rmx jetting question

[apb]

My '90 rmx250 is running really rich right now, and fouling plugs after about an hour or so of running time (I'm at lower to mid throttle positions much of the time). The factory jetting for this bike (I have the stock mikuni tmx 38mm carb) is 420 main and 50 pilot. I have gone down to a 370 main (same pilot, planning to go down to a 45 though). Still way too rich. I have a fmf pipe and silencer, and boyesen rad valve. FMF's website says to use a 260 main with 45 pilot (for 0-1500' altitude and 50/50 mix of race gas and super unleaded @32:1). I realize this is just ballpark recommendations, but the 260 main sounds way, way leaner than the factory 420 main.. Does this sound weird? Or is it common to rejet this far from stock settings after adding a fmf exhaust system? I'm wondering if its safe to try going this lean, or if I should work my way down from my current jetting in gradual increments (this could get pricey though!).. Or is a better first step to go to a hotter heat range plug? I'm using a BR8ES currently. By the way, I'm at 2000' altitude, running a 32:1 mix with 91 octane premium gas.. Sorry this is so long, but I'd really appreciate any help from the jetting experts on here.

 

[steve.emma]

mate im not the jetting master sorry, BUT if you are riding at mid to low throttle positions and fouling plugs then concentrate on your needle clip position and pilot jet size.
try going leaner on both.once that is sorted then worry about main jet size etc (see jetting links for how to do this..)
also that plug is a bit cold unless you are thrashing it and will be prone to fouling, try a grade hotter. hope this helps.

 

[apb]

thanks for the help steve. should I first rejet as best I can with the stock 8 range plug, and see if it stops fouling? Or should I go to the hotter plug 1st, and see if it takes care of the problem or at least improves it? what do you guys think?

 

[steve.emma]

try going 1 heat range hotter on the plug and do it at the same time, from the sounds of it you are unlikely to have a problem with excessive heat in the cylinder so no harm in doing both at once.
good luck.
steve.

 

[RM_guy]

Is the plug fouling a new development? If so then I'd check for a leaky right side crank seal or a dirty choke plunger before messing with the jetting. I chased a plug fouling problem until I realized that both the seal was leaking and the choke plunger was dirty (it wouldn't seal and let extra fuel through).

 

[apb]

yeah, I'll put the hotter plug in and further lean the pilot, etc, as needed. As for the crank seals, the motor just got a new bottom end, so I'd be surprised if theres any leak yet.. Unless they weren't installed properly. The fouling is a new development since the rebuild, when I also replaced a faulty air boot (carb to reeds). I haven't had much riding time on the bike since buying it over the summer, since the crank bearings seized not long after I got it. It wasn't running nearly this rich, or fouling plugs before the seize, but my theory is that it occurred because of the damaged/leaking carb to reed intake boot. That was leaking air and so the lean run let the bike seize, plus maybe kept the jetting artificially leaner before it did seize?... Not really sure about any of that though.

I'm still wondering if you guys think the jetting fmf gave me for this pipe sounds safe to try? A 260 main, compared to the 420 that suzuki gives stock with the bike, seems like a huge change to me. Maybe its because this bike came stock with such a restrictive intake and exhaust system, that the big change is needed.. Should I try it? Or work towards it more gradually? Thanks again for any help with this.