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2110
Joined
4/1/2008
Location
CA
US
Edited Date/Time
11/2/2020 10:50pm
Ever since bolting on an FMF Factory Fatty expansion chamber on my YZ250, the bike runs weird. It's hard to describe. The issue only manifests itself at constant throttle openings. Under load and accelerating it seems fine (better than with the OEM pipe). But at constant throttle openings and light load, the motor seems unsettled. The RPMS kind of dance around a bit.
At first I thought it was maybe surging (lean?). Per MXA recommendation, I swapped the 178 main for a 180. I've also played with needle position (raising needle to richen it up) but it didn't seem to change anything.
Then I thought maybe the FMF pipe made the bike more sensitive to my sloppy throttle hand...whereby any minute movement of the wrist at minimal load, caused an immediate response in engine RPM.
Or maybe it's simply signing off and I need another gear to keep it under load. Is it possible that the stock OEM pipe has that much more over-rev than the FMF? It doesn't feel like I'm in the upper stratosphere of RPMs when this happens.
Bike ran fine with the stock pipe but I wanted a little more oomph so I sprung for the FMF. Has anyone experienced anything similar to this?
I run 91 octane pump gas and Yamalube 2R at 44:1
I have a 13 oz Steahly flywheel weight for what its worth.
Running a 50 pilot, air screw at 3/4 turn out. 180 main. Stock needle which I ended up running with the clip in the 2nd groove from the top (3rd groove didn't seem to do anything). I'm riding at about 200 feet above sea level.
I tried https://www.fmfracing.com/Jetting but they don't list the YZ250 2-stroke in their pull-down menu.
Anybody got any thoughts/insight?
At first I thought it was maybe surging (lean?). Per MXA recommendation, I swapped the 178 main for a 180. I've also played with needle position (raising needle to richen it up) but it didn't seem to change anything.
Then I thought maybe the FMF pipe made the bike more sensitive to my sloppy throttle hand...whereby any minute movement of the wrist at minimal load, caused an immediate response in engine RPM.
Or maybe it's simply signing off and I need another gear to keep it under load. Is it possible that the stock OEM pipe has that much more over-rev than the FMF? It doesn't feel like I'm in the upper stratosphere of RPMs when this happens.
Bike ran fine with the stock pipe but I wanted a little more oomph so I sprung for the FMF. Has anyone experienced anything similar to this?
I run 91 octane pump gas and Yamalube 2R at 44:1
I have a 13 oz Steahly flywheel weight for what its worth.
Running a 50 pilot, air screw at 3/4 turn out. 180 main. Stock needle which I ended up running with the clip in the 2nd groove from the top (3rd groove didn't seem to do anything). I'm riding at about 200 feet above sea level.
I tried https://www.fmfracing.com/Jetting but they don't list the YZ250 2-stroke in their pull-down menu.
Anybody got any thoughts/insight?
What year bike? Up to 2006 needle was N3EJ. 2007 and later are N3EW (1 step richer straight section).
All in all, your setup seems to be on the rich side and I suspect you are getting a rich burble or stutter.
I've ran the Factory Fatty on my 2006 YZ250 and here are my specs:
pilot 50
main 178
needle N3CJ (1/2 clip leaner grooves than N3EJ)
clip 2
Air screw 1.75
Fuel/oil VP MS-100 with Maxima Super-M at 32:1
So, my main is leaner, my needle and clip are leaner, my air screw is leaner, and the 32:1 leans out the fuel also. The 100 octane is allowing me to run this and I am at 1000 - 4000ft elevation. If you tried these settings at sea level with pump gas you'd likely get pinging. But the takeaway is that most people end up having to run 95-100 octane in YZ250's in order to allow you to lean it out enough to run correctly.
Your specs suggest that I'm not dealing with a lean carb setting. Hmmm, back to the drawing board I guess.
The Shop
The 44:1 also adds to an already confused jetting set up.
Paw Paw
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