Motorcycle Repair: 2004 KX 125, air fuel mixture, crisp throttle response

Question
hi,
I just got a procircuit works pipe and moto tasinari vforce 3 reeds for my bike, It ran really good for about 3 days then it still had a little power because it would rev in to the start of the powerband but no higher (and higher is when it make its best power) and if i tried to rev it any higher it would kinda pop and sputter.
i worked with it for a few days playing with combos of the stock and afmtermarket parts and finaly i got it to run right w/ stock reeds and whih the pro ciriut pipe, it dident run nearly as good as it did with both the pipe and reeds but at least it ran ok.

well between the days it ran good and bad i never changed any thing
i am just figureing that i need to rejet it to make it run properly
i have a 410 main jet and a 35 piolot jet the needle is on the 3rd pos the airscrew is 2.5 turns out (stock).
i use klotz r50 oil @ 40:1  w/ a ngk br9eg plug

do you have any ideas what the problem is?


thanks
Josh
D-A-S-H@comcast.net

Answer
Hi Josh,

The problem lies within tuning the air-fuel mixture to obtain the optimum mix. You're on the right track. Install the parts you desire and start tuning the carb one change at a time until you get crisp throttle response and the engine makes power through out all ranges of the carburetor. If you make more than one change at a time, you won't know which change caused which effect. You'll end up  starting over and re-doing the work, or giving up all together.

I cannot tell you the "right" jets to drop in. It doesn't work that way. We all must read the powerband and spark plugs to tune. At racetracks such as Daytona or Talladega, I use expensive weather equipment and formulas to get the air-fuel mixture matched to the elements of the hour. Otherwise, it's usually repeated on and off with the carb and plug until it's right.

Read spark plug colors to know what effects the changes have on the air-fuel mixture. Ideally, you want light shades of brown on the plug's insulator.


I suspect the carb is running a little rich as described.

If you need assistance, email me: mshively1@woh.rr.com

Respectfully,
Mark Shively