Gunson Colortune keeps indicating lean condition?

Have a 1999 KZ1000p that is stock engine except for Dyna coils and shorty bullet mufflers that has been riding fairly well. Recently moved from ~7000' elevation to ~4500', and noticed that I seemed to be losing power on the top end (max speed ~85-90 mph on 15/41 gearing). Also an occasional stumble off idle. I decided on a Colortune to try to sort this out, but when first installed I couldn't get any carb to show "Yellow" even with AS turned all the way in unless enrichener knob was pulled- should indicate overly lean. This was all on new stock size jetting of 37.5 and 127.5. Ordered and installed 40/130 to try to correct with same result, no Yellow showing.

I can't imagine on a generally stock bike that I'd need to go any bigger than this, so what am I missing? Seems like it should be an easy enough tool to use, and a search didn't turn up any obvious problems using Colortune. Intake boots and carb holders are new and pliable, and all vacuum ports have fresh caps, so I don't think its an air leak. Carbs were recently gone through and cleaned, and all internals are new and floats set. I do have the next two sizes of main to try as well, but it seems like a big step for no clear reason.

Thanks

At lower altitudes the air is denser and contains more oxygen.
This will lean off the mixture.
You need to increase the fuel ratio to compensate.
Colour tunes can be hit and miss with their readings.
I'd go by seat of the pants riding and plug colour etc.

Agreed. I've had no luck with the Colortune.
Steve

Thanks for the quick responses. Without getting into a whole thread about jetting, does going up one on both pilot and main to 40/130 seem reasonable for a generally stock bike, or do I need to go back and start again?

Thanks

Check this out

Check valve clearances. Synchronize the carburetors. Inspect the gas filter.

Read the spark plugs for jetting clues.

You can richen the pilot up on the mixture screw and i would have thought you only need to go up a size on the main.

Broken Wheel, #1 tuning tip, put Colortune back in the box it came in and try and sell it. Once it's gone, Viola! You will have all this extra time to find your problem.

Again, thanks for the input. Seems not everyone loves the Colortune. SWest, thanks for that guide, I hadn't seen it before. I'd mostly been using this one for the BS34.


I think I've been reading too many carb tuning guides at this point and am starting to confuse myself. Can someone please confirm what way to turn the Pilot Mix screw (between carb and head) on these BS34's to lean/richen. Seems like the guide I attached contradicts itself, but both make sense?