I guess I get to go have a little chat with my street bike mechanic...or maybe I just completely misunderstood his line of explanation when we dialed in the Yamaha. Freakin' 4 strokes. I would be embarrassed but I'm too old to waste time on that.

So without a dyno run how are you guys determining AFR with any level of accuracy? Feel of the bike through the RPM range, looking for flat spots or bogging or random spikes? Don't think I'd trust old skool spark plug reading much.

 

OK, just got a PM with some great links and I found a great page from Mikuni that goes through the circuit progression really clearly. Hell, I've only had the bike a month - plenty to learn. Thanks for the wisdom.

KLXster, question. You said use the #132 main and "raise the needle one notch above standard for Stage II" and you said that was still a touch lean. The Stage II instructions said to use the 4th notch down and both spacers in the kit, which I did. So are you recommending using the 3rd notch down? And still using the spacers?

Reason I'm asking is the Mikuni info says that going up a notch would lean the mix. From their page: "If the engine needs to run leaner, the clip would be moved higher. This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer."

If I tear back into it I want to move the right way!

 

They are not accurately determining AFR - but we all think we are because we are all special like that. Hell I've only been tuning carbs for 34 years and I thought the standard Stage II settings had my bike dialed in perfectly. What a surprise when the Dyno showed a damned AFR above 15:1 - see my threads with the AFR charts....

 

When it comes to needle positions, I always use the needle position in relation to the main jet. Raise the needle out of the main jet and everything gets richer - set the needle lower into the jet and you go leaner. You put that clip near the top of the needle and you will have it lower in the jet - lean... Put the clip near the bottom of the needle and it will live higher in the jet n your bike runs richer..

 

There's lots of ways to narrow jetting down:

Doing roll-on races through a few gears with a friend who's not modifying his bike at the same time you are.

Running at certain throttle positions up a long and steep enough hill to load the engine in that gear and compare max mph before and after jetting.

For wide open throttle testing (WOT) they say to get the bike running high revs for a few seconds, then back off to aprx 7/8 throttle. If the engine speeds up-you're too lean; if it bog/hesitates-you're too rich.

If your jetting is very close then the revs will just slow down some after you back off to 7/8 throttle.


- There may be an unusual case where the main jet's effective range extended through a larger than typical range of the throttle position; but it'd be something like where the needle jet was way too large and the needle wasn't able to meter fuel properly etc

 

I'd try tack welding a little divider into the header mouth-just to see what happens..

 

So by that reasoning I want to move the clip to 5th notch down (richen), NOT move it up to 3rd notch (lean) if I change to the #132 main and remove the airbox lid per your recommendation.

 

That sounds like a pretty reasonable ad hoc feel-of-the-bike assessment. I know I need to ride this bike a bunch more before I can feel the minor differences. It definitely responds and accelerates better now!

 

If you go richer from 128 to 132 on the main you may want to go a step leaner on the needle by moving the clip higher. But it just takes experimentation.

 

Yes Hoots, if Stage II calls for notch 4 then notch 5 (from the top of the needle) is what you'll need.. It's been too long ago for me to remember the Stage II settings - if you say it's #4 notch then so be it - I thought I remembered it as notch 3... Count em carefully!

These settings are not my opinion - they are from the results of Dyno testing my bike. No belief in me or my tuning skills are at play here - just cold hard facts from the AFR on my dyno charts - see them for yourself on my threads.