Two Stroke Motocross
Two Stroke Motocross Forum => Technical => Topic started by: yo_marc on May 23, 2011, 07:17:21 AM
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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??
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What is your altitude, year of bike, pipe type, and fuel octane?
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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.
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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
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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..
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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.
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FMF specs have never failed me on anybike I have every owned or jetted.
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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. ;D
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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 ;)
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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.
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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