1981 GS 850G A Little Flat @ 2000-2400rpm

[SteveR]

Hello all,
I rebuilt my carbs... It seems that just about the time I forget how, I need to do it again. I have done all the details and balanced them with a Carbtune.
It idles really great but, I notice a little vibration at 2000-2400 rpm. I think that one of the carbs is rich or lean during this transition. Also the balance does not stay the same as I increase the throttle. Are the carb main needles sticking... How accurate do the springs need to be relative to each other?

I'm really close...
I have had these carbs setup Super good in the past, so I know it's possible.

[Dave8338]

Have you adjusted the valves yet? If you have everything set correctly, there should be no change in the balance. The 400 RPM band that you mentioned is very narrow and at the bottom most range of the bikes natural power curve (if not below it). It could be a slide hanging up slightly, have you checked the diaphragms for slight cracks or pinholes? All orings replaced? New plugs?

[bwringer]

I'd also quadruple-check float height -- assuming valve clearances, airbox sealing, intake boots/o-rings etc. are all good, float height has a profound effect in the RPM range you mention.

All floats (and both sides of each float) must be set EXACTLY the same, not just somewhere in the range given in the manual. I've found that changing float height by as little as 1/2 mm makes an easily perceivable difference.

On my 850, I ended up setting float height 1/2mm below the range given in the manual. (There's a range of two numbers given -- after several rounds of road testing, I ended up at 1/2mm less than the lower number.)

You also have to be careful to measure float height at the step in the float. I found one machine where someone mistakenly set float height using the overall height of the float.

I've also had trouble with shoddy aftermarket float needles -- they had weak springs, which allowed the floats to compress the springs and made it impossible to set float height. With the carbs upside-down, the spring-loaded nubbins in the float needles should not compress at all.

[SteveR]

Float height

Thanks,
I am suspicious of the float height. I have installed new or corrected all of the above mentioned items. I played musical valve shims about 3500 miles ago and the setup was working fine.
1/2mm is quite a small tolerance... If the float height is too high what is the symptom... and too low in this rpm range?
I read about the clear tubes connected in the bottom of the float bowls to check the fuel level. I guess that would be the last word on float height.

[SteveR]

I reread the post and I think you set your float level to give you a slightly richer mixture. Am I understanding your setup correctly?
1/2 mm under the book range.

[bwringer]

I reread the post and I think you set your float level to give you a slightly richer mixture. Am I understanding your setup correctly?
1/2 mm under the book range.

Yes -- with the carbs upside-down, less float height equals higher fuel level when right-side-up, and thus slightly richer.

If it's too lean, you'll get a slight stumble in this range - slightly richer will make this RPM range a bit smoother.

Also, I've found that twiddling the idle mixture screws to give a slightly richer idle mixture helps smooth out the just-off-idle transition. Set for best idle speed/quality, then add 1/4 to 1/2 turn further out and road test. Mine are close to three turns out.

Fine-tuning the off-idle and low-mid ranges like this makes things much smoother in hard cornering. Smooth = fast, you know. :twisted: Getting both of the above juuuuuuuust exactly right makes tight, twisty roads a lot more fun -- you can get back in the power a lot sooner in tight corners and leave those pestersome CBRs behind.

Too many people fuss over maximum power, but smooooooooooth low end gives the greatest gains in ridability and speed when the road or track contains curves.

Exhibit A:

[SteveR]

OK, I'm going to set float height and check the main jet pistons for smooth action.
I will report back.

[49er]

Too many people fuss over maximum power, but smooooooooooth low end gives the greatest gains in ridability and speed when the road or track contains curves.

Too true. What's the point in having another 5-10 more hp with a power band starting at 6K. When the road is undulating and twisty, you're gonna get left behind big time.
I ended up setting my float levels at the bottom of the suggested range (leaner), to improve the response just off idle. I guess the range accounts for your engine's state of tune, ie 4-1 or 4-2, airbox or pods and how closely matched your pilot jet is sized to the stock setup!!

[koolaid_kid]

...you can get back in the power a lot sooner in tight corners and leave those pestersome CBRs behind.
Exhibit A:

Don't let him fool you, folks. Brian is running from the fashion police.