Pilot circuit jetting question.

[yo_marc]

This is probably going to sound pretty dumb.  Bear in mind I think I have jetting OCD, and I desire a lot of throttle control for slippery woods riding.

I've been fooling with my jetting, and am running a different needle with a slightly (correction) SMALLER root diameter.

I'm getting a slight stumble at 1/8 throttle that I can't clean up.  Off the pipe it manifests as a 'tat tat tat', until the motor snaps to life.  On the pipe, things work pretty well, with only a slight hesitation.

I've heard that 'tat tat tat' called "four stroking".   Is that caused by a rich condition, (a lean condition?), or is it just a byproduct of the engines porting design and being off the pipe?

I tried many air screw settings, and the further in I went, the fatter and worse the issue was.  I tuned the air screw out, and it worked best at 2 to 2 1/4 turns, so I put in one size smaller pilot.  It helped, but didn't make it go away.  Still running the air screw around 2 turns out.  (Keihin PWK, FWIW)

Bike runs clean up on the pipe, and will come down very cleanly when you chop the throttle from a high speed top gear blast.  I think I am close, but not quite there on the jetting.  What would you do??

[MyckMcClung]

What is your altitude, year of bike, pipe type, and fuel octane?

[yo_marc]

Its a '00 RM250..  stock pipe and basically a stock motor.

93 octane pump gas, mixed at 32:1 

Running at 600 feet, and it was about 70*F out.

I'm basically running a '99 RM250 carb right now.  (needle and choke assembly are different).

Jetting is real clean and power is smooth and linear.. except under 1/8 throttle.

[MyckMcClung]

 Here are the specs from Pro Circuits  page

2000 RM250
MODEL      MAIN PILOT NEEDLE POS SCREW POWER SLIDE NOZZLE LEAK OTHER
RM250    158    42    N3WK   3RD   1 1/2    55    6              38 PWX KEHIN
Pro Circuit    155    STD    STD      STD    1 1/2                        CHOKE 3 TURNS OUT

1999 RM250
RM250    158    45    NIEG    3RD   1 1/2    55                   
Pro Circuit    155    42    STD      2ND    1 1/2               
     
FMF specs
    Main Jet
    155

    Pilot Jet
    45

    Needle
    STOCK

    Needle Position
    2ND

    Power Jet
    N/A

    Air Screw
    1.75

    Fuel Screw
    N/A

    Comments

[yo_marc]

Thank you!!

I'm running a 158 main, N1EG on 2nd clip, and now a 40 pilot.  Started with a 45, ran a 42, and ended up putting in a 40.

I don't recall getting that 'tat-tat' running the N3WK / 2nd clip / 42 pilot combo --- but stupid me... I somehow seem to have damaged the needle while it was out of the bike and I was trying the N1EG.  (I'm still trying to figure out how the clip-end seems to have got bent.)

I'm wondering half out of curiosity / half out of lazyness in finding another needle, if I'm going the right direction in going with a 38 pilot to clean up that 1/8 throttle area.

The N1EG's root diameter is slightly smaller than the N3WK's..

[TMKIWI]

If it wa running properly with the other setup I would go back to it.
The last thing you want with woods riding is a too rich mixture just above idle.

[MyckMcClung]

FMF specs have never failed me on anybike I have every owned or jetted.

[SachsGS]

When I'm dealing with a jetting issue such as you are I tend to pay careful attention to the profile of the needle.I've had success in situations like yours by selecting a needle with a more gradual taper  that allows for a more subtle transition from pilot to needle.

[yo_marc]

I like that idea, Sachs.  I'm definitely interested in checking out some additional needle profiles.  If anyone has any good sources for them I'd love to take a look.  (JD, Sudco, etc.)

I did end up finding a few minutes to drop the 38 pilot in the bike last night.  The motor tinks away cleanly at idle on the stand, and seems to respond correctly to choke, warm-up procedures, and cracks of the throttle.  I want to say it's the pilot the motor needs, but I probably won't be able to get it out for a ride to verify for another week, easy.

I wish I had a way to put up video. It'd be immensely helpful in describing all this

[yo_marc]

Just to follow up.. I ran the 38 pilot / N1EG-2 combo... and it was rather lousy.  I couldn't clean up the 1/8 throttle response with the air screw.  Best I could do was lean it out at the air screw, but then I'd detonate and suffer at that throttle setting when the rpm's were up.  1/4 throttle was great tho - had tons of torque.

Anyway, I'm giving up on the N1EG and going back to the N3WK and 42 combo.

[chump6784]

my wifes rm 125 had fmf's jetting in it when i got it. i pulled real hard up top but was rich off the bottom. went to pro circuits jetting and the thing is perfect all the way through