proper AFR's and carb tuning on new big bore GS750 920cc build???

[49er]

Swapped to the gs1000 carbs to find that my "too rich" 110 mains in the 750 carbs were just about right on the larger air jet gs1000 carbs. I think I may bump to 112.5 again, as my afr's hit 14-15 at high rpm when we dipped into cooler ravines, or got hit with big cross winds.

My o-4 needle jets in the package with the o-4 suzuki part numer actually gave 0-3 stamped into them, turns out. I'm pretty good on afr's at half to 3/4, around 12's until 7500-8000+ it leans out in the 13's.

My big tuning problem is that I was having a hanging idle, so i richened the pilots. Then it was too rich to start (12:1). 1/8 throttle held constant would be decent at low rpm, but would lean out to 14-16 depending in how high I let the rpm's get (3700+). I need to take the pilot screw back to about 7/8 or 1 turn to get the good starting that I had with afr's in the 13's. The air screw really made a difference in rpm's turning it when the fuel screw was at 3/4-1. No difference noticed on the air adjustment with the fuel screws above.

Is this leaning on 1/8 throttle okay as the rpm climbs? Or is it a sign that I need richer than an o-3 and maybe then drop the needle a half clip postion to fix the 1/2-3/4 that was perfect before?

Don't get too hung up on relatively static AFR's. You should be more focused on the readings you get when under a moderate to high loading (whilst accelerating). Short bursts of 12.8 -13.2 AFR at WOT won't hurt your engine, provided you're not over advanced, nor running in very high ambient air temps.

Have you compared the O-6 and 5DL36 combination to your present test setup?

[Chuck78]

The first of these two Mikuni charts shows what I believe to be more pertinent, that the needle jet dictates more of what the lower midrange throttle does, and the upper midrange can be adjusted with the needle clip position.

I pulled some gs550 vm22 needle jets, & they are o-6's but are a 258 series, which isnt quite as long as the 250 series that the 0-6 gs750 needle jets were. This means they wouldn't stick up as far into the venturi, which I am guessing is rrally just a guide for the needle when at WOT & set in the lowest clip notch for the highest needle height.

Due to the different pilot and main air jet sizes between the 750 and 1000 carb bodies, I think the o-6 that was too rich on the 750 carbs should be just right on these 1000 carbs.

The gs750 needle jets are a 250 series o-6, by the way. I believe the gs1000 had 250's also. I will be swapping in the o-6 and suspect i may need to shim the needle to 2.5 as Ian thought initially, & I should be good. 110 or 112.5 mains. 15 pilots set at 7/8-15/16 out on the fuel screws.