Yamaha TRX850 Enthusiasts Forum

Hi I have a part throttle mid/high rpm lean stumble. On non CV carbs I know what circuit to look at but not CV carbs. What part should I adjust on these carbs float? Needle? I have no idea as the slide could be half way up on near a closed throttle with CV carbs.

setting up carbs is something of a black art but ....

does the bike have any stumbling on WOT? And only at mid / high revs - 5k plus?

if the answers are no and yes, try lfting the needle a notch and report back

The stumble is only at low throttle but 4000rpm + WOT is perfect. My floats are set at 15mm. needles are set at the highest already

Kenny wrote:The stumble is only at low throttle but 4000rpm + WOT is perfect. My floats are set at 15mm. needles are set at the highest already

Have you had the needles at different heights? Option 2 is to drop the needle - it may be fluffy due to over fuelling in which case dropping will help.

of course, all of this is pure guess work. Can you confirm what size jets you've got, standard air filter, exhausts etc?

K&N in airbox, Scorpion cans on std headers, 140 mains, mixture screw 3 turns, needle bottom clip.
Its a deffinate lean spot as when it happens and I open the throttle more it gets worse then picks up.

Kenny wrote:K&N in airbox, Scorpion cans on std headers, 140 mains, mixture screw 3 turns, needle bottom clip.
Its a deffinate lean spot as when it happens and I open the throttle more it gets worse then picks up.

the symptons you describe are often attributed to lean running but very often it is the opposite - i.e. it's running rich.

However, I suspect you are right. IIRC 142 is standard for main and you have freed up the breathing through the addition of cans and a K&N.

Either way, your plugs should give you an indication. If they don't, try up jetting a bit (or get it on the dyno for a definitive answer)

I suspect you've probably seen this;

http://www.factorypro.com/tech_tuning_p ... gines.html

Works for me.