Carb sync and tuning

[Sailorcto]

Great advice Eric. I usually leave 2-3mm slack in the cables. To each their own I suppose, but 6-10mm seems too loose to me. 

—Tony

On Oct 6, 2021, at 16:16, Zwicky, Eric <ezwicky2 at gmail.com> wrote:

Also, it would be helpful to read up a bit on the procedure(s). There's a few.   Shorting sticks, various gauges (I have the Harmonizer and I love it).

But basically, after you have the timing and the valves sorted, you want to baseline your carbs.

Start with the idle stop screws, which open the butterfly in the carb body, at *just touching* the lever.  or Maybe one turn in.   But both sides set the same way.

Then turn in the idle mixture screws (at the bottom of the carb body) til they bottom out, then back them out a full turn, or a turn and a half.   Different years  models have different starting points.   You'll have to look at the Bing site, or maybe Snowbum's site, for specifics.   Now you've got the baseline set.

Also, make sure your throttle cables at each side have roughly the same amount of play, maybe 6 - 10mm, at the place where the outer cable sheath sits in the little adjuster on the top of the carb body.

Then, I like to use the shorting-stick method to get the idle where I want it (on my R90/6, around 1000rpm), and even on both sides.  This is adjusted at the idle adjustment screws, not the idle mixture screws... the ones that you level-set at the beginning. (The terminology can be confusing).    Once it's roughed-in like this, I use the Harmonizer from here on.      But I am assuming you don't have one, so let's assume shorting sticks.

One I am happy that the idle is at the right rpm and both sides are contributing equally, I will use the idle mixture screws to set each side at their highest rpm.    You might have to turn one in a bit, and the other one out a bit.   Turn the screw an 8th of a turn at a time, slowly, waiting to hear an increase or decrease in rpm.    Or use the gauge to see the rpms rise or fall.

Once you've got the idle mixture optimized on each side, go back to step one and re-balance the idle stop screws (butterfly valve) as both sides might no longer be in balance since you've optimized the idle *mixture*.

You might have to keep going back and forth a few times between idle speed and idle mixture adjustments til you have them both optimized.

Once you're happy with those steps, you want to adjust the throttle cable where it attaches at the carb.

start the engine and twist the throttle just off idle, and adjustments to the adjusting nut until both sides are balanced *with the throttle engaged a bit*.    Some people make the cable adjustments with the throttle just off idle, some recommend running it up to 3000rpm and checking.

This is much easier with a gauge than with shorting sticks btw.    For somebody new to it anyway.   Old-timers with a lot of experience, and a good feel and good ears love the shorting sticks method.    Me, my ears are shot from a lifetime of loud music so I like the Harmonizer.

There's probably a lot of minor details that the gurus will pick apart here, but this is the gist.

The thing is to read up and get a feel for it before you start.    Just a few for starters:

https://bmwmotorcycletech.info/synchcarbs.htm

https://www.youtube.com/watch?v=5bmJ0iYBYgQ

And this pdf from Bing to see where to set your idle-mixture screws to start:

https://www.bingcarburetor.com/uploads/9/8/7/9/98794296/bmw_jetting_chart_cv_carbs.pdf

Good luck.

eric


On 10/6/2021 3:42 PM, Zwicky, Eric wrote:
> Mac is right.   You need to get the timing and valve adjustments sorted before doing the carb sync.
> I don't know where Lutherville is but there's a concentration of very helpful and knowledgeable airheads in the Annapolis area, and further west in Westminster.
> If you were in Richmond I'd come on over :)
> On 10/6/2021 3:37 PM, Mac Kirkpatrick wrote:
>> Just an FYI, ALL other parts of a tune up must be satisfactorily completed prior to doing a carb sync. Particularly adjusting the valves, or the sync is useless.
>> 
>> Mac Kirkpatrick
>> 
>> On Wed, Oct 6, 2021 at 15:02 Porter Gieske via MDAirheads <mdairheads at casano.com <mailto:mdairheads at casano.com>> wrote:
>> 
>>     Does anyone have a carb sync tool and would be willing to help me
>>     get my carbs in tune?
>>     I have bought a kit of gauges and they are being delivered in a few
>>     days- I am looking more for advice on mixture and idle.
>> 
>>     I am in Lutherville, 21093.  Bike is 1981 R100.  Bing 40’s.
>>     Thanks
>>     porter
>> 
>> 
>> 
>> 
>> 
>> -- 
>> 
>> 
>> Mac Kirkpatrick
>> Glenmoore, PA
>> 
>> "After all, what is adventure, but inconvenience, properly regarded?"
>> C. Donahue