250ex jetting.

[250EXking]

im running a wb e-seires with 10 disc. 112 jet, with kn air filter, 2 1/2 on air screw. my darn bike keeps back firing everytime i let off the gas. it pisses me off because its loud when it back fires. ive tried everything to make it stop but it doesnt work. any suggestions. i know they back fire once in a while but its all the time. i need some help.

[honda270ex]

same thing happened to me. i put a larger jet in and i adjusted the idle speed screw wabout 1 turn out

[White_Knuckles]

King, remove some of those discs to increase back pressure. Try almost another turn out on the fuel screw closer to 2 & 3/4 out. Try lowering your idle speed screw to the slowest possible idle (this worked for me). I've heard others claim raising the idle speed helped. In all cases it will never completly go away just find where it's reduced.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This Happens:
1) 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.
2) 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.
3) 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.
4) 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.
5) 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.

[White_Knuckles]

I have a Procircuit T-4 and it backfires mainly on quick throttle(snap) let-off. If I roll it off it rarely backfires. I thought this was something I could correct so I set out to fix it. After a lot of experiments I became frustrated in missing the magic. I called Procircuit and spoke with a lead tech with real world Dyno time. He explained the above posted theory of scavenging exhaust and the dread (normal) backfire result.

Our home built hot rods (of the '70's) were plagued with Ka-Blam backfire when we ran glass packs straight off the headers. It all came back to me and I realized the magic fix may not be there.

If you find a cure please post it as others (like me) have resolved it's not gonna happen. 400's are different and may work with a larger pilot etc., been there, tried that, no go. I do think on your pipe 10 discs are close to free flow so try 5 and see if the result is your goal. I doubt you will see a performance hit.

[ryanh250ex]

i have the mods listed below and i had the EXACT same problems as you.

here is what you do (takes 5 minutes). open up the carb top (on that's right, the top of the carb- very good! :lol ). take out the throttle valve. while compressing the throttle valve spring (pressing it as much as possible to the carb top), remove the throttle cable , then flip the throttle valve upside down and you'll see the end of the throttle cable...the end is a little metal thing which i call the "Ball"....while you have the spring compressed manuever the cable so can move the ball out of its current place, up through the slit in the valve and out of the round part. after that you can decompress the string and put it down somewhere. now you need to take off the jet needle retainer....that's the little white thing inside the throttle valve that holds the jet needle there. take a screwdriver try to pluck it out (this is probably the hardest part, besides putting it back in).....after that, push out the jet needle. the needle will have a little clip on it called the "e" clip. there should be 6 grooves on the clip in all. the stock position is that the clip is on the 3rd groove from the top. try moving it down 1 groove. i had the exact same problem as you, i went ahead and moved it 2 grooves and while it did get better, it's still there.

try that, and see what happens.

[White_Knuckles]

ryanh250ex, good tip- richening the mid-range by raising the needle. It's richening the low idle circuit and the path to it (mid range) that can prevent the lean scavenging Ka-Blams.

He should know that you have a Dynojet/K&N needle with extra grooves. The stock needle has 3 grooves and ships in the bottom most position. In other words he can't raise the needle only lower or lean the mid-range.

Try the low idle speed trick I mean blub, blub, blub ...way low. A friend raised his idle rather high to prevent backfire on a full race cafe bike. I didn't have luck with that, it just idled high and backfired. Grrrrrr :mad:

[250EXking]

thx for all the suggestions knuckles and ryan. ill adjust the needle and take off some dics see how that works. i guess its a never ending battle. can to much backfiring hurt the engine thought. thats my only concern.

[White_Knuckles]

Naaa, just hurts your ears. Not a problem, I've heard it can burn your exhaust valve - not true, a super lean condition and straight through pipe can fry the valve though. The pop is contained within the pipe.