Poping 250ex

[250EXking]

i have a 2002 250ex. mods at the bottom. i have a poping problem. i can rev the crap out of it at idle and it doesnt pop. but once i hit like third of fourth and let of it justs pops all to hell. i have a 106 jet. 2 down on the needle. stock piolt. 2 1/2 turns on the air screw. what do i need to do to make it stop poping. ps. white brothers pipe with 12 disc.

thx kyle.

[White_Knuckles]

By poping do you mean muffled pops or loud gunshot backfiring? Two down on the needle means clip position toward the pointed end effectivly raising the needle position? Is this the stock needle or K&N/Dyno?

Total backfire elimination on a freeflow pipe is nearly impossible. The e-series is more forgiving than others as removing discs increase backpressure helping but the quick let-off is the cause.
2.5 turns out seems to be the best setup, 3 or more will cause off-idle stumble - you will feel. A larger pilot may work on the 400's but not for yours.

I run a 105 Keihin main, stock pilot & needle, 2.5 out on the fuel screw with a T-4 and it will backfire under the same conditions. I found setting the idle speed low as possible helped but the 2.5 idle mixture adjustment helped the most. She'll still pop every now in then under high RPM to idle transition but the big bangs went away.

[White_Knuckles]

Mine does the same thing, it won't duplicate the problem in neutral only in motion (gear selection doesn't matter you can pop it in first or second if you try). I did all kinds of jetting tweaks and I found many ways to make it worse. I think your mid-range setting on the needle e-clip position is wrong, if it's the stock needle move it to the lowest (toward point) position. This is what convinced me why it occurs (from Pro Circuit):

When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high airflow rate through the small gap formed by the throttle valve and carburetor throat.
Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.

[cals400ex]

look at the spark plug and see what color it is. what is the factory mainjet on the 250ex? with the needle, did you move the clip down two positions or the needle down two positions? the clip down will richen the bike out while the needle down two positions would lean the bike out. also, what brand of jets are you running?

[250EXking]

plug is fine. i moved the clip on the needle to the second spot from top and im using kn jet kit.

[White_Knuckles]

Clip should be in the middle or down 1-2 positions from middle richening the mid range. If it still is backfiring try removing 6 discs from the exhaust.

Post your results.

[250EXking]

ill try richening it. see the thing is that it just started all the sudden. it was fine all winter and at the beggining of the spring here. one day i get it out and it starts popping when i let off the throttle. i might bump the needle down and richen it to see if it helps. thx white knuckles.

[White_Knuckles]

Could be the change in air temp. Make sure your backing out the fuel screw 2.5 turns from lightly seated. Some of us count 1/2 turns as one. ;)

[250EXking]

thx knuckles ill try it

[mudderchic]

It's more than likely a jetting problem. It took me a long time to correctly jet my 250ex. Before I did though, it was popping in different gears depending on the jet i was using. The 250ex is VERY picky when it comes to jetting.

[BaD_250ex]

Where is the fuel screw located at?

[sprtrx300ex]

Its on the bottom of the carb.

Does turning the fuel screw on a 300ex only adjust the rich/leaness of the bike at an idle or throughout the RPM range?

Im a bit confused. :confused: