perfect jetting.... look to help me :)

[jeremy_283]

i am going to try for perfect jetting............ everything thing is great right now but for when i am at idle and slap the throttle all the way to wide open it will die before even hitting mid rpm's......... which circuit should i check for this?

[boricua]

try stopping before hitting full throttle, i.e. between 1/4 and 3/4 throttle. if it hesitates at all in this range, you might to adjust your needle. its trial and error..... good luck.

[ryanh250ex]

your jetting will never be perfect. theres too many variables that affect jetting that change from day to day to have consistently "perfect" carburetion.

that said, i'd check your main. sounds like you're lean, if its dieing out on you when cracking the throttle wide open

care to tell us what jetting specs you have currently?

[jeremy_283]

sorry........ i get dont like it when people ask jetting questions without listing their current spec's but then i go and do the exact same thing.......... I have Dynojet 170 main clip in the standard position 42 pilot 1500 feet above sea level....... temperature is right around 75 F right now........... K&N powerlid........ full yoshi exhaust.............. tat shuld cover her :)

[cals400ex]

are you using the stock needle or the dynojet needle? i would for sure go with a smaller mainjet if it was me.

[GPracer2500]

Bogs, stumbles, misses, etc. that occur when the throttle is whacked open quickly from idle is often (usually?) the accelerator pump circuit. I don't believe it is a tunable circuit on a stock 400EX carb. Note that not all carbs even have an AP (the 400EX carb does though).

In many cases such a bog will only surface when the engine is in neutral and you're really whacking the throttle open with vigor but won't really happen much (or ever) in real-world riding.

If the bog is troublesome you can try turning the fuel screw a 1/4 turn in one direction or the other and see if that helps. Also, turning the idle speed up a few hundred RPM can help.

The AP is tough to tune perfectly--like I said above some AP equipped carbs don't even have any adjustment points for the AP. Even carbs that do have leak jets and AP timing adjustments can be hard to get perfect.

You can read up on AP tuning here: http://www.thumperfaq.com/ap.htm. It's specific to the FCR carb on Yamaha dirtbikes but much of the info is relevent to all APs.