die if off the throttle, and no start with choke down.
So, i'm stumped....
I'd like to add i'm totally new to carbs, but i've done a bit of reading.
Any input and help would be great. I dont really want to have to pay someone to do this after all i've done already.
Thanks in advanced.

 

[impaled]

what air filter are you running?

 

[DAN400ex]

ATV Carburetor Jetting Instruction - ATV Technical Articles

Spark Plug Colors Reading Instructions - ATV Technical Articles

I don't know a hole lot about jetting either, and like you I did a lot of reading and found nothing to help out so I finally joined this forum for help. Carbs are frustrating. It sounds to me like you pilot jet is screwing you over. I place a 42 pilot jet in my 400ex and had the same starting issue you are having to a lesser degree and had to play with my fuel screw. It sounds like you are running really rich and thats why it will fire up when you have the choke on. Also check you spark plug and compare it to the above spark plug link. I think if I were you I'd replace the stock pilot jet and go from there for the time being, you might just have a pilot thats to big but I'm not sure. Moose was of great help to me with my jetting issues, and there are a lot of great guys that should be helping you solve you problems shortly. Like I said earlier I'm not a jetting master but from what I've leaned so far and what happen to me personally it sound like your running rich in the pilot jet. Hope this helps.

 

[Reece616]

I'm running a uni filter.

I went back to the shop to get some different pilot jets to try.
My friend and I will be working on that later today. Ill post on my findings.
it did seem like it was too rich though.

 

[DAN400ex]

I'm running a uni filter.

I went back to the shop to get some different pilot jets to try.
My friend and I will be working on that later today. Ill post on my findings.
it did seem like it was too rich though.

Good luck brother. Hope it goes well.

 

[Reece616]

So it seems like most of my jetting problems are fixed but I have a new one.
When it's idling, if I push the throttle all the way or 3/4 real quick it like sputters and sometimes stalls. It seems like its not getting enough fuel, but it has no problem when I gradually push the throttle wide open.

 

[Redracer]

FYI, the numbers that Mikuni uses and the numbers that Keihin uses are not the same. A 105 mikuni is not the same as a 105 keihin.

KEIHIN DYNOJET MIKUNI
160 146 137.5
162 148
165 150 140
168 152 142.5
170 154 145
172 156
175 158 147.5
178 160 150

 

[Reece616]

FYI, the numbers that Mikuni uses and the numbers that Keihin uses are not the same. A 105 mikuni is not the same as a 105 keihin.

KEIHIN DYNOJET MIKUNI
160 146 137.5
162 148
165 150 140
168 152 142.5
170 154 145
172 156
175 158 147.5
178 160 150

Yea, I realized that and we tried to eye it for a basic size range. I looked it up the other day and found that all the mikuni jets I had were way too big. I was able to exchange them for proper keihn sizes and installed a 102.5 today.
it seemed to be much better but the quick throttle problem remains.

 

[TRX2fiveO]

you shouldn't have had to go any higher than a 100 on the main and up one size on the pilot for the decell backfiring bro. 105 in a kehin or mikuni is way too big your blip at 3/4 throttle isn't lack of fuel it too much fuel pull the plug and check it I bet it's dark

 

[Reece616]

I tried some different stuff today.
I changed the main down to a 100 (keihn)
and tried that, still got a ton of backfiring/ popping,
and the quick throttle issue remained.
Then i decided to try the stock jet (95), and that didn't have any difference in either area.

 

[TRX2fiveO]

the popping and back firing isn't from the main jet, that comes from the idle circuit remove the slowjet and clean it or go up one size.
You didn't by chance cut or remove the clear plastic tube that runs from the top of the frame to the air box did you?

 

[Reece616]

the popping and back firing isn't from the main jet, that comes from the idle circuit remove the slowjet and clean it or go up one size.
You didn't by chance cut or remove the clear plastic tube that runs from the top of the frame to the air box did you?

So the popping would be from being lean?
I went up on the pilot, it's a 42 now, i was planning on trying to open the pilot screw a little to see if that helped. I was gonna last night when i was trying but the engine was too hot to get my hand in there to do it.

the clear plastic "snorkel" is still very much intact.
Thanks for the responses good sir.

 

[TRX2fiveO]

did you cut it any? if you did put iti back like it was use duct tape if you have to and yes its way too lean

 

[Reece616]

The snorkel looks to be good. What would any cuts or anything on it do?

I adjusted the pilot screw about 1/2 to 3/4 turn out and it seemed much better

my brothers work 3rd shift so they're sleeping now so i cant really take it for a ride, but i'll post back when i can.

 

[1987_250x]

So it seems like most of my jetting problems are fixed but I have a new one.
When it's idling, if I push the throttle all the way or 3/4 real quick it like sputters and sometimes stalls. It seems like its not getting enough fuel, but it has no problem when I gradually push the throttle wide open.

Every quad or dirtbike I have ever owned has done this, but you will never ever find yourself in a situation requiring that kind of throttle response. I know with the 90 trx's it is a safety device so that some kit doesn't overreact and hurt them self. The 250ex's have a both a centrpetal and manual clutch, as I am sure you know. Maybe it has the same sort of device as the lil, 90's, but that I do not know.

 

[TRX2fiveO]

The snorkel looks to be good. What would any cuts or anything on it do?

I adjusted the pilot screw about 1/2 to 3/4 turn out and it seemed much better

my brothers work 3rd shift so they're sleeping now so i cant really take it for a ride, but i'll post back when i can.

The end of the snorkle at the top should only have about a quarter sized hole in the end of it any larger close it off, I just worked on one of these and I couldn't for the life of me figure out why it was popping back until I saw somone had cut the end off the snorkel, I closed it off and it cured the problem. If you're only 1/2 to 3/4 turn out on the air screw your idle circuit is partially clogged bro.

 

[Reece616]

Every quad or dirtbike I have ever owned has done this, but you will never ever find yourself in a situation requiring that kind of throttle response. I know with the 90 trx's it is a safety device so that some kit doesn't overreact and hurt them self. The 250ex's have a both a centrpetal and manual clutch, as I am sure you know. Maybe it has the same sort of device as the lil, 90's, but that I do not know.

Yea, i didn't notice it before but that doesn't mean it didn't happen before the exhaust and jetting. I'm probly just being picky and want it to be gone, it's fine if i just go a bit slower.

The end of the snorkle at the top should only have about a quarter sized hole in the end of it any larger close it off, I just worked on one of these and I couldn't for the life of me figure out why it was popping back until I saw somone had cut the end off the snorkel, I closed it off and it cured the problem. If you're only 1/2 to 3/4 turn out on the air screw your idle circuit is partially clogged bro.

I checked it out, the snorkel only has the little hole at the top.
I adjusted the air screw 1/2 to 3/4 out from where it was. I cant remember how far out it was before, i just further adjusted it.
If i do have further problems i'll look into cleaning it out.
Thanks for the help guys.

 

[TRX2fiveO]

No problem bro keep us posted

 

[Redracer]

So it seems like most of my jetting problems are fixed but I have a new one.
When it's idling, if I push the throttle all the way or 3/4 real quick it like sputters and sometimes stalls. It seems like its not getting enough fuel, but it has no problem when I gradually push the throttle wide open.

Every quad or dirtbike I have ever owned has done this,

That means every quad or dirtbike you have owned was not jetted for optimum performance. If your machine is jetted properly for temp. and altitude you will not have ANY hesitation throughout the power band. A spot that bogs would be absolutely horrible to a racer.

 

[1987_250x]

That means every quad or dirtbike you have owned was not jetted for optimum performance. If your machine is jetted properly for temp. and altitude you will not have ANY hesitation throughout the power band. A spot that bogs would be absolutely horrible to a racer.

I mean if you are sitting in neutral and suddenly press the throttle from all the way out to full bore. Must of read something wrong, or I'm just an idiot and have no idea what I'm talking about!:icon_ rofl: