where are the air fuel screws on a 250ex?

[ryanh250ex]

the title of the topic says it all. i think i want to adjust the air screw to eliminate the backfire i get when i let off the gas. i know it isn't the pilot jet, because i put in a larger one than stoc. starts much easier now but i still get a backfire when i let off the gas. not a loud one, but it crackles somewhat.

any help would be appreciated.

ryan

[White_Knuckles]

The fuel screw lives under the carb. Look for the recess on the float bowl toward the engine side. It's reachable by removing the left side plastic and is facing straight up. Make sure you count full turns carefully, 1/2 turns are often counted as 1. Seat the screw gently inward and come out 2.5 for the stock pilot 1-1.5 for the 40.

You should have seen it when you pulled the float bowl and added a 102-105 (Keihin) main jet. Uh, you did bump the main up didn't you? Smaller size main like a 98-100 if you still run the airbox lid on. Open up the snorkle end to increase air flow. I also added 4 UNI pop-in filters into the lid.

You may never totally eliminate popping on throttle transition from high RPM to idle with a high flow pipe like the T-4. Richening the idle circuit by adjusting the fuel screw will make it much better.

Mine still pops and gurgles a little but the big BANG's went away. If you have off idle stumble go back to the stock pilot.

[ryanh250ex]

Originally posted by White_Knuckles
The fuel screw lives under the carb. Look for the recess on the float bowl toward the engine side. It's reachable by removing the left side plastic and is facing straight up. Make sure you count full turns carefully, 1/2 turns are often counted as 1. Seat the screw gently inward and come out 2.5 for the stock pilot 1-1.5 for the 40.

You should have seen it when you pulled the float bowl and added a 102-105 (Keihin) main jet. Uh, you did bump the main up didn't you? Smaller size main like a 98-100 if you still run the airbox lid on. Open up the snorkle end to increase air flow. I also added 4 UNI pop-in filters into the lid.

You may never totally eliminate popping on throttle transition from high RPM to idle with a high flow pipe like the T-4. Richening the idle circuit by adjusting the fuel screw will make it much better.

Mine still pops and gurgles a little but the big BANG's went away. If you have off idle stumble go back to the stock pilot.

actually, i wasn't the one who rejetted it. i had a shop do it for me. i know for sure that they put a bigger pilot in because i was having some trouble starting up the machine. it starts very easily now.

the main jet has been replaced with a bigger one, but what size i do not know. i am running the stock lid (no holes or anything), and the stock filter.i really don't think the main jet is the issue, as it only does it when i get off the gas, not when i'm on it. when i go from simply idling along in gear to nailing it WOT, there is no hesitation. when i initially had the bike re-jetted (it has been jetted twice now, since they were a little ways off the first time around), there would be a sputtering and then it would take off. i'm pretty sure the combination of the bigger pilot and a different main (not sure if the main was initially lean or rich, i'm thinking rich) was what fixed it.

how do you remove the plastic? i once tried to pull off those fasteners but i couldn't get it.

which way do i turn the fuel screw to make it richer? i dont have to take apart the entire carb to get to it, correct? can i simply turn it with a screw driver?

the backfires only really happen when i go from high rpms to idle. like i said, they aren't huge bangs like they were before, but they can be heard. and i'd like to keep the crackling/popping/gurgling to a minimum before i tear apart the bike and discover that my piston is toast.

sorry for so many questions, but all this jetting stuff is new to me. thanks for the help.

[brokeitin3]

it is not necessary to remove any plastic to adjust the air screw. and its easier to do while the engine is cold. you need a short flat screwdriver to do it tho.

[ryanh250ex]

but does it matter what i adjust? i would think that increasing the fuel would give the same results as if you restricted the air.


ARG!!!! sorry, but i'm just confused and i wanna do this right the first time out.

[White_Knuckles]

Okay adjusting the fuel screw outward will richen the idle circuit which is a must for those who have the stock pilot and are trying to reduce backfire. In that you have a shop installed larger pilot it does the same thing as cranking out the screw - richens, so... you may try slight adjustments to further increase idle mixture to a richer yet mixture but I doubt you will eliminate backfire.

Read carefully: The backfire will never go away on the T-4 on throttle let off. The disc pipes like white bros or stock baffled pipes have more back pressure and are more forgiving but still gurgle on let off. The pops even loud backfires are contained in the pipe and will not damage your engine, exhaust valve or piston in any way.

To benifit from your pipe however punch some holes in the airbox lid, open up the snorkle end and check the main it has a number on it use what I suggested in the other post. The thing will pull secomd gear power wheelies and haul arse. Trust me, the 250EX I modded for my kid is amazing.

[ryanh250ex]

Originally posted by White_Knuckles
Okay adjusting the fuel screw outward will richen the idle circuit which is a must for those who have the stock pilot and are trying to reduce backfire. In that you have a shop installed larger pilot it does the same thing as cranking out the screw - richens, so... you may try slight adjustments to further increase idle mixture to a richer yet mixture but I doubt you will eliminate backfire.

Read carefully: The backfire will never go away on the T-4 on throttle let off. The disc pipes like white bros or stock baffled pipes have more back pressure and are more forgiving but still gurgle on let off. The pops even loud backfires are contained in the pipe and will not damage your engine, exhaust valve or piston in any way.

To benifit from your pipe however punch some holes in the airbox lid, open up the snorkle end and check the main it has a number on it use what I suggested in the other post. The thing will pull secomd gear power wheelies and haul arse. Trust me, the 250EX I modded for my kid is amazing.

thanks alot for the help.

i have recently noticed how straight through this particular exhaust is. once you have the bend around the cylinder head it's essentially a straight pipe.

i'd love to mod my EX more, but i think i'm done. i'm almost 16 and as soon as i can save up enough money i will be buying junky dirtbikes, fixing them and then selling for a profit, and then i will sell my 250, and then i can afford a nice used 400ex.

with the mods i have now, i can actually beat stock and piped blasters (could be the rider skill though), but what kills me is the suspension. i can barely keep all 4 wheels on the ground when i'm flying on the track. sure, i could buy aftermarket shocks, but the stock 400ex shocks would easily out perform even the best shocks out for the 250 (elkas, which are a rare find and cost a fortune). and after riding 2 400ex's in the last month, i know this for a fact. the 250 only has so much potential.

enough of my babbling. thanks alot

Ryan

[WhY_ZeE]

By the way guyz where did you pick up your pilot jet #42 ??
I visited two honda dealer and none are able to get one... :confused:
Honda don't make them right ? They make 38-45...
any help please .. :huh

[WhY_ZeE]

*talking about the 400ex...

[White_Knuckles]

By the way guyz where did you pick up your pilot jet #42 ??


Re-check with your dealer (they lie like dogs) Keihin shows 38,40,42 on their site, get ready for $18-20 a pop for pilots. Perhaps repost this as a question in a new thread.

[WhY_ZeE]

Okay i'll give it a try