[sak384]

My 11yr old son is new to riding 2 wheels. He is having issues with takeoffs and has not mastered feathering the clutch. If I raise the idle a little higher than norm he does much better. Riding the trails with him can be a bit tiresome to get off my bike and kickstart his
The bike is pretty much dialed in. New piston and rings. All valves and valve seats checked and replaced. Only 7hrs on since rebuild. Runs flawless.
But for some strange reason I can't get 500 rpms extra to stay steady. The bike per the manual says 2000 -2100 rpm for idle and no matter what slow jet I use it stays below 1500rpm. If I increase the idle screw it goes over and the idle hangs. If I adjust idle lower it goes it goes from 1150 - 1500rpm.
I have tried every slow jet from 40-52. Currently using a 48. Adjusting the fuel screw has some effect but will not get it spot on. Too much tampering with the fuel screw causes throttle response issues.
I pulled in the hot start lever ever so slightly while idleing the bike to lean out the idle mix and the rpms went up!!!! I could even get to the 2100 rpms steady. This got me thinking that I have the correct slow jet for starting but the "slow air jet" may be a bit too small.
I figured i was too rich for idle so I went down in slow jet size. If I go too low the bike has issues starting and runs lean. Too high a slow jet and starting issues again.
I want to try using a slightly larger slow air jet or a adjustable slow air screw for this carb. If i can slightly lean out the idle fuel mixture using the 48 slow jet i believe i will get a higher idle rpm's higher and steady .
Does anyone have any experience with a adjustable slow air screw for this FCR carb. What else am I missing? Below are my current jetting info and temps.

Year: 2007
Elevation: sea level -200'
Temperature Range: current 65
Backfire Screen: yes
Air Filter: twin air
Pilot Jet: 48
Main Jet:140
Needle: jd red
Needle Clip Position:5th from top
Fuel Screw Type: flex screw
Fuel Screw Setting: 2.25
AP Modified: yes
Leak Jet: 60
AP Cover: QS2
Exhaust Type: stock
Exhaust Modified: fmf powercore 4

[riddler9]

I battled with my son's '08 for a while. Finally traced the hanging idle to a bad carb boot (engine side). I will also add that with some guidance from this site, I was able to get things dialed in, but also learned that the 150 has a rather small margin for adjusting the fuel screw and idle before the idle surges and hangs.

Unless you put the flex needle in with the carb off the bike or used needle nose of similar to help 'guide' the needle into place, something else to look at is the flex screw on the 150 can have a false fit feel. Meaning you think it is in but if you give the needle a tug it just may pull straight out.

[crfsonly]

any chance that you installed the floating valve upside down?

are you sure there's not an issue with your fuel screw? does your fuel screw have the spring, o-ring and washer installed correctly? i once had a small piece of the previous o-ring lodged in the fuel screw area. it caused similar issues.

do check the boots and make sure they are well seated.

ken

[sak384]

Thanks so much for responding guys. The bike is running flawless in all other respects. No hanging idle and perfect response.
Has anyone ever installed or used an adjustable slow air screw in place of a fixed slow air jet?
On sudco's website directly below their listing for the #100 slow air jet which is installed in the fcr mx carbs is a listing for an adjustable slow air screw.
This slow air screw gives you the ability to adjust the slow air circuit richer or leaner by turning the screw in or out. While the #100 slow air jet is fixed. (i.e, no fine tuning). I wanted to give this a shot.
The other possible cause for my dillemma may be due to a incorrectly set up or faulty tach device. Maybe I am already at or above the 2100 rpm threshold. I am using a works connection tach gauge. I thought I followed the directions carefully. I am using the 1.0 scale. i wrapped the sensing wire on the tube like their you tube post shows.
I know that these are really not the best way to test rpm. Does anyone have any ideas on how I can confirm that this tach is working properly?